qid int64 1 4.65M | metadata listlengths 3 3 | prompt stringlengths 31 25.8k | chosen stringlengths 17 28.2k | rejected stringlengths 19 40.5k | domain stringclasses 28
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|---|---|---|---|---|---|
15,034 | [
"https://quantumcomputing.stackexchange.com/questions/15034",
"https://quantumcomputing.stackexchange.com",
"https://quantumcomputing.stackexchange.com/users/14072/"
] | Suppose I have the state <span class="math-container">$\frac{1}{2}(|0\rangle + |1\rangle)$</span>, how can I refer to ket 0 and ket 1 in general within a state? Would "term 1" and "term 2" work?
| If you have <span class="math-container">$|\psi \rangle = \alpha |0\rangle + \beta|1\rangle$</span> then you can refer to <span class="math-container">$|0\rangle$</span> and <span class="math-container">$|1\rangle$</span> as the basis states. This is generalize to <span class="math-container">$n$</span> qubit system as... | Generally, a quantum state <span class="math-container">$|\psi\rangle$</span> is composed of basis states <span class="math-container">$|0\rangle$</span> and <span class="math-container">$|1\rangle$</span>. This come from linear algebra where any vector can be written as a sum of basis states multplied by an appropriat... | https://quantumcomputing.stackexchange.com |
13,410 | [
"https://datascience.stackexchange.com/questions/13410",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/15412/"
] | I am trying to build a model in scikit-learn. I used <code>RandomForestClassifier</code> as my method for classification. In order to improve the score and efficiency of my model, I thought about using GridSearchCV.
Here is the code:
<pre><code>import pandas as pd
import numpy as np
from sklearn.cross_validation impo... | Well the error message is quite clear. GridSearchCV accepts only lists. Therefore <code>'random_state': [7]}</code> will solve the issue.
However when you have only one value with this parameter, it makes more sense put it directly into the classifier as you did with <code>n_estimators</code>.
| I would say that you have to remove <code>random_state</code> from the parameter grid. That, or put something like [7, X] which will work but that doesn't make sense I think. If you want to use fixed <code>random_state = 7</code>, you should write it when you instantiate the estimator just as another hyperparameter (ne... | https://datascience.stackexchange.com |
112,991 | [
"https://cs.stackexchange.com/questions/112991",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/108781/"
] | In my experience, sometimes a computer boots normally, and in a time span of days, the computer won't boot, then again, it will boot normally again. I am not a computer science expert so I assume that if a computer boots normally one time and then it doesn't, <strong>it must be</strong> due to any changes made into the... | <blockquote>
Can anything change in a computer without the user modifying something, also while not having internet connection?
</blockquote>
Yes, definitely - computers update their own states all the time, and some of these changes will be persistent from one boot cycle to the next.
The computer itself is a compl... | <blockquote>
Can anything change in a computer without the user modifying something
</blockquote>
Yes, definitely. Most obvious example: the clock, it changes all the time!
Other examples of environmental factors that change without the user actively manipulating them, are temperature and humidity. Temperature infl... | https://cs.stackexchange.com |
3,492,417 | [
"https://math.stackexchange.com/questions/3492417",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/386082/"
] | I'm a little confused here.
On the one hand, <span class="math-container">$\mathbb{R}/\mathbb{Z}$</span> must be free as an <span class="math-container">$\mathbb{R}$</span>-module, since any module over a field is free (being a vector space it has a basis).
One the other hand, it can't be free, as it has torsion - e... | One big problem here is that you are trying to form the quotient module <span class="math-container">$\Bbb R/\Bbb Z$</span>, even though <span class="math-container">$\Bbb Z$</span> is not an <span class="math-container">$\Bbb R$</span>-submodule of <span class="math-container">$\Bbb R$</span>. So this quotient doesn't... | Because <span class="math-container">$\mathbb{R}/\mathbb{Z}$</span> isn't an <span class="math-container">$\mathbb{R}$</span>-module in the first place.
(It is a <span class="math-container">$\mathbb{Z}$</span>-module - that is, an abelian group - but not an <span class="math-container">$\mathbb{R}$</span>-module.)
T... | https://math.stackexchange.com |
113,404 | [
"https://electronics.stackexchange.com/questions/113404",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/44168/"
] | I have a problem with a question about a MOS transistor in common source setting.
Using the provided formulas I have calculated several quantities: \$V_G\$, \$V_{GS}\$, \$I_D\$, \$V_D\$ and \$V_S\$.
I'm supposed to calculate an operation point (the operation point?), but no definition was provided, and my google skil... | The operating point is nothing more than the values that you already found (mainly \$I_D\$, \$V_{GS}\$ and \$V_{DS}\$).
The small signal AC properties (like trans-conductance) of the transistor depend on its DC operating point.
The reason is that the curves that model its behavior are not straight lines, but for sm... | According to the book <em>Microelectronic Circuits</em> by <em>Sedra</em> & <em>Smith</em> (\$5^{th}\$ Edition, Page 154) for a diode "<em>The load line intersects the diode curve at point \$Q\$, which represents the <strong>operating point</strong> of the circuit. Its coordinates give the values of \$I_D\$ and \$V... | https://electronics.stackexchange.com |
681,421 | [
"https://physics.stackexchange.com/questions/681421",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/299408/"
] | Consider a parallel-plate capacitor with the standard parameters <span class="math-container">$Q$</span> (charge), <span class="math-container">$V$</span> (Potential Difference), <span class="math-container">$A$</span> (Area), <span class="math-container">$d$</span> (distance between the plates), <span class="math-cont... | Inside a dielectric, the value of the electric field becomes <span class="math-container">$$E = \frac{\sigma}{\varepsilon}$$</span> where <span class="math-container">$\varepsilon = \kappa \varepsilon_0$</span>. As <span class="math-container">$Q \rightarrow \kappa Q$</span>, <span class="math-container">$\sigma \right... | <em>E = σ/<span class="math-container">$\LARGEε_{\Large \color{red}0}$</span></em>
Are you sure about the <span class="math-container">$\color{red}0$</span> ?
| https://physics.stackexchange.com |
2,299,781 | [
"https://math.stackexchange.com/questions/2299781",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/450228/"
] | I have to prove that for every natural number $n > 1$ that is not a perfect square, $x_{0}$ is the unique integer satisfying $(x_{0} − 1)^2 < n < x_{0}^2$.
I know I have to prove existence and uniqueness of $x_{0}$. If I can show that $(x_{0} − 1) < \sqrt{n} < x_{0}$ is true for all $n > 1$, I should... | Given $n>1$, let $A=\{\,x\in\Bbb N\mid x^2\ge n\,\}$. Then $A$ is not empty because for example $n^2\ge n$ implies $n\in A$. As a subset of $\Bbb N$, $A$ has a minimal element $x_0$. Note that $x_0-1\in\Bbb N$ because certainly $x_0>1$. Now $x_0-1\in\Bbb N\setminus A$, $x_o\in A$ translates to $(x_0-1)^2<n\le ... | As the comments indicated, we need to replace at least one $<$ with $\le$. Then, however, this is true for
$$[0, \infty) = \bigcup_{x \in \mathbb N_0} [x^2, (x+1)^2) = \bigcup_{x \in \mathbb N_0} (x^2, (x+1)^2] \cup \{0\}.$$
(This equality is immediate: If $x$ is maximal such that $x^2 \le n$ then $n < (x+1)^2... | https://math.stackexchange.com |
394,771 | [
"https://mathoverflow.net/questions/394771",
"https://mathoverflow.net",
"https://mathoverflow.net/users/175893/"
] | Let <span class="math-container">$LG(n,2n)$</span> be the Lagrangian Grassmannian parametrizing Lagrangian subspaces (so of dimension <span class="math-container">$n$</span>) of <span class="math-container">$\mathbb{C}^{2n}$</span>. Then <span class="math-container">$LG(n,2n)\subset G(n,2n)$</span>, where <span class="... | Note first that, if <span class="math-container">$L$</span> is a Lagrangian contained in <span class="math-container">$H$</span> then <span class="math-container">$L^\perp = L$</span> contains <span class="math-container">$H^\perp$</span>. So <span class="math-container">$X_n$</span> is non-empty only when <span class=... | Co-isotropic subspaces of dimension <span class="math-container">$n+k$</span> are in bijection with isotropic subspaces of dimension <span class="math-container">$n-k$</span>.
The variety of isotropic subspaces of dimension <span class="math-container">$n-k$</span> of a symplectic vector space of dimension <span class=... | https://mathoverflow.net |
37,706 | [
"https://datascience.stackexchange.com/questions/37706",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/57724/"
] | Tutorial problems come in the form of binary or mult-class classification where data are all properly labelled. In real-life applications, there are incoming data that do not belong to any category and cannot be classified. How can we handle these data which fall into "unknown" category? The universe of "unknown" can b... | I think this is one of those topics with the most frustrating answer - <em>it depends</em>.
To your questions:
<strong>How can we handle these data which fall into "unknown" category?</strong>
There are many ways of doing this. Some are very simple and some are more complex, but they all depend on you understanding ... | I suggest these methods:
<ol>
<li>do oversample or undersample</li>
<li>instead of accuracy consider AUC or F1-score.</li>
<li>define the cost of the different class of prediction vs actuality.</li>
<li>try to find some rules to find 'unknown' then remove them from training data.</li>
</ol>
good luck.
| https://datascience.stackexchange.com |
246,363 | [
"https://dba.stackexchange.com/questions/246363",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/188638/"
] | I want to find Schema creation date and user who created schema in SQL server,
I am unable to find information in system views.
Note: Not asking about schema scoped objects
| Okay, so I have now managed to "solve" this issue.
After failing to work out why my backup files were not being removed, I decided to test this scenario on my Replica. I copied over a backup file to the Replica and ran the delete command;
<pre><code>DECLARE @BackupDir NVARCHAR(4000) = (SELECT BackupDir = dbo.fn_SQLSe... | Since you mentioned this is in an AG, are you sure you're running the command and checking the backup folder on the same server?
If you are connecting to the Availability Group primary through the AG listener, then running the <code>xp_delete_file</code> command will run on whichever physical server happens to be the ... | https://dba.stackexchange.com |
33,036 | [
"https://stats.stackexchange.com/questions/33036",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/9560/"
] | I've fitted a lognormal model using R with a set of data. The resulting parameters were:
<pre><code>meanlog = 4.2991610
sdlog = 0.5511349
</code></pre>
I'd like to transfer this model to Scipy, which I've never used before. Using Scipy, I was able to get a shape and scale of 1 and 3.1626716539637488e+90 -- very diff... | I fought my way through the source code, to arrive at the following interpretation of the scipy lognormal routine.
<span class="math-container">$\frac{x-\text{loc}}{\text{scale}} \sim \text{Lognormal}(\sigma)$</span>
where <span class="math-container">$\sigma$</span> is the "shape" parameter.
The equivalence ... | The lognormal distribution in SciPy fits in to the general framework for <em>all</em> distributions in SciPy. They all have a scale and location keyword (which default to 0 and 1 if not explicitly provided). This allows all distributions to be shifted and scaled from their normalized specification with clear implicat... | https://stats.stackexchange.com |
51,874 | [
"https://electronics.stackexchange.com/questions/51874",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/10707/"
] | I've toying with V-USB for a few days and got a circuit working in a breadboard with a Attiny85 and 3.6 Zener diodes.
However, when I tried to get it done in a stripboard, it doesn't work.
Assuming the soldering is correct, the only think that can be a problem is a modification I did to make it easier to solder.
Is... | Although USB uses 5V as power supply, both datalines are rated at
<ul>
<li>min. 2.8V - max. 3.6V for HIGH</li>
<li>min. 0.0V - max. 0.3V for LOW</li>
</ul>
This means that the inputs on host side, your PC/hub/..., must not be driven higher than 3V6. Your controller (AVR) on the other hand, when fed from 5V, attempt t... | Another potential problem with your relocated 68 ohm resistors is that when the USB signal is being driven from the X1 side of your diagram the 68 ohm and the 1.5K ohm resistor form a voltage divider which results in a slightly elevated low level of the signal making it out the location on the right side of the diagram... | https://electronics.stackexchange.com |
128,152 | [
"https://mathoverflow.net/questions/128152",
"https://mathoverflow.net",
"https://mathoverflow.net/users/2821/"
] | A virtual currency called <em>bitcoins</em> has been in the news recently. It is said that in order to "mine" bitcoins, you have to solve hard mathematical problems.
Now, there are two kinds of mathematical problems. The difference is best explained by the following beautiful quotation from Langlands :
<blockquote>... | Bitcoin mining is based on hash functions. Specifically the SHA-256 hash function, which maps arbitrary bit strings to 256-bit outputs in such a way that nobody knows how to find a collision (two inputs with the same output), although the pigeonhole principle implies collisions exist. Bitcoin mining doesn't involve fin... | Another way to earn bitcoin is not to mine them, but to have them wired to you from other bitcoin accounts. The transactions are signed using ECDSA. So if you solve the discrete logarithm problem in an elliptic curve (for bitcoin this is the curve Secp256k1), then you potentially can earn <em>many</em> bitcoins (illega... | https://mathoverflow.net |
119,168 | [
"https://softwareengineering.stackexchange.com/questions/119168",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/32505/"
] | This is a somewhat subjective quesiton but I'd love to hear feedback/opinions from either interviewers/interviewees on the topic.
We split our technical interview into 4 parts. Write Code, Read & Analyse Code, Design Session & Code on the white board.
For the last part what we ask interviewees to do is write ... | In my view, It is very appropriate. If you are wanting a job to do a particular skill, then it is entirely appropriate to be expected to demonstrate that skill at interview.
The effect of this technique on the recruitment process is very noticeable. 90% of candidates fail this task. but the developers recruited are ... | <blockquote>
My question is "Is it innappropriate / unreasonable to expect a programmer to write a code snippet on a whiteboard during an interview ?"
</blockquote>
It's very reasonable. An alternative to a whiteboard might be a laptop and a beamer, since programmers are more used to writing code on a keyboard than ... | https://softwareengineering.stackexchange.com |
87,662 | [
"https://stats.stackexchange.com/questions/87662",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/40724/"
] | I have fit a random forest regression model to training data (used 65% of data for training). The data has approximately 40,000 observations and 100 features.
I fit a random forest regression in R with the following parameterization:
<pre><code>randomForest(formula = Response ~ .,
data = crs$dataset[, c... | "nodesize: Minimum size of terminal nodes. Setting this number larger
causes smaller trees to be grown (and thus take less time).
Note that the default values are different for classification
(1) and regression (5)."
Check out this parameter in the randomForest package. By upping the nu... | The Caret package enables you to run randomForests with a range of parameters for .mtry so you can optimise, using ROC values, the number of features that gives you optimal classification. This package also includes straightforward methods to carry out multiple iterations of k-fold cross validation to avoid overfitting... | https://stats.stackexchange.com |
490,976 | [
"https://physics.stackexchange.com/questions/490976",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/130319/"
] | In Landau’s <em>Statistical Physics (part 1)</em> , section 5, he writes:" In particular, it would be quite incorrect to suppose that the description by means of the density matrix signifies that the subsystem can be found in various ψ states with various probabilities and that the averaging is over these probabilities... | I think what he means is with reference to his equation (5.1) <span class="math-container">$$\psi = \sum_n c_n \psi_n. $$</span> These <span class="math-container">$\psi_n$</span> states are in a superposition and this superposition is not the same as saying the system has some probability of being in one state or the... | If I interpret what the author had in mind correctly, he would like to distinguish between the uncertainty (and related probability distribution) inherent to quantum mechanics, and the incomplete statistical description given by a density matrix.
In fact, a (non-pure) density matrix describes a (quantum) system on whi... | https://physics.stackexchange.com |
4,351,610 | [
"https://math.stackexchange.com/questions/4351610",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/1010788/"
] | I'm trying to solve this problem, I took the basis <span class="math-container">$B=\langle(2,7)^T, (1,3)^T\rangle$</span> and then build the matrix of <span class="math-container">$f$</span> application in <span class="math-container">$B$</span>:
<span class="math-container">$$
M_b(f)=\left(\begin{array}{cc}
7&4\\5... | If <span class="math-container">$f$</span> is a linear map which I am assuming is the case then you can (as you tried) express the linear map in terms of a matrix. The "mistake" you did is that you wrote the images of the basis <span class="math-container">$B$</span> as columns so your matrix is the matrix re... | I assume <span class="math-container">$f:\Bbb{R^2}\to \Bbb{R^2}$</span> be a linear map.
Given <span class="math-container">$f(2, 7) =(7, 5) $</span> and <span class="math-container">$f(1, 3) =(4, 1) $</span>
To find <span class="math-container">$f(3, 5) $</span> , first we have to write <span class="math-container">$(... | https://math.stackexchange.com |
818,702 | [
"https://math.stackexchange.com/questions/818702",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/154821/"
] | if a function has equal directional derivatives in all directions at a specific point, so is the function differentiable in it?
I think it is correct, and i think if the function is f(x,y) so the tangent plane in this point is parallel to the XY plane.
But I don't know how to prove it or how to find en example that d... | Before I forget, $(x^2 \pm y)^2 \geq 0, 2 x^2 |y| \leq x^4 + y^2,$ so, below, $f \leq x^{2/5} \; |y|^{1/5} /2.$
Take $$ f(0,0) = 0, \; \mbox{otherwise} \; \; f(x,y) = \frac{x^{12/5} \; y^{6/5}}{x^4 + y^2} $$
Using polar coordinates, with $r$ for usual positive radius, letter $c$ for $\cos \theta,$ letter $s$ for $\... | Here is a simpler counterexample with a discontinuous function $f$:
$$
f(x,y) = \begin{cases}
1 & \text{ if } y = x^2,\ x\ne0\\
0 & \text{ else. }
\end{cases}
$$
Then $f$ is not continuous in $(0,0)$, but all directional derivatives in $(0,0)$ are zero.
| https://math.stackexchange.com |
203,138 | [
"https://mathoverflow.net/questions/203138",
"https://mathoverflow.net",
"https://mathoverflow.net/users/69763/"
] | Apparently one identifies the configuration space in physics often with a manifold $M$. The tangent bundle $TM$ is then the space of all possible positions and velocities.
Furthermore, many sources seem to claim that $T^*M$ can be regarded as the phase space, where $(q,p) \in T^*M$ satisfies by definition that $p \in ... | The Lagrangian is a function on the tangent bundle $L:TM\rightarrow\mathbb{R}$. Given a point $q\in M$ and a Lagrangian, we can define a function $L_q:T_qM\rightarrow \mathbb{R}$ using the simple formula $L_q(v_q)=L(v_q)$, where $v_q\in T_qM$ is a tangent vector at $q\in M$. Notice that $L_q$ is a mapping between the v... | $p$ is the differential of L with respect to the second variable ($\dot q$), so it represents a linear functional on the tangent space at $q, \dot q$), given by
$$ (u,v)\to \frac{d}{dt} L(q,\dot q+tv)|_{t=0}.$$
| https://mathoverflow.net |
7,450 | [
"https://scicomp.stackexchange.com/questions/7450",
"https://scicomp.stackexchange.com",
"https://scicomp.stackexchange.com/users/3691/"
] | This might be a naive question, but when applying a implicit discretization to a PDE with a source term, should the source be averaged in time? For example if we take the diffusion equation with a non-linear source term,
$$
u_t = u_{xx}+s(x,t,u)
$$
We can apply the following central difference implicit scheme to the ... | If the right hand side were independent of $u$ then one would generally use the averaged form
$$
(1-\theta)s_(x,t^{n+1}) + \theta s(x,t^n).
$$
In the nonlinear case you can't do that easily, as you note, but you can at least use some kind of extrapolation, for example approximate
$$
(1-\theta)s(x,t^{n+1},u^{n+1}) +... | The most robust way is solving it implicitly, otherwise for stiff nonlinearity in the source function $s(u)$ you will have to use very small time step to maintain numerical stability.
Consider
$u_t = s(t,u)$
Then
$u^{n+1} = u^n + dt \; s(t+dt, u^{n+1})$
Here one can use average between n and n+1 in the RHS to in... | https://scicomp.stackexchange.com |
80,723 | [
"https://physics.stackexchange.com/questions/80723",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/26708/"
] | <img src="https://i.stack.imgur.com/Qgmfv.jpg" alt="enter image description here">
The snip shows a part of the method shown by my teacher, to describe the reflection formula for spherical surface. However I do not understand how the relation of <em>AB</em> and <em>BI</em> (which I have highlighted) is coming. Please ... | The equations comes form the law of cosines, but we will see there is an error in the equations, as can be easily observed with dimensional analysis.
The law of cosines says that if you have a triangle with three vertices $A$, $B$ and $C$, and if the lengths of the sides connecting the vertices are $AB$, $BC$ and $CA$... | Well it seems that everyone who tries to teach geometrical optics, as far as ray tracing goes, seems to feel they have to create their own procedures, ignoring the fact that several hundred years of prior work, has established efficient ways to do it. You state the problem is to establish the law of reflection from Fe... | https://physics.stackexchange.com |
1,433,966 | [
"https://math.stackexchange.com/questions/1433966",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/270453/"
] | I am fairly new to statistics so bear with me.
Two players are playing a game with a fair dice that has $n>6$ sides. The sides are labeled $1,2,3,..,n$.
The one who throws a larger number wins but if the throws are equal then player one wins. What are the chances that player two wins? (the probability is a function... | We have two players. They have been given the labels one and two. I would rather call them Alicia and Beti.
Alicia wins if she gets a greater number than Beti, or if she and Beti get the same number. We want to find the probability that Beti wins. In order for Beti to win she must get a number <strong>larger</strong>... | Hint: If the two throws are unequal, then the probability of player one winning is $1/2$ (Why?). The only other way player one can win is if the two throws are equal. Can you calculate that probability?
| https://math.stackexchange.com |
193,205 | [
"https://physics.stackexchange.com/questions/193205",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/14087/"
] | The original question was to prove
<blockquote>
$$e^{i\sigma_z\phi} \sigma_y e^{-i\sigma_z\phi}=e^{2i\sigma_z\phi} \sigma_y.$$
</blockquote>
Since left hand side is equal to $\sigma_y$, I thought in order for right hand side to be equal to $\sigma_y$,I have to show that $$e^{2i\sigma_z\phi}=1$$
For this,I tried to ... | LHS: $(\cos\phi+i\sigma_z\sin\phi)\sigma_y(\cos\phi-i\sigma_z\sin\phi)=\cos^2\phi\sigma_y-i\cos\phi\sin\phi[\sigma_y,\sigma_z]+\sin^2\phi \sigma_z\sigma_y\sigma_z=(\cos^2\phi-\sin^2\phi)\sigma_y+2\cos\phi\sin\phi\sigma_x=\cos 2\phi \sigma_y+\sigma_x\sin2\phi=(\cos 2\phi+i\sin 2\phi\sigma_z)\sigma_y=e^{2i\phi\sigma_z}\s... | Use fact that the matrices $i\,\sigma$ form the Lie algebra $\mathfrak{su}(2)$ and then the adjoint representation "braiding relationship" that:
$$\exp(\phi\,\mathrm{ad}(Z))\,Y = \mathrm{Ad}(e^{\phi\,Z})\,Y\tag{1}$$
and here $Z$ and $Y$ are $3\times 1$ column matrices that stand for superpositions of basis matrices i... | https://physics.stackexchange.com |
251,585 | [
"https://mathoverflow.net/questions/251585",
"https://mathoverflow.net",
"https://mathoverflow.net/users/47136/"
] | Let $F$ be a number field such that $[F:\mathbb{Q}]=n$ and with ring of integers $O_F$. Let's put $B=\operatorname{Spec } O_F$, then an Arakelov divisor is an element of:
$$Div(X)\times \bigoplus_\sigma \mathbb R[\sigma]$$
namely it can be written as
$$\bigg(\sum_{\text{$\mathfrak p$ prime $\neq 0$}}n_{\mathfrak p}[\m... | You would usually want the principal Arakelov divisors, i.e. those of the form $(\sum_{\mathfrak{p}}{\rm ord}_{\mathfrak{p}}(a), \sum_\sigma -\log|\sigma(a)|)$ for $a\in F^\times$, to be cocompact in the group of degree $0$ divisors — the volume of the quotient should be the familiar product ${\rm Reg}(F)\cdot{\rm h}(F... | I think the natural definition is the one using archimedean places (or equivalently, all embeddings, with the pairs of complex conjugates considered as a single embedding), and I'm very curious of the context where you have seen the definition using complex embeddings only.
Consider for example the basic result that t... | https://mathoverflow.net |
470,646 | [
"https://physics.stackexchange.com/questions/470646",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/222579/"
] | Why the expectation value of momentum <span class="math-container">$\langle p \rangle$</span> is zero for the one dimensional ground-state wave function of an infinite square well? And why <span class="math-container">$\langle p^2 \rangle = \frac{\hbar^2 \pi^2}{L^2}$</span>? I am not asking for a proof. I am trying to ... | To be <span class="math-container">$0$</span> on average, a quantity must either be always <span class="math-container">$0$</span> as you suggest or else it must have positive or negative outcomes, for instance <span class="math-container">$\{-2,-1,3\}$</span>. Of course it is possible for a particle in <span class="m... | So when trying to build intuition about a subject, ideally you want something experiential to help you understand it. By the statement of your question you seem to get why the math woks out the way it does (please correct me if this is not the case) but don't get what it's telling you about the system. Let's see if we ... | https://physics.stackexchange.com |
162,564 | [
"https://electronics.stackexchange.com/questions/162564",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/58649/"
] | <img src="https://i.stack.imgur.com/qgZgE.jpg" alt="A simple diode circuit">
I am referring to the symbol with +4V next to it. Seems like a voltage source to me. I just haven't seen this anywhere before.
| Strictly speaking, it is not a full circuit, but an equivalent full circuit can be easily derived from it, because it has the full information. It has two "labels" (which are not an actual components), which are denoted with the GND symbol (the bottom one) and the triangle symbol (the top one). Both are denoting the vo... | I assume you are referring to the top symbol, the triangle pointing up to "+4V". That is showing that the net is connected to a +4 V supply. The other symbols with "R" designators are resistors. The symbol with the "D" designator is a diode, cathode to the left, anode to the right. This means current can flow thru ... | https://electronics.stackexchange.com |
125,494 | [
"https://dba.stackexchange.com/questions/125494",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/46951/"
] | I have to get selection for a given string (with comma separated values) passed to the cursor. There can be multiple values that could be passed via single variable. My code is somewhat like:
<pre><code>CURSOR My_Cursor( vsStr1 )
IS
SELECT some_field
FROM some_table
WHERE txtfield1 IN ( vsStr1 ); --this ... | The simplest little dirty trick I know for this is <code>xmltable()</code>. With <code>xmltable()</code>you can convert a comma-seperated list of values into rows, for example:
<pre><code>select * from xmltable('1, 2, 3')
COLUMN_VALUE
------------
1
2
3
</code></pre>
Notice how the column is named <code>column_value... | Here is a solution using subquery factoring, regular expressions, and a hierarchical query on dual.
<pre><code>CURSOR My_Cursor( vsStr1 ) IS
WITH
RawInputData As (
SELECT vsStr1 sStr1 FROM dual
),
InputData As (
SELECT regexp_substr((SELECT sStr1 FROM RawInputData),'[^,]+',1,... | https://dba.stackexchange.com |
280,325 | [
"https://dba.stackexchange.com/questions/280325",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/128056/"
] | I am trying to understand why my update statement on my local server is failing on my remote server. I am receiving the following error:
<blockquote>
OLE DB provider "SQLNCLI11" for linked server "WEBDB" returned
message "Multiple-step OLE DB operation generated errors. Check each
OLE DB status... | You already are not using the "same license" for both servers. SSRS is part of the SQL Server license (Standard Edition and above), but it's only covered by your SQL Server license <em>on the same server</em>. IOW, if your report server is on Server A and SQL Server is on Server B, you need to license them bo... | By default this is not the best place to ask about licensing questions. You should contact a Microsoft partner to get a better answer.
Said that, if you have two SQL Server licenses, you can install a new SQL Server using one license and keep the actual SSRS using the old license.
IICR, you cannot install two different... | https://dba.stackexchange.com |
29,280 | [
"https://softwareengineering.stackexchange.com/questions/29280",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/11390/"
] | Is coding important to be good at computer science? Should one implement the algorithm to know it well ?
I remember one cs professor's idiom that "<strong>I never code"</strong>
| <strong>You won't really know the algorithm <em>well</em> until you code it.</strong>
| If you're talking about being a pure Computer Science academic specializing in abstract, foundational Computer Science concepts, then not necessarily.
To bend an analogy: this is a bit like asking if every rocket scientist at NASA should have to fly in space to be a "good rocket scientist". Of course not. Being an ast... | https://softwareengineering.stackexchange.com |
4,736 | [
"https://security.stackexchange.com/questions/4736",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/2666/"
] | I am interested in how reliable the information determined from doing forensics on a SOHO router is. Most of these routers don't seem to be very advanced, so I question if it would be possible to detect tampering.
If logfiles had been changed for example, could this be detected? Given that they use flash memory, would... | Forensics comes down to how well trained and trustworthy the person doing the forensics is and is unrelated to the design of hardware.
If your router's software is compromised, there's no book of things that can be changed since anything the mind can conceive, can realistically be done. The processors on routers these... | <blockquote>
If you need to make a case that a SOHO
router was tampered with in any way,
what steps could you take to establish
this and then what would you provide
as evidence?
</blockquote>
The only way I can think of is if you had logs being duplicated to a separate log host, a quick comparison of the log... | https://security.stackexchange.com |
141,055 | [
"https://security.stackexchange.com/questions/141055",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/128837/"
] | I am new to PKI, digital certificates etc and will be the first time to connect a medical wireless device to a wireless network using EAP-TLS.<br>
I am connecting a medical wireless device to our customer's hospital wireless network with the below set up:
<pre><code>Root CA > Intermediate CA > Intermediate CA &g... | <strong>CA-CERT:</strong> <em>I would presume the CA-CERT would contain the hospital's certificate issued to this device (possibly with the entire chain)</em>
No, this is actually the root CA certificate for which the server will send a certificate chain to authenticate to the client. This prevents the client from con... | In general as in most of the cases generic terminology means :-
CA-CERT : It is the Root certificate of the certificate chain the EAP server is going to test , it helps user to protect from EVIL-TWIN type of attack by not allowing TLS handshake from non trusted Authentication servers
Client Cert : - It is the Chain... | https://security.stackexchange.com |
14,808 | [
"https://biology.stackexchange.com/questions/14808",
"https://biology.stackexchange.com",
"https://biology.stackexchange.com/users/4705/"
] | It seems so simple that if my brain can tell my right hand to do something, then it should be able to tell equally well to the left ? This seems like a pretty major evolutionary advantage. Why haven't we evolved so during millions of years ?
| Having a dominant side is advantageous. Instead of seeing it as the brain influences our fine motor skill, think of it the other way round. The way we use our hands or any other part of our body sends feedback to the brain to increase the portion of the brain devoted to that region of the body and of course with disuse... | A simple answer to this is that at some point our brain's architecture was wired to have a dominant side, at which point it became too expensive evolutionarily to have drastic brain re-organisation or size changes.
Another answer to this is that the two sides have many different specialisations and functions. While ha... | https://biology.stackexchange.com |
16,719 | [
"https://stats.stackexchange.com/questions/16719",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/5407/"
] | I have a situation where I measure something for a long time, and occasionally a seperate event of interest will occur. I want to determine if these events have a significant effect on the signal I am measuring. So I am thinking of taking the signal value at each event time, and comparing it to times when there was no ... | Any of the methods that you mentioned would work. I would avoid the 2-sample where you compare to a subset of those values where the event did not occur, using less data will cost you power and not give you any advantages. I would probably go with the 2-sample t-test where the 2nd group is all the observations that d... | I would think you would want to do some sort of paired test, comparing the values before and after the event. But the nature of your data is not entirely clear.
| https://stats.stackexchange.com |
202,179 | [
"https://stats.stackexchange.com/questions/202179",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/1871/"
] | I need to use the ratio of two variables as the dependent variable in a regression. Both variables are normally distributed but with positive values. I can either center them or use as it is.
If I center them: the ratio of two centered normals is a Cauchy distribution. Is it therefore meaningful to do a "Cauchy" regre... | Recall that the expectation of $Y$ is conditional on $x$, so every $E(\cdot)$ below is actually $E(\cdot|x)$. Thus all terms with only $x$ are constants inside the expectation.
Also, remember that $E(\epsilon) = 0$ and thus $E(\epsilon^2) = Var(\epsilon)$.
\begin{align*}
E(Y - \hat{Y})^2 & = E\left[(f(x) - \hat{f... | Adding to @Greenparker, the first term of the last line is decomposed as below
$=(f(x)-\hat{f(x)}+E(\hat{f(x)})-E(\hat{f(x)})^2+Var(\epsilon)$
$=(\underbrace{E(\hat{f(x)})-f(x)}_{Bias})^2+(\underbrace{E(\hat{f(x)})-\hat{f(x)})^2}_{Var(\hat{f(x))}}+Var(\epsilon)$
| https://stats.stackexchange.com |
1,719,780 | [
"https://math.stackexchange.com/questions/1719780",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/279073/"
] | Suppose a R.V has distribution function $F$. Then I believe I can write
$$\int_{-\infty}^0|x|^tdF(x)=\int_{0}^{+\infty}y^tdF(-y).$$But seriously, I don't know how to rigorously justify such formula. I know this is a Lebesgue-Stieljies integral and I failed to find any good reference on change of variable theorems for ... | Let $\{X_n:n=0,1,\ldots\}$ be a sequence of iid random variables with distribution
$$\mathbb P(X_0=S)=\frac25,\ \mathbb P(X_0=C)=\frac25,\ \mathbb P(X_0=R)=\frac15. $$
Then $\{X_n:n=0,1,\ldots\}$ is a Markov chain on state space $\{S,C,R\}$ with initial distribution that of $X_0$ and transition probabilities $$P=\begin... | Well we can think of it in this way.
On the first day we have at least 1 weather block with probability 1. On the next day, the probability of the weather changing and giving us another block is $$\frac{4}{5}\times\frac{3}{5} + \frac{1}{5} \times \frac{4}{5}$$
This occurs $n-1$ days and thus the expected number of w... | https://math.stackexchange.com |
1,728 | [
"https://mechanics.stackexchange.com/questions/1728",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/953/"
] | I need help trouble shooting my honda civic. It will not start.
I was driving it earlier today and I went to drive it again when it would not start. I'm glad I was at home when it decided to quit.
Turning the key goes normally until I try to get it to start.
Upon the regular engine starting key turn area the dash li... | <blockquote>
Upon the regular engine starting key turn area the dash lights dim and
there maybe a single 'tch sound or no sound at all coming from the
engine block.
</blockquote>
Sadly, I know this sound well. This sounds like a dead battery.
Here are the steps that I would suggest:
<ol>
<li>Charge the batter... | If the dash lights dim and the starter doesn't turn, it's usually either the battery going bad or the starter that's at fault. I'd check the battery voltage both without any consumers and with someone trying to operate the starter motor and would expect at least 12.8V (no load) and about 10V (trying to start), otherwis... | https://mechanics.stackexchange.com |
66,186 | [
"https://dba.stackexchange.com/questions/66186",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/39215/"
] | Before making substantial changes to the content, functionality and design of my website (I'll be refactoring everything, almost a complete rewrite), I'm setting up a new PostgreSQL database to hold all the data previously in a MySQL database. I also have a heavily-customized WordPress blog that I'm going to move over ... | I would go for separate tables for each type. With so few rows performance will not be a consideration either way. Individual tables will only have a page or two each. Similarly for the combined table.
Either way you will have to join a "type" table to complete your views. Inserts are infrequent at this level of a... | The table layout of your Database Depends on how granular the information you want to store is. I suggest you use an entity diagram software package to layout your new database in theory before you fill it with live data, and then realize you "oopsed" and forgot a column or table. At some point if you want some piece... | https://dba.stackexchange.com |
210,760 | [
"https://mathoverflow.net/questions/210760",
"https://mathoverflow.net",
"https://mathoverflow.net/users/48893/"
] | Let $V$ a vector space of dimension $2$ over a field $k$ of characteristic different from $2$ and $3$. Let $S^{3}V$ the third symmetric power of $V$.<br>
How to construct a symplectic form on $S^{3}V$ such that elements coming from the linear group of $V$ are similitudes for this form?<br>
I believe it's some miraculo... | There's nothing miraculous about this: Here is an explicit formula: Let $x$ and $y$ be a basis for $V$. Then $A,B\in S^3(V)$ can be written in the form
$$
A = a_{-3}\,x^3+3a_{-1}\,x^2y+3a_1\,xy^2+a_3\,y^3
\quad\text{and}\quad
B = b_{-3}\,x^3+3b_{-1}\,x^2y+3b_1\,xy^2+b_3\,y^3
$$
where $a_i$ and $b_i$ are in $k$. Set... | Here is a construction that gives a way from where this formula comes from. I do not fully understand what the OP is trying to find in his last sentence. But this is what I think the question is about. Please downvote or comment if I misunderstood the intent of the question.
<hr>
Let $V = k^{2^*}$. Denote by $X,Y$ t... | https://mathoverflow.net |
98,695 | [
"https://physics.stackexchange.com/questions/98695",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/40358/"
] | I have solved this question below from my textbook. My answer differs from the one given in the book. I have tried my best to see where I am wrong. I need your help to find the truth of this question.
<strong>Q.</strong> Two stones are thrown up simultaneously from the edge of a $200\,\mathrm{m}$ high cliff with initi... | Relative position versus time is linear when relative velocity is constant i.e. when relative acceleration eqauls zero. In this case, both stones are acted upon by gravity only and hence their acceleration always equals $g$ directed downwards. Therefore, relative position varies linearly for the entire duration of moti... | Hint: The speed of the object number $i$ ($i=1$ or $2$) will be $$v_i(t)=v_i(0)-gt.$$
| https://physics.stackexchange.com |
9,706 | [
"https://cs.stackexchange.com/questions/9706",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/6569/"
] | I'm looking at my textbook here from Michael Sipser and he says that a <strong>nondeterministic Turing machine is a decider if all its computation branches halt on all inputs.</strong> I think I recall seeing somewhere what you'd call a nondeterministic Turing machine that halts on at least one branch for all inputs, b... | The idea is that a <em>deterministic</em> TM will always answer Yes/No in finite time (else the whole idea makes no sense). And to do that, the deterministic simulation of the NTM can't just go off into lala-land on some branches, i.e., every branch must end in yes/no at a finite depth. It <em>decides</em> (gives you a... | The strings accepted by a NTM <code>M</code> is the language of <code>M</code>, noted <code>L(M)</code>
Let us say that <code>M</code> for any input is not guaranteed to halt on all branches.
Then <code>M</code> clearly cannot be a decider, and is thus only a recognizer. <code>M</code> recognizes the language of all s... | https://cs.stackexchange.com |
3,227,754 | [
"https://math.stackexchange.com/questions/3227754",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/431631/"
] | So i have solved a system of linear ODEs problem given the matrix
<span class="math-container">$\begin{bmatrix}-1 & -2\\5 & -3\end{bmatrix}$</span> and initial conditions
<strong>y</strong> <span class="math-container">$(0)$</span>= <span class="math-container">$\begin{bmatrix}2\\1\end{bmatrix}$</span>
but d... | We have
<span class="math-container">$e^{\lambda_1 t}v_1 = e^{(-2+3i)t}\begin{bmatrix}1+3i\\5\end{bmatrix} = e^{-2t}e^{3it}\begin{bmatrix}1+3i\\5\end{bmatrix} = e^{-2t}(\cos 3t + i \sin 3t)\begin{bmatrix}1+3i\\5\end{bmatrix} = \begin{bmatrix} e^{-2t}(\cos 3t -3 \sin 3 t) + i (3 \cos 3t +\sin 3 t)\\ e^{-2t}(5\cos3 t ... | Hint:
<span class="math-container">$\tilde{y_{1}}=\frac{y_{1}+y_{2}}{2}, \tilde{y_{2}}=\frac{y_{1}-y_{2}}{2i}$</span>
| https://math.stackexchange.com |
35,335 | [
"https://mathoverflow.net/questions/35335",
"https://mathoverflow.net",
"https://mathoverflow.net/users/-1/"
] | Suppose I have a category C. And there are two objects X and Y with no morphisms between them. I've checked up "quotient category" on wikipedia, but there I can only make isomorphic objects with morphisms between them.
Is there a categorial notion available that I can use in this case?
| Short answer: yes. There is a special class of 2-categorical limits called iso-inserters that does the trick. The paper to check is M. Kelly's "Elementary observations on 2-categorical limits", Bull. Austr. Math. Soc. 39 (1989), 301-317.
Instead of explaining what these are let me go about explaining how you would mak... | Domenicos comment lead to the following idea (I am posting this as an answer, as it is too long for a comment):
Let $CAT$ denote the category of small categories and $CAT'$ denote the category, whose objects are small categories except for the fact that the composition needn't be defined on the whole of $Mor(A,B)\time... | https://mathoverflow.net |
479,433 | [
"https://stats.stackexchange.com/questions/479433",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/287323/"
] | Is it OK to combine categorical and continuous features into the same vector for training deep neural networks? Say there is a categorical feature and continuous feature that I want to feed into a deep neural net at the same time. Is this the way to do it?
<pre><code>categorical feature (one-hot encoded) = [0,0,0,1,0]
... | Yes, that is one typical way of doing it. But, you need to standardize your features so that gradient descent doesn't suffer, and the regularization treats your weights equally. One way is to standardize the numerical features and then concatenate the one-hot vectors, and the other way is standardizing together. As far... | Yes, this is absolutely standard.
| https://stats.stackexchange.com |
2,511 | [
"https://earthscience.stackexchange.com/questions/2511",
"https://earthscience.stackexchange.com",
"https://earthscience.stackexchange.com/users/898/"
] | Where is obsidian found?
Is it typically found on the surface or underground?
If underground, how far under (meters or feet would be perfect)?
Also, is it found everywhere on Earth, or just in areas where volcanic activity is (or was recently) high?
| Obsidian is formed when a rhyolitic (or felsic) lava flows cool rapidly. This must mean that it's mostly available on the surface (and I think if you go near volcanos you can find pieces of Obsidian on the ground) because molten rock cools much faster above ground than it does below, allowing the melt to cool with sma... | The processes forming obsidian are not well understood because an active obsidian-forming eruption has never been recorded by humans. However, we can make many inferences from the composition of the rock and settings in which it is found.
Obsidian is more than 70% weight percent SiO<sub>2</sub> (i.e. rhyolitic), but h... | https://earthscience.stackexchange.com |
630,795 | [
"https://physics.stackexchange.com/questions/630795",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/296804/"
] | Is there a loss of mechanical energy when a permanent magnet falls trough a coil?
Given a magnet with mass m and a coil with N windings. The coil is placed vertically so the magnet can fall trough. The magnet is dropped from a height H, at this point the mehchanical energy is equal to mgH. I want to calculate the mecha... | Assuming no friction (from the coil or air) and no resistance in the wire, the difference between the mgH at the top of the coil and the mgH at the bottom would indeed be equal to the amount of energy transferred via eddy currents into the coil. However, unless done in a vacuum, there would be energy lost to air resist... | Part of the energy will be used up in driving eddy currents (induced currents) in the coil, so the final energy of the magnet will not be <span class="math-container">$mgH$</span>. The total energy of the magnet <span class="math-container">$+$</span> coil system will, however, be conserved. Exact calculations will req... | https://physics.stackexchange.com |
149,450 | [
"https://dba.stackexchange.com/questions/149450",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/62356/"
] | I have a table <code>Person</code>. AFAIU, the data for this table is stored in <code>/usr/local/mysql/data/my_database/person.ibd</code>.
Now, I make an <code>insert</code>. Then I check out the contents of <code>person.ibd</code> with vim. The data I have just entered is indeed there.
Then, I move the <code>person.... | Do <em>NOT</em> edit a .ibd file! Things are likely to go wrong:
<ul>
<li>The layout of the file is complex, and you are likely to make it unreadable by MySQL.</li>
<li>The data is probably cached in RAM, so your changes are not immediately recognized and/or they will be overwritten when MySQL decides to rewrite the ... | Did you shut down MySQL and start it again? For shure it won´t work when you try to start it again!
The answer is you are playing with memory buffers, until the database needs to write to disk the error is not present, depending on the buffer size the raise of the error is going to take some time, but the error is warr... | https://dba.stackexchange.com |
196,109 | [
"https://mathoverflow.net/questions/196109",
"https://mathoverflow.net",
"https://mathoverflow.net/users/303/"
] | More generally, I expect that the following is true:
<blockquote>
Let $D$ be a diagram quasicategory, let $d \in D$ be a vertex, and use this to define $D' = D \amalg_{\Delta^{\{0\}}} \Delta^1$. Then $\mathrm{Fun}(D',C) \to \mathrm{Fun}(\Delta^{\{1\}},C) \cong C$ is a cocartesian fibration.
</blockquote>
(This fol... | This follows from (the coCartesian version of) HTT Corollary 2.4.7.12, applied to the "evaluation at $d$" functor $\mathrm{Fun}(\mathcal{D}, \mathcal{C}) \rightarrow \mathcal{C}$, since we can identify $\mathrm{Fun}(\mathcal{D}', \mathcal{C})$ with $\mathrm{Fun}(\mathcal{D}, \mathcal{C}) \times_{\mathcal{C}} \mathrm{Fu... | In fact, it can be shown by (a whole lot of) brute force that the morphism of cospans towards the bottom of the question is actually a Reedy acyclic fibration -- at least with respect to the Reedy structure on $\Lambda^2_2$ described by
$$ (\deg = 2) \rightarrow (\deg = 1) \leftarrow (\deg = 0) . $$
Since this induces ... | https://mathoverflow.net |
214,411 | [
"https://stats.stackexchange.com/questions/214411",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/116767/"
] | I have a data frame that has various capital and metropolitan cities with various economic, market and performance data. One set of variables is each total annual non-domestic visitor volume and the recent annual growth rates in that volume.
As you might expect cities with smaller annual total volumes tend to see larg... | There is no standard protocol for addressing your question that I'm aware of.
A key point of clarification is that absolute and relative metrics are getting somewhat confounded -- city size is absolute whereas growth rate is relative. Equilibrating them to create a "fair comparison" is a daunting task for which there... | Run a regression using the growth rates by annual number of visitors (predictor); and then analyze the residuals. If the residual is positive it suggests the growth rate is greater than otherwise expects based on annual number of visitors.
| https://stats.stackexchange.com |
5,244 | [
"https://datascience.stackexchange.com/questions/5244",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/8310/"
] | Many of us are very familiar with using R in reproducible, but very much targeted, ad-hoc analysis. Given that R is currently the best collection of cutting-edge scientific methods from world-class experts in each particular field, and given that plenty of libraries exist for data io in R, it seems very natural to exte... | Speed of code execution is rarely an issue. The important speed in business is almost always the speed of designing, deploying, and maintaining the application. An experienced programmer can optimize where necessary to get code execution fast enough. In these cases, R can make a lot of sense in production.
In case... | R and most of its CRAN modules are licensed using the GPL.
In many companies, legal departments go <em>crazy</em> if you propose to use anything that is GPL in production... It's not reasonable, but you'll see they love Apache, and hate GPL. <strong>Before going into production, make sure it's okay with the legal depa... | https://datascience.stackexchange.com |
49,710 | [
"https://security.stackexchange.com/questions/49710",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/38284/"
] | If a person uses VPN and authenticates herself to some social network through the VPN, doesn't she lose her anonymity for as long as the VPN session is used ? By losing her anonymity, I mean that her real name may be linked to the IP of the VPN and subsequently the IP of the VPN to torrent downloading (for instance).
... | As noted, VPN tunnels permit secure and private communication through untrusted networks. VPN "anonymity" services, fundamentally proxies on sticks, can also provide limited anonymity.
However, using a particular VPN service for authenticated social-network activity irrevocably destroys whatever anonymity it had provi... | Indeed, anonymity disappears if you authenticate... using a VPN is like wearing a mask; people don't see your face and cannot guess your true identity. But if, wearing a mask, you shout your own name and proceed to demonstrate that you are really the person you claim, then your anonymity no longer exists.
When you use... | https://security.stackexchange.com |
2,244 | [
"https://security.stackexchange.com/questions/2244",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/1270/"
] | So I know what SHOULD be in an AUP. This question circulates more around how do I explain to my bosses the importance of having an AUP and ensuring everyone not only knows of it but understands the value and consequences of it.
We are an eCommerce company based on the East Coast that ship nationally and to several oth... | The strongest case for AUP is that you can address or discipline risky behaviour before it bites you rather than having to mop up afterwards.
On the compliance front there is the issue of employer liability for employee actions on the Internet (which I believe is an issue in the US). If the workstations handle or shar... | Two key points that really should sell this:
<ul>
<li>If you don't have a policy, the law appears to assume that you implicitly approve all behaviour by your staff. So you may take on liability for your staff committing crimes using your systems. (caveats - I am not a lawyer, and I really have only seen this in the UK... | https://security.stackexchange.com |
71,072 | [
"https://physics.stackexchange.com/questions/71072",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/26059/"
] | I apologize in advance for the nature of this question.
I have been thinking lately of methods to increase the quality of my self-study. I have been meaning, but never actually got to, in addition to solving numerous problems in the textbooks, to construct and solve my own problems, based on the material taught in ea... | I completely disagree with udiboy. I think you should certainly devise your own problems, precisely for the reasons he's stating. As udiboy is saying, textbook problems are made so that there is a solution. But real world problems are not of this kind. Once you'll become a researcher, if that is the path you want to ta... | Making problems and solving them by yourself defeats the purpose of a good problem. A good problem can only be made by someone who knows what is actually happening in the problem described, i.e. you should know the solution beforehand to make a <strong>good</strong> problem.
What you could try is picking up problems f... | https://physics.stackexchange.com |
125,984 | [
"https://mathoverflow.net/questions/125984",
"https://mathoverflow.net",
"https://mathoverflow.net/users/16888/"
] | As we can see in the plot below, Mertens function: $M(x)\equiv \sum_{n=1}^{x}\mu(n)$ has wild swings from positive to negative and back again.
<img src="https://i.stack.imgur.com/FGOGa.jpg" alt="merten's plot">
When we use: $$x=\frac{1}{2+\frac{1}{\phi+2}}\text{, where }\phi \text{ is the golden ratio,}$$ as th... | I'll try to flesh out the first comment to give an answer: for all $n\ge 4$ there is a representation $\rho:A_{n+1}\to \rm SL_n(\bf Z)$ whose image normally generates $\rm SL_n(\bf Z)$ (because it injects into all $\mathrm{PSL}_{n-1}(\mathbf{Z} / N)$ for $N\ge 1$ and $\rm SL_n(\bf Z)$ has CSP). But for $n\ge 5$ there i... | $SL_n(\mathbb Z)$ is generated by elementary matrices. In general let $E_n(R)$ be the subgroup of $SL_n(R)$ generated by elementary matrices. Let's show that if $m\lt n$ and $n\ge 3$ then the only homomorphism $E_n(R)\to GL_m(F)$ is the trivial one, if $F$ is a field. Wlog $F$ is algebraically closed.
Let $T_n(R)\subs... | https://mathoverflow.net |
467,736 | [
"https://physics.stackexchange.com/questions/467736",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/226035/"
] | I tried looking for other questions but I couldn't find any. (if this is a duplicate, then I'm sorry, I just signed up, so I'm not sure what to search for)
I was wondering, how do we navigate into deep space without knowing the position of everything in space? Doesn't all matter have a gravitational affect on all ot... | There are two key effects which help deal with this.
The first is that, were we to want to land <em>precisely</em> on a point on an asteroid, we would indeed need to account for every last bit of matter in the universe. Fortunately, in most situations we don't mind being nanometers off. In fact, we often don't mind ... | Well, when going to Mars it typically works like this:
Launch services deliver the bus to low-earth-orbit (LEO) for a once-around. Then the spacecraft (s/c) is launched on a course to Mars.
"Cruise" takes over at his point. They rely heavily the Nav[igation] team. Of course these guys have a pretty good idea what the... | https://physics.stackexchange.com |
549,817 | [
"https://electronics.stackexchange.com/questions/549817",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/277601/"
] | How long should a 240V 45W appliance run on on an inverter connected to an 18V 5Ah (90Wh) battery?
The appliance is a CPAP connected to a Ryobi 18v lithium battery inverter.
| Sure, but you don't need to.
Just write
<pre><code>a - b
</code></pre>
wherever you would instantiate the full substractor. Your code will be more readable and synthesis hasn't had difficulties with this construct since the 90s.
| If you actually want to do addition or subtraction then use the built-in + and - operators, that is what they are there for.
Still if we want to design a multi-bit adder from first principles, we can do it by operating on vectors. So we essentially build a bunch of 1 bit adders in paralell, then wire them up to each ot... | https://electronics.stackexchange.com |
329,053 | [
"https://softwareengineering.stackexchange.com/questions/329053",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/182925/"
] | I understand that CSS only supports multi-line comments like such
<pre><code>/* foobar */
</code></pre>
Why is there not support for single line comments.
<pre><code>// foobar
</code></pre>
They are just as common in programming and seems particularly useful for a language like CSS where each rule is on it's own li... | <strong>Backwards compatibility</strong>
Introducing single-line comments to the CSS syntax could change the meaning of files which currently uses the token <code>//</code>. Browser vendors are <em>very</em> reluctant to introduce changes which could potentially break existing pages.
CSS is defined with very precise ... | Supporting yet another syntax element isn't that easy: there are plenty of tools which should be able to handle yet additional comment style. Actually, I wouldn't be surprised to see that most tokenizers/parsers simply ignore newlines, probably replacing them by <code>;</code>.
If it would be essential to the language... | https://softwareengineering.stackexchange.com |
581,615 | [
"https://electronics.stackexchange.com/questions/581615",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/275581/"
] | I have an ADC, and, in order to avoid sync problems, I have used a FIFO with two different clocks: one for the write operation, that works at the ADC clock frequency and the other, FPGA clock that is higher than the ADC clock and is for the read operation. The depth of the FIFO is 8192, but i can set to more depth: 163... | If your FPGA logic reading the data can immediately stop when the FIFO is empty, then you don't have a problem. For example, if you use an AXI-Stream FIFO, the read port will give a not-valid condition when you read 'till the end. (That's a good reason to consider using AXI-S by the way: it solves this problem very nea... | <strong>Is important to keep the FIFO from becoming empty?</strong><br/>
Be sure you are not trying to solve a problem that doesn’t exist. Your question doesn’t provide enough details regarding your application, but unless you need regularly spaced samples, just use the EMPTY flag.
<strong>If you must keep synchronizat... | https://electronics.stackexchange.com |
14,417 | [
"https://chemistry.stackexchange.com/questions/14417",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/7159/"
] | Here are the MO schemes of $\ce{N2}$ (left) and $\ce{O2}$ (right).
<img src="https://i.stack.imgur.com/kSj3G.gif" height="325">
<img src="https://i.stack.imgur.com/9sfpZ.gif" height="325">
Why is the $\sigma$-MO formed by the $p$ AOs energetically above the $\pi$-MO for $\ce{N2}$ but not for $\ce{O2}$?
Can it be exp... | This phenomenon is explained by s-p mixing. All the elements in the second period before oxygen have the difference in energy between the 2s and 2p orbital small enough, so that s-p mixing (combination) can occur lowering the energy of the σ(2s) and σ*(2s) and increasing the energy of the σ(2p) and σ*(2p) molecular orb... | The strength of interaction of sigma and pi-kind developing differently with distance between two atoms.
The pi-interaction, which usually is weaker, is favoured at shorter atomic distance, because it leads to better side overlapping of orbitals. The maximum of energy for pi is by shorter atomic distance than for sigm... | https://chemistry.stackexchange.com |
153,738 | [
"https://dba.stackexchange.com/questions/153738",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/108119/"
] | I know how to write a stored procedure with output parameters. But I don't know the reason why I would use it over just using a simple <code>SELECT</code> statement. A normal stored procedure can still return an output in a grid (see below examples).
Can anyone give a real life example where I can use an SP with output... | Output parameters in stored procedures are useful for passing a value back to the calling T-SQL, which can then use that value for other things.
Let's say you have a stored procedure that returns a state given the input of a city, with state as the output parameter:
<pre><code>CREATE PROCEDURE [dbo].[GetStateFromCity... | Assuming this question pertains to SQL Server: It comes down to context and efficiency.
<strong>Context = App Code</strong>
When executing the Stored Procedure from app code, it isn't much different in terms of the amount of code. When returning a result set, just call <code>ExecuteReader</code> and then <code>SqlDat... | https://dba.stackexchange.com |
19,635 | [
"https://mathoverflow.net/questions/19635",
"https://mathoverflow.net",
"https://mathoverflow.net/users/4975/"
] | Given a smooth projective complex variety $X$, instead of using Mumford's GIT to construct the moduli of rank $n$ topologically trivial vector bundles, we can also take the gauge theory approach.
To classify all topologically trivial vector bundles is same as to classify all possible complex structures on the topolog... | In the case of a nonsingular algebraic curve, I guess this is the point of Atiyah and Bott's Yang-Mills on Riemann surfaces, or the work of Narasimhan and Seshadri. Choose a Hermitian pairing on your complex bundle and then search for a unitary connection with central curvature. There is a really great book by Kobaya... | As Charlie Frohman says, for curves this is the Narasimhan-Seshadri correspondonce. For Kahler manifolds of higher dimensions it is called the Hitchin-Kobayashi correspondence, proved by Donaldson-Uhlenbeck-Yau. You search for a Hermitian metric on your vector bundle which is Hermitian-Yang-Mills, i.e. it's curvature f... | https://mathoverflow.net |
441,810 | [
"https://softwareengineering.stackexchange.com/questions/441810",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/422077/"
] | I've written a web scraper and would like it to run as quickly as possible. The scraping isn't trivial; I scrape a few web-pages, gather links from them, scrape those, then gather links from those, then scrape those again (illustrated by the three different functions, showing the process to be multi-level). Since stand... | The design you have chosen definitely works, though it is more reminiscent of traditional multithreading designs than of async designs. Your current design would also work perfectly well with a blocking HTTP library, assuming that your 20 <code>process_queue()</code> instances were each running in a separate thread (bu... | <em>Note: this answer was originally posted on Code Review SE</em>
<code>get_page_{1,3}</code> are almost identical. You can replace them with one function that takes <code>depth</code> as a parameter. You call it with <code>depth=2</code> from <code>main()</code> and then do the recursion while decreasing <code>depth<... | https://softwareengineering.stackexchange.com |
168,044 | [
"https://dba.stackexchange.com/questions/168044",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/120366/"
] | Ubuntu 16.04.2. MySQL 5.7.17.
During peek hours, my PHP sites suffer fatal error trying to connect to MySQL. Apache says
<blockquote>
Error 24 Out of resources when opening file
</blockquote>
Ok makes sense, I get lots of traffic. I read up, and seems my <code>open_files_limit</code> var of <code>1,024</code> is ... | It depends on your db architecture. If you have many tables in separate files in the MyISAM, ARIA, or Innodb with files per table you will want a higher open file limit. If you have everything in the Innodb / xtradb in a single tablespace then upping your file limits will not benefit.
Check current system limits
<pre... | Once you have changed the ulimit in the OS, change/add
<pre><code>[mysqld]
open_file_slimit = 2048
</code></pre>
and restart.
Note: it must be in the <code>[mysqld]</code> section of <code>my.cnf</code> (or whichever file).
| https://dba.stackexchange.com |
886,223 | [
"https://math.stackexchange.com/questions/886223",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | Can anyone help me generate all the triplets solution of the Diophantine equation: $a^2+b^2=c^2$ by factoring using Complex numbers? thanks.
| It's a standard procedure.
First you reduce to triples in which $\gcd(a, b) = 1$.
Then you prove, arguing modulo $4$, that $a$ and $b$ have different parities.
Now you have $c^{2} = a^{2} + b^{2} = (a + i b) (a - i b)$.
Prove that $\gcd(a + i b, a - i b) = 1$ in the ring of Gaussian integer. (The fact that $a$ and... | Here’s one way: Consider a $\mathbb Z$-prime $p$ with $p\equiv1\pmod4$, like $13$, which according to a well-known theorem is the sum of two squares, $13=3^2+2^2$. Factor it into $\mathbb Z[i]$-primes, $13=(3+2i)(3-2i)$. Now divide one of these primes by the other, $(3+2i)/(3-2i)=(5+12i)/13$. Note that you see the thre... | https://math.stackexchange.com |
550,685 | [
"https://electronics.stackexchange.com/questions/550685",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/29243/"
] | I have a setup where I'd like to select one (or neither) of 2 LEDs for reflected or transmitted illumination, but it's under vacuum and I only have a single BNC feedthrough (at least until more parts come). Easiest would be flipping the polarity of the cables from the power supply.l
I recall two facts: first, white LE... | Yes, it's fine. Most LEDs are rated at 5V reverse voltage, which is much less than the Vf of a blue LED.
Multiplexing LEDs also exposes them to similar reverse voltages, so every blue or white LED numeric display is probably operated the same way.
| Reflective sensors may range from UV & yellow for paper money to IR for most other things.
It is possible to operate at +/- 5V with floating current limiting to drive the emitter and have a logic level drive the 0V floating to earth ground with an open collector and Zener limiter. I.e. two way Tx/Rx on one cable a... | https://electronics.stackexchange.com |
14,430 | [
"https://security.stackexchange.com/questions/14430",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/9544/"
] | I've seen referenced in literature the need to have a "break glass" mode in Healthcare IT applications. In this mode, the access controls in an application can be bypassed for an emergency when it is critical for a user to gain access to imaging data. Typically, the facility will have some sort of policy in place to ... | My healthcare organization uses Break The Glass with our EMR. I work on the IT side of things so I'm not sure how it applies to the clinicians, but for us it is setup so that if we access any patient record we are required to Break The Glass. In doing so we are forced to enter a comment as to why we need access to th... | There is no broad requirement for that specific feature as far as I'm aware. However, as a medical application you will normally consider the hazards of the system and mitigations.
If one of your hazards is "can't get at the images when I need immediate access for emergency diagnosis" then it's quite likely that one... | https://security.stackexchange.com |
81,515 | [
"https://mathoverflow.net/questions/81515",
"https://mathoverflow.net",
"https://mathoverflow.net/users/19092/"
] | This question came up when I was reading over some information about sheaves; specifically, that if $\mathscr{F}$ is a sheaf on the topological space $X$, $x\in X$, and $Z\subseteq X$, then $(\mathscr{F}|_Z )_x =\mathscr{F}_x$. I don't know if this is supposed to be trivial, and while it definitely seems to be a desira... | Dylan's comment is right. More generally, if $D \colon J \to C$ is a diagram and $L \colon J' \to J$ a functor, then the colimit of D is isomorphic to the colimit of DL if and only if L is (co)final. See Mac Lane, <em>Cats Work</em>, section IX.3.
| Is $I$ is a small category then you have $I=\bigcup_{c\in \pi_0(I)} I^{(c)}$ i.e. I is the (disjoint) union of the family $\pi_0(I)$ of its connect components, (full categories) (connection is the equivalent relation on objects generated by "have some arrow betwenn them" ).
Then if in some category $\mathcal{C}$ e... | https://mathoverflow.net |
133,532 | [
"https://cs.stackexchange.com/questions/133532",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/129412/"
] | I am given with this information:
<em>"CMP OP1,OP2 will compare registers OP1 and OP2 if they are equal,
flag values will be ZF=1, CF=0,
if the first
operands value is greater flag values will be ZF=0, CF=0,
if the first operands value is less than the
second operand flag values will be ZF=0,CF=1."</em>
I nee... | <span class="math-container">$\DeclareMathOperator\s{size}\def\f#1{\lfloor#1\rfloor}\def\c#1{\lceil#1\rceil}$</span>As already pointed out by gnasher729, the statement is not literally true when <span class="math-container">$n\equiv1\pmod3$</span>: if <span class="math-container">$n=3k+1$</span>, there are binary trees... |
Let us prove that in a binary tree with <span class="math-container">$n\gt1$</span> nodes, there is a node <span class="math-container">$u$</span> such that <span class="math-container">$\frac{n}3\le size(u)\le\frac{2n}3$</span>.
Proof: Run the following algorithm.
<ol>
<li>Let <span class="math-container">$u$</span> ... | https://cs.stackexchange.com |
46,793 | [
"https://mathoverflow.net/questions/46793",
"https://mathoverflow.net",
"https://mathoverflow.net/users/10987/"
] | Just today I had a bet with my friend over the following problem:
How many winning configurations can you have in a nxn Tic-Tac-Toe game where players win if a they get n/2 in either a row or column, consecutively.
n is even.
For example, in a 4x4 game, players win if a they get 2 of their symbols in either a row or... | There is a construction for both Thom isomorphisms, homological and cohomological, via classical stable homotopy theory. You find the details in Rudyaks book "On Thom spectra, orientability, and cobordism", chapter V, §1. The Thom class is a map $X^{\mu} \to\Sigma^{n} H \mathbb{Z}$. Moreover, there is a map of spectra ... | Johannes points out that it works quite generally, and perhaps it is useful to see that generality. Pardon me for going back to basics here. There are three ingredients:
<ol>
<li>Given an n-plane bundle $\xi \to B$ we have the Thom diagonal
$T(\xi) \to T(\xi) \wedge B_+ $
induced by the diagonal map $B_+ \to B_+ ... | https://mathoverflow.net |
155,997 | [
"https://physics.stackexchange.com/questions/155997",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/68803/"
] | I've noticed that when I use a cup that is square shaped with convex sides, it spills more easily than a circular cup. Why does this happen? What is the most spill-prone cup shape? What is the most spill-resistant basic cup shape?
| When a square cup is displaced slightly from its position, the water inside it moves to a diagonal corner of the cup. If we look at the direction of motion of the fluid then we will see that the fluid moves from lower corner to upper diagonal corner. You can imagine your room as the cup and consider water in it. Suppos... | Look at the bottom of a round cup. The distance from the center of mass,
is the same in all directions from a point that is the center of the
circle that forms the bottom of the cup. Thus when a force is applied
to tip over the cup, the direction that the force is applied is
unimportant because the distance from th... | https://physics.stackexchange.com |
388,071 | [
"https://softwareengineering.stackexchange.com/questions/388071",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/321339/"
] | A colleague, and frankly better software engineer that me, is telling me that this pattern
<pre><code>let variable = someDefaultVariable();
if (some_boolean) {
variable = someOtherValue();
}
</code></pre>
is better than this pattern
<pre><code>let variable
if (some_boolean) {
variable = someOtherValue();
} e... | This is more of a style issue than anything else, and very susceptible to personal opinion.
The second example in the question more clearly shows intent, which increases the readability of the code, however with the first example, there is no way to have the variable become uninitialised by further code changes breaki... | With the first approach you have a variable that is initialized to a value the moment it is created. Adding additional lines between the declaration and <code>if</code> statement means you have some meaningful information already in the variable.
<pre><code>let variable = someDefaultVariable();
// We know `variable` ... | https://softwareengineering.stackexchange.com |
4,385,753 | [
"https://math.stackexchange.com/questions/4385753",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/663571/"
] | I know three different but similar proofs of the statement:
<blockquote>
If <span class="math-container">$f:\mathbb{R}\to\mathbb{R}$</span> is an increasing function, then there are at most countably many discontinuities.
</blockquote>
But each of the proofs relies on A.C. Therefore I am wondering if there is a way to ... | You don't need AC for the proof n°1 (or n°3, I'm not exactly sure what's distinguishing them) because the rationals are well-orderable (this can be done without AC), therefore you can consider a well-ordering <span class="math-container">$(\Bbb Q,\preceq)$</span> and assign to each jump discontinuity the <span class="m... | Partition <span class="math-container">$\mathbb{R}$</span> into disjoint bounded intervals <span class="math-container">$(I_m)_{m\geqslant1}.$</span> For example, <span class="math-container">$[0, 1), [-1, 0), [1, 2), \ldots$</span>; the details don't matter. Take any strictly decreasing sequence <span class="math-cont... | https://math.stackexchange.com |
110,624 | [
"https://electronics.stackexchange.com/questions/110624",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/42932/"
] | There are active styli out there, and they take 3A or 4A batteries and don't require any wireless connection. How do those emulate human touch?
EDIT: I am specifically interested in iPad touch screens. After some research, I have realized that there are many types of capacitive sensor tech, but most people seem to be... | <blockquote>
How do those emulate human touch?
</blockquote>
Capacitive touch screens don't work from the supposed "aura" that some folk believe emanates from humans - they'll work with dogs, cats, medium sized rodents, pieces of metal and probably wet wood. Maybe they'll also work with high dielectric plastics too.... | A capacitive touch screen is one half of a capacitor. To make it work, the equivalent of the remaining capacitor plate is needed. This will be a conductive element that is capable of absorbing charge. Your finger is connected to you and your body is large enough to absorb the small amount of charge to be sensed by th... | https://electronics.stackexchange.com |
87,346 | [
"https://physics.stackexchange.com/questions/87346",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/29184/"
] | The Minkowski metric transforms under Lorentz transformations as
\begin{align*}\eta_{\rho\sigma} = \eta_{\mu\nu}\Lambda^\mu_{\ \ \ \rho} \Lambda^\nu_{\ \ \ \sigma} \end{align*}
I want to show that under a infinitesimal transformation $\Lambda^\mu_{\ \ \ \nu}=\delta^\mu_{\ \ \ \nu} + \omega^\mu_{{\ \ \ \nu}}$, that $... | Note that if you lower an index of the Kronecker delta, it becomes the metric:
$\eta_{\mu\nu}\delta^{\mu}_{\rho}=\delta_{\nu\rho}=\eta_{\nu\rho}$
And in your last step you got a wrong index. It should be $\omega_{\rho\sigma}$, not $\omega^{\rho}_{\sigma}$.
Then, the metric terms cancel and you neglect cuadratic ter... | Since the Lorentz transformation is valid for any $x\in M_{4}$, it can be rewritten as $\Lambda_{\rho}^{\mu}\eta_{\mu\nu}\Lambda_{\sigma}^{\nu}=\eta_{\rho\sigma}$. Substituting the infinitesimal form of the Lorentz transformation into the previous formula we get
$$(\delta_{\rho}^{\mu}+\omega_{\rho}^{\mu})\eta_{\mu\nu... | https://physics.stackexchange.com |
2,064 | [
"https://scicomp.stackexchange.com/questions/2064",
"https://scicomp.stackexchange.com",
"https://scicomp.stackexchange.com/users/486/"
] | As far as I understand, the order in which non-blocking point-to-point MPI messages (Isend and Irecv) are received is consistent with the order in which they are sent. Are there any techniques for giving certain messages priority over others?
For example, we have a multi-level algorithm in which the high resolution s... | I think the answer to this is no. Once you've pushed them into the MPI stack, they're out of your control, and the MPI semantics govern the way the messages are sent.
You could certainly prioritize messages by queuing them in your code before sending them, and then checking frequently which are the most important to ... | Currently MPI has no provisions for prioritisation of messages and neither has the upcoming MPI 3.0 standard. It is up to the MPI implementation to decide how to transmit the messages. E.g. smaller messages <em>might</em> get sent faster because of certain bypasses in the communication machinery (highly implementation ... | https://scicomp.stackexchange.com |
405,183 | [
"https://softwareengineering.stackexchange.com/questions/405183",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/218080/"
] | I've been assigned to a code base responsible for millions of dollars of transactions, per quarter, and has been in use for over a decade. Sifting through the solution, I see <strong>doubles</strong> used everywhere to represent money and arithmetic is done on these variables; on the rare occasion is a <strong>decimal<... | Using single precision float for money is fatal. Float has only 24 bits of precision, so if you are using dollars, then anything above 2^18 dollars has a resolution worse than one cent. So anything above $263,000 or so has big problems.
Using double precision is much much less of a problem. If you have an amount of o... | You're right in being concerned about the use of floats for monetary amounts. Unless they are just used to calculate something which is then properly represented as a rounded scaled decimal (or an integral number of cents, which is equivalent,) they shouldn't be used to represent money.
However, the damage that could ... | https://softwareengineering.stackexchange.com |
479,901 | [
"https://physics.stackexchange.com/questions/479901",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/-1/"
] | <span class="math-container">$$
\rm
{}^{12}C +{}^{12}C\rightarrow {}^{20}Ne + {}^4He
$$</span>
Why doesn't this reaction occur on Earth but can occur in big stars?
| The (nuclear) reaction you quote is highly exothermic, but that in itself is not enough for the reaction to proceed. It needs to be initiated and this requires certain physical conditions to be met.
Most importantly, the carbon nuclei need to be brought close enough together that the strong nuclear force can fuse them ... | The reaction has a positive <span class="math-container">$Q$</span>-value but can only occur if two carbon-<span class="math-container">$12$</span> nuclei are close enough together.
To get the nuclei in that position the coulomb repulsion between the nuclei must be overcome ie the nuclei must gain electric potential... | https://physics.stackexchange.com |
181,952 | [
"https://electronics.stackexchange.com/questions/181952",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/76826/"
] | The open-collector adds inconveniences like requiring an external pull-up resistor and slows down the throughput. Why did the I2C designers choose to use open-collector/open-drain?
| Open drain allows there to be multiple masters on the same bus. If two masters try to transmit at the same time with push-pull drivers, they can damage each other, and even if they don't it's hard to tell which master will win. Open drain acts as a "wired AND", which makes it easy to share the line and arbitrate collis... | Using open collector gives the bus a lot of flexibility also it is required because the bus is bidirectional on a single wire.
For example imagine if every device had an internal pullup of 10K ohms. If there are two devices then there is only 5K ohms on the bus and the internal drivers have no problem pulling the line... | https://electronics.stackexchange.com |
213,124 | [
"https://softwareengineering.stackexchange.com/questions/213124",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/68808/"
] | I have noticed where I work people are keen on storing information in file names, and parsing the file names.
To me this doesn't seem to be especially good practice. I already see the occasional issues with scripts globbing for a file, and getting the wrong one because another file matches first.We are also discussing... | Yes I think it's bad practice. It is subject to all sorts of problems - for example length limits, encoding issues and conflicts due to duplicate data.
Better is to use a "master file" (sometimes called manifest or index) that contains metadata and paths to the files. Or something similar in a database, register or wh... | It sounds like you need a database.
There are lots of security issues with putting user data in file names. Let's say that you have a file for each user ("username.txt"). What happens what someone registers the username "../../../../etc/passwd" depends on how you are filtering user input.
Database frameworks will so... | https://softwareengineering.stackexchange.com |
17,809 | [
"https://mechanics.stackexchange.com/questions/17809",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/876/"
] | My father in law tells me if I leave the radio on in my car when I shut off the engine this will cause a bit of battery drain when I start the car. He says this will actually reduce the life of my battery.
Is this true?
Is this only true with 'older' cars? If so, before what year?
I'm surprised a modern car doesn't... | This step is a carryover from days gone by and is not needed in modern vehicles. Batteries from 50 or 60+ years ago were not as powerful or reliable as modern ones. Older batteries had difficulty handling the load of the starter motor alone. Engines cranked longer before starting and were cranking large displacement en... | It very much depends on how your radio is wired up. A typical car radio takes two feeds from the battery; switched live and memory live.
Switched live is usually attached to the ignition switch and powers the radio up when the ignition is switched on. Memory live always takes a feed of power from the battery and is ... | https://mechanics.stackexchange.com |
289,316 | [
"https://electronics.stackexchange.com/questions/289316",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/79263/"
] | I'm making a circuit including a 555 timer controlled by an RC combination, where R is fixed and C is one of several capacitors (values around a few pF to a few hundred pF). I would like to connect the capacitors to the timer circuit using an analog-friendly multiplexer IC (like the 4051). This way, the 555 frequency c... | You should read the datasheet of the analog multiplexer and look for capacitance values, especially input capacitance. Those unwanted and unavoidable capacitances will influence the frequency of the 555 timer, especially if the capacitor connected to the 555 is very small, about a few pF. The unwanted capacitances are ... | <blockquote>
Would this actually work?
</blockquote>
yes.
but much easier to simply use a switch on the low-side to switch in / out a capacitor.
if two such switches + two capacitors give you four combinations of capacitance. ...
| https://electronics.stackexchange.com |
424,404 | [
"https://physics.stackexchange.com/questions/424404",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/197536/"
] | <b>Do we actually feel fictitious forces acting on our bodies?</b> My conclusion is that yes we do, since when I am accelerating upwards in an elevator, I feel a lot of pressure on my knees. I have an example though (that certainly has some logical error in it) that proves the opposite:
A car is moving at constant vel... | Fictitious forces are introduced in a non-inertial frame of reference so that we can use Newton's laws of motion.
<blockquote>
Do we actually feel fictitious forces acting on our bodies? My conclusion is that yes we do, since when I am accelerating upwards in an elevator, I feel a lot of pressure on my knees.
</... | <blockquote>
I am accelerating upwards in an elevator, I feel a lot of pressure on my knees.
</blockquote>
How is that fictitious? The floor is moving up against you, making you the non-moving object into a moving object, and you feel that force until such time as you moving the same speed as the floor.
| https://physics.stackexchange.com |
692,630 | [
"https://math.stackexchange.com/questions/692630",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/11792/"
] | Let $p$ be an odd prime number.
Can the set of squares modulo $p$ be invariant under translation?
I.e. given $p$, let $S = (\mathbb{F}_p^\times)^2 \cup \{0\} \subseteq \mathbb{F}_p$. Can there exist $\delta \in \mathbb{F}_p^\times$ such that $$S + \delta := \{x + \delta \mid x \in S\}$$ is again equal to $S$?
I su... | Assume by contradiction that $S=S+\delta$ for some $\delta \neq 0$.
Then, as $0 \in S$ we get $\delta \in S$ and then by induction that $n\delta \in S$.
But the additive group $(\mathbb F_p,+)$ is cyclic and generated by any non-zero element. Thus
$$F_p=\{ n \delta | z \in \mathbb N \} \subset S \,.$$
Contradiction... | No: The squares of $\mathbf F_p$ are the roots of
$$f(X) = X^{\frac{p+1}{2}} - X.$$
If $\delta$ is as you describe, then
$$f(X) = f(X+\delta).$$
Remark that $$f'(X) = \frac{p+1}{2}X^{\frac{p-1}{2}} - 1.$$
Differentiating the equation $f(X) = f(X+\delta)$ we find
$$\frac{p+1}{2}X^{\frac{p-1}{2}} - 1 = \frac{p+1}{2... | https://math.stackexchange.com |
83,569 | [
"https://mathoverflow.net/questions/83569",
"https://mathoverflow.net",
"https://mathoverflow.net/users/2837/"
] | I find the definition of constructible $\bar{\mathbb Q}_l$-sheaves (or their derived category) on a variety of positive characteristic quite involved and ad Hoc. Roughly it goes as follows:
First one defines constructible sheaves modules over torsion rings in the naive way, then over finite $\mathbb Z_l$ extensions as... | I believe the goal is to force the category of l-adic sheaves to keep some reasonable connection to geometry. No doubt someone more knowledgeable can say more about that, but I think if you start by understanding that torsion local systems are, by definition, representable by etale covers by finite relative group sche... | One reason why one needs a large coefficient group like $\overline{\mathbb{Q}_\ell}$ instead of $\mathbb{Q}$ is that a reasonable cohomology theory should be a functor. Sometimes varieties over finite fields admit morphisms that cannot be captured by those smaller coefficients.
The standard example due to Serre is th... | https://mathoverflow.net |
667,163 | [
"https://physics.stackexchange.com/questions/667163",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/314322/"
] | Say I have a <span class="math-container">$F=kQ_{1}Q_{2}/r^{2}$</span> and a direction vector <span class="math-container">$(x, y, z).$</span> How can I find the component forces <span class="math-container">$F_{x}$</span>, <span class="math-container">$F_{y}$</span>, and <span class="math-container">$F_{z}$</span>?
| It's somewhat unclear from your question, but I interpreted <span class="math-container">$F$</span> to be just the magnitude of the force (a scalar), and you want to construct a force of that magnitude pointing along the given direction vector.
If the direction vector is a <em>unit vector</em> (a vector of length 1), t... | To find the components of any vector <span class="math-container">$\bf F$</span> using unit vectors,
you can use the dot product between the vector and each unit vector.
So the x-component of <span class="math-container">$\bf F$</span> is <span class="math-container">$\bf F\cdot \hat i$</span> the y-component is <span ... | https://physics.stackexchange.com |
253,351 | [
"https://stats.stackexchange.com/questions/253351",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/143489/"
] | I have created a random forest object in R (using the <code>randomForest</code> package) with <code>ntree = N</code>. Now I would like to predict some new data on it using a subset of <code>N</code>, that is using only <code>n</code> trees for the prediction. Is this possible?
For the random forest object the forest... | Sounds like you want to set <code>predict.all = TRUE</code>. This will cause <code>predict.randomForest</code> to return a list containing a vector of the aggregate predictions and a matrix of the individual tree predictions. You can then ensemble the individual trees at your leisure.
<pre><code>library("randomForest... | Ok, so for your usecase you can set <code>predict.all = TRUE</code> when predicting on new data (see the help file via <code>?predict.randomForest</code>). Then you get a list element called <code>individual</code>, which contains the prediction of each tree.
| https://stats.stackexchange.com |
109,863 | [
"https://stats.stackexchange.com/questions/109863",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/21961/"
] | What is the difference between parameter estimation which includes system identification and learning in machine learning perspective?
Let say the model is y= Ax.
x is the input and y is the output. In estimation, I have seen parameters to be estimated, maybe in this case it is A and the samples are also estimated (un... | Basicaly same, but flavor of terms is a little different - by estimation people usually mean that you specify underlying distributions and then estimate their parameters. Learning may be distribution free - just optimizing some target function and it applies in situations with complex structured data when it not reason... | Estimators are rules for estimating a value based on observed data. We have estimators like GMM or MLE which estimate certain values, whether parametrically or not. These rules must satisfy certain properties like consistency, unbiasedness, efficiency, etc.
Learning is more general, and includes methods with or without... | https://stats.stackexchange.com |
2,027,709 | [
"https://math.stackexchange.com/questions/2027709",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/386821/"
] | I'm not sure if this question is too broad, but here goes: What does the word "linear" mean in mathematical models? In my econometrics class, one of the Gauss Markov assumptions for running regressions, as I understand it, is that the model is linear in parameters. What would a model that is non-linear in parameters lo... | Edit: this answer addresses the case $n=1$.
If $a\equiv 1$ (mod $p$) then $1-\zeta^{a-1}=0$. Otherwise $b=a-1$ will be prime to $p$.
It turns out that if $(b,p)=1$ then $1-\zeta$ and $1-\zeta^b$ are associates. Indeed, we have
$$ \frac{1-\zeta^b}{1-\zeta}=1+\zeta+\dots+\zeta^{b-1}\in\mathbb{Z}[\zeta]$$
and since $(b,... | For a general $n$. I call $\zeta_{p^k}$ the $p^k$-th roots of unity.
Take $(a,p^n)=1$ we have:
$$ \begin{cases}
1-\zeta_{p^n}^{a-1}=0 & \ \ \text{if} \ \ a\equiv 1 \ \ (\mathrm{mod} \: p^n) \\
1-\zeta_{p^n}^{a-1}=u(1-\zeta_{p^n}), \ u\in\left(\mathbb{Z}[\zeta_{p^n}]\right)^{\times} & \ \ \text{if} \ \ (a-1,... | https://math.stackexchange.com |
436,682 | [
"https://softwareengineering.stackexchange.com/questions/436682",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/116474/"
] | I have multiple VMs set up that have the same repo and with slight differences. I use these VMs to prototype or run different experiments. Sometimes I sync or merge between them and from different branches.
I want to avoid polluting the repository's commit history as a way to rsync between them. Rsync is too dangerous ... | It sounds like you are running your app direct from the code base and skipping the build and deploy stages?
These stages would normally force you to commit changes to a branch and be a bit more rigorous about stuff like not cherry picking bits of code from all over and choosing what branches you deploy.
Generally I wou... | From git's point of view, there is nothing special about the copy of the repo on GitHub. All changes do not have to pass through it. You can push directly from one VM to another VM, or from your local copy to a VM. Essentially every clone/copy of a repo is a fork.
There's also no real reason to avoid having lots of bra... | https://softwareengineering.stackexchange.com |
341,252 | [
"https://electronics.stackexchange.com/questions/341252",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/56642/"
] | I need to set a temperature between 20 °C and 300 °C. I am using a PWM output with a resolution of 1% duty cycle. Can the overall PWM period, and PID loop timing, get better than 1% accuracy of the temperature span in the load?
| It depends on the time constant of the load. A heated load will often respond in 10s of seconds.
If you generate 30% PWM for one second, then 31% PWM for the next second, that will look like 30.5% averaged over several seconds.
Oven thermostats tend to operate a minute on, a minute off, and still achieve oven temper... | <h2>Yes.</h2>
Given the fact that relative to other regulations (voltage, current, etc.), temperature regulation is quite slow, what you can do is use a microcontroller (i'm guessing you already do), but add a cyclic variation of the PWM signal's width.
imagine that for your system to be stable, you need to have a p... | https://electronics.stackexchange.com |
732,749 | [
"https://physics.stackexchange.com/questions/732749",
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"https://physics.stackexchange.com/users/21456/"
] | It seems like 2 eyes is enough “wetware” to do interferometry inside brain. Can you definitely see some reason why this could not be happening, or some way to test if it does happen?
| To do interferometry in post-processing after detection of radiation, the detector must be able to record the phase of the radiation. The eye cannot do this: the photochemical reactions that record the radiation are insensitive to phase.
In instrumentation, radio interferometry may be done post-detection because phase-... | I guess you’re imagining an interferometer like that used for long baseline radio interferometry. In this case it is necessary that the detectors can sensing the relative phase difference between waves arriving at each detector. Our eyes (and most visible EM detectors) only detect the amplitude of visible EM waves and ... | https://physics.stackexchange.com |
195,177 | [
"https://softwareengineering.stackexchange.com/questions/195177",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/68683/"
] | I am a beginning programmer who has written a spider application in PHP. Currently there are three parts:
<pre><code>1) The Spider (spider.php)
2) The Harvester (harvest.php)
3) The Configuration file (for example, craigslist_config.php)
</code></pre>
I use the spider to search the web for items I want to buy. An ite... | <blockquote>
Ultimately, what I am trying to achieve is the ability to simply drop in a new configuration file into my config directory
</blockquote>
First, add the related web address to your config file:
<pre><code>$conf = array(
'url' => ' specific_site url or url pattern',
'title' => ' specific_s... | Keep the configuration files as simple as possible. That means: avoid PHP code and redundancy (like XML). A suitable format is the INI format, which can be read by PHP into an array.
<pre><code>[specificwebsite1.com]
title="title pattern"
price="price pattern"
[anotherwebsite.com]
title="title pattern"
price="price p... | https://softwareengineering.stackexchange.com |
592,274 | [
"https://math.stackexchange.com/questions/592274",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/90332/"
] | I've tried many things with this question, and just can't seem to get it quite right, can someone please show me how to answer this question? Thank you in advance.
$$g(x) = \ln(5x+25) \qquad g^{-1}(x) = \frac{\boxed{\phantom{X}}}{\boxed{\phantom{X}}} e^{x}\,\boxed{\phantom{XXX}}$$
| If you just want to find some $x$, not all, you can try to find $x$ such that $\sqrt{x+7} = 7$, as then
$$ \sqrt{x + \sqrt{x+7}} = \sqrt{x+7} = 7 \in \mathbb N.$$
Can you see such an $x$?
| $$x + 7 = ((n^2)-x)^2 = n^4 - 2n^2x + x^2$$
$$x^2-(2n^2+1)x+n^4-7=0$$
$$x_{1,2}=\frac{2n^2+1 \pm \sqrt{(2n^2+1)^2-4(n^4+7)}}{2}\\
=\frac{2n^2+1 \pm \sqrt{4n^4+4n^2+1-4n^4+28}}{2}\\
=\frac{2n^2+1 \pm \sqrt{4n^2+29}}{2}\\$$
This is an integer if and only if $ \sqrt{4n^2+29}$ is an integer (the converse follows immediate... | https://math.stackexchange.com |
100,453 | [
"https://stats.stackexchange.com/questions/100453",
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"https://stats.stackexchange.com/users/21674/"
] | I am running a glm in R on data with quite many predictors (~50), both initially continuous and factors. The response is binary and the volume of the data is OK (~100K rows), in order to model non-linear relationships, I convert the continuous variables to factors as well. In the end this results in some levels of some... | For those initial factor predictors, it is arguable whether those insignificant levels should be "merged". Note that your approach seems to be simply dropping those insignificant levels, this approach is incorrect.
For a significant factor predictors (at least one level is significant), the number of its significant l... | It is not correct to categorize continuous variables and it is not correct to drop "insignificant" variables from a regression model. Allow continuous variables to have smooth nonlinear effects using regression splines. Dropping variables leads to bias, especially in estimating standard errors. If you want to drop v... | https://stats.stackexchange.com |
3,036,223 | [
"https://math.stackexchange.com/questions/3036223",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/625547/"
] | Quoted from a popular book,
<blockquote>
For instance, if we flip a fair coin twice, knowing whether the first
flip is Heads gives us no information about whether the second flip is
Heads. These events are independent.
On the other hand, knowing whether the first flip is Heads certainly
gives us informat... | You are misinterpreting the expression a little bit.
When we say both flip heads, we are referring to
<span class="math-container">$$P(H_1,H_2)=P(H_1)P(H_2)$$</span>
which we can directly apply the formula.
But when we take it one step further with "first flip head and both flip tails" we are finding
<span class="ma... | You have already given yourself the answer.
The events <span class="math-container">$TT$</span> and <span class="math-container">$H\cdot$</span> <strong>are dependent</strong>. Independence of two events <span class="math-container">$E,F$</span> would mean that
<ul>
<li><span class="math-container">$P(E|F)= \frac{P... | https://math.stackexchange.com |
322,009 | [
"https://electronics.stackexchange.com/questions/322009",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/158879/"
] | I am trying to use ESP8266 with STM32L476RG Nucleo. The ESP8266 requires up to 300 mA to transmit properly. I tried to connect my ESP to an Arduino Mega, and it responds correctly, but when I brunch the ESP to the STM micro controller, the AT command was sent correctly, but the response message is not correct (if ... | You should not power the ESP off of the STM's 3.3 V pin. You probably have a 500 mA fuse on there and if your STM is running full load, maybe a couple of LEDs or other components on the board and the ESP together could exceed that fuses rating.
Get the LM1117 instead; it's a solid 3.3 V regulator. Also ... | The Nucleo should happily give you 300 mA of 3.3V current - it's regulator is rated at 500 mA so even with those few LEDs and running the MCU at 80 MHz ypu should hace enough. There IS one caveat though - it cannot do so if powered through the programming USB - you need to power it through E5V or VIN for it to work. Or... | https://electronics.stackexchange.com |
55,640 | [
"https://mechanics.stackexchange.com/questions/55640",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/38585/"
] | My car's horn only works during the morning in transit to work. After the engine and the weather heats up, i need to apply force to the horn in order for it to work. During the midday sun, it doesn't work at all. What is the source of the issue? What are some potential fixes?
Car: Nissan Note 2006 (JDM)
| Car engines are definitely made better today, but transmissions aren't. Stick your arm out the window as you drive and feel the wind resistance at different speeds. That's the best clue to what your transmission endures. As for the high rev operation...your computer helps the engine, but not the transmission in most... | What @Old-School Engineer says is correct. Wind resistance (aka drag) increases exponentially with speed. The (simplified) formula is <code>drag = (constant)*(velocity^2)</code>. So for example, going from 30kmh to 100kmh, the drag is <strong>11 TIMES HIGHER</strong>. As a result, even though the engine is spinning at ... | https://mechanics.stackexchange.com |
340,428 | [
"https://stats.stackexchange.com/questions/340428",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/38069/"
] | I am trying to understand why the hazard function is not a PDF. For a random variable T, people often define the PDF of this random variable as:
$$f(t)=\lim_{\delta \to 0} \frac{P(t\leqslant T <t+\delta)}{\delta}$$
By this definition, the hazard should also be a conditional PDF.
$$\lambda(t)=\lim_{\delta \to 0} ... | The <em>argument</em> of a conditional pdf cannot depend on the <em>conditioning event</em> in any way, shape or form. In
$$f_{T\mid A}(t\mid A) = \lim_{\delta\to 0} \frac{P\{t < T \leq t+\delta\mid A\}}{\delta},$$ $A$ can be a <em>fixed</em> event such as $\{T>5\}$ but not something that depends on $t$ such as... | I think the counterexample Björn suggested is a enough to answer the question. Let me write it out in more detail:
<h3>Björn's Counterexample</h3>
Let $T\stackrel{d}{=}\mathrm{Exp}(\lambda)$ for any $\lambda > 0$ then $f_T(t) = \lambda e^{-\lambda t}$ which implies $S(t) = \mathcal{P}(T>t) = 1-(1-e^{-\lambda t}... | https://stats.stackexchange.com |
257,563 | [
"https://electronics.stackexchange.com/questions/257563",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/121673/"
] | On the datasheet says minimum Upper Switch Current Limit at minimum duty cycle is 4.0A (min) 5.8A (typ).
How can i calculate the maximum output current? It will be higher or lower than 3A?
| Datasheet 1st page says
<pre><code>3A Continuous Output Current
4A Peak Output Current
</code></pre>
<blockquote>
The peak current is usually limited by choice of L and IC heat loss and is exceeded by upper switch max current capacity.
</blockquote>
| In theory you could get 4-A output current, assuming very large heatsink pad and inductance enough large to minimize current ripple. But maximum duty cycle is 90 %, so there will be also switching at 340 kHz and total losses will probably overheat the poor IC. Nobody uses components at their maximum ratings, even if it... | https://electronics.stackexchange.com |
49,714 | [
"https://mechanics.stackexchange.com/questions/49714",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/28703/"
] | Sometimes when I'm driving, when i press gently on the gas pedal [like you should be doing] the acceleration doesn't work at all, and when i press down on it harder it works and revs violently, and then the next time i stop the acceleration stops working entirely and the battery light comes on. I have to restart the ca... | I would start by testing the full sweep of the Throttle Position Sensor (TPS).
These can go bad in a way that creates intermittant problems. The wiper area of this "potentiometer" can wear, especially around the idle position, and create all sorts of havoc with driveability.
This can occur without throwing a OBD dia... | I would agree with the TPS suggested earlier except for the mention of the battery light coming on. Assuming this is not because the car has stalled, I would wager your car's electrical system has a bad ground. The negative battery cable can look fine but have lots of corrosion internally. Replace it with a new one, bu... | https://mechanics.stackexchange.com |
164,444 | [
"https://chemistry.stackexchange.com/questions/164444",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/122804/"
] | To give some background on the different tests I have went over: The Iron (III) Chlorine Test tests for the presence of phenols. However, phenols contain both the alcohol group and the aromatic group and not the aromatic group individually. The Bromine Water Test also tests for double bonds and single bonds, however, d... | It is easier to think of the heat capacity at constant volume as the slope of the internal energy with temperature. This is effectively your first definition. The correct interpretation of the heat capacity is only possible via quantum theory, that is by realising that atoms and molecules have discrete energy levels.
T... | As @Poutnik wrote in the comments, the heat capacity <span class="math-container">$C$</span> quantifies the influence of energy on the temperature of the system. There is no contradiction to write <span class="math-container">$C = k_\mathrm{B}$</span> since at the scale of a single molecule <span class="math-container"... | https://chemistry.stackexchange.com |
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