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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143,674 | [
"https://physics.stackexchange.com/questions/143674",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/57610/"
] | A person of mass $m$ is attached to a spring that its loose length is $l_0$. He is standing on a bridge and jumps off it, so he travels down a distance $2 l_0$ when measured from the bridge up to the moment the spring pulls him up.
Find the spring constant.
So, If I divide the jump into 3 parts then the man is in fre... | From the moment that the spring (bungee) comes under tension, the motion of the jumper can be described as a combination of simple harmonic oscillator and a constant acceleration - that is, a sine wave with an offset (the offset is the "equilibrium point" you mentioned).
But you don't need to go there. If you say that... | While it may be interesting and instructive, you don't really need to know much about the motion of the jumper. You can answer the quesetion by using a simple energy balance.
At the bottom of the fall where the velocity of the jumper is 0, all the potential energy of the jumper from there to the starting point has be... | https://physics.stackexchange.com |
1,419 | [
"https://quantumcomputing.stackexchange.com/questions/1419",
"https://quantumcomputing.stackexchange.com",
"https://quantumcomputing.stackexchange.com/users/580/"
] | Grover's algorithm is often described as a way to <em>search a database</em> in $O(\sqrt{N})$ time. For using it we need an oracle gate that represents some function $f$ such that $f^{-1}(1)$ is the answer. But how do you actually make such a “<em>database oracle</em>”?
Suppose I have an array of numbers $a$ that cont... | $\newcommand{\xtarget}{\boldsymbol{x}_{\operatorname{target}}}\newcommand{\bs}[1]{{\boldsymbol #1}}\newcommand{\on}[1]{{\operatorname{#1}}}$No, it does not.
The "<em>oracle</em>" in Grover's algorithm is a function that, given any element $\boldsymbol x_k$, checks whether $\boldsymbol x_k$ is the element we are lookin... | You would build a function $f$ such that $f(x)$ first accesses the $x$-th item of your array and then compares it to $w$. An actual implementation might access the array encoded in extra (parameter) input qubits as if they were bits.
| https://quantumcomputing.stackexchange.com |
142,894 | [
"https://physics.stackexchange.com/questions/142894",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/1075/"
] | So the energy potential of a Hydroelectric dam is the difference between the head and base. Before the dam was built where does all that energy go? I can understand that for a long valley that is dammed up the the energy was released from the headwaters to where the dam was built over a large area. However in a possibl... | The energy is converted to heat. The friction of the water with the river bed, and with itself, converts the energy to heat.
| Two energy sinks:
<ul>
<li>Heat, as stated in the other answer.</li>
<li>mechanical work on solids: rocks get ground to gravel, gravel to smaller grave etc.,</li>
<li>mechanical work on the fluid, it will mst likely flow faster downstream, unless all that energy is dissipated as mechanical work as described above</li>... | https://physics.stackexchange.com |
291,974 | [
"https://dba.stackexchange.com/questions/291974",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/228193/"
] | I took a look at my old accounting system, and it seems that performance is taking a role in the daily labor of the employees using it. So, I discovered that using a subquery was the problem, I've been reading, testing and, it seems that using a JOIN is like 100x faster as the data that we have in our Databases is huge... | I would not do what another Answer suggests -- it involves the "explode-implode", which is even slower.
I would start by seeing if this helped much:
<pre><code> orders: INDEX(order_state, order_time)
</code></pre>
Your correlated subquery is not necessarily inefficient <em>in this case</em>. (That 100X quot... | A simple way to do this would be like so:
<pre><code>SELECT ord.order_id, ord.orden_time, ord.orden_user, ord.payment_state, ord.order_state,
ord.area_name, COUNT(itm.item_id) AS items_number
FROM orders ord INNER JOIN order_item itm ON ord.order_id = itm.order_id
WHERE ord.order_state = 1
and ord.order_ti... | https://dba.stackexchange.com |
2,237,950 | [
"https://math.stackexchange.com/questions/2237950",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/356308/"
] | When we use change of variables for integration, we are required to also multiply the integrand by a scaling factor:
For change of coordinates with double integrals, the scaling factor is found by $dxdy = \left| \det \left( \dfrac{\partial g(u,v)}{\partial(u,v)} \right) \right| dudv$.
For change of coordinates with t... | For the pure $\epsilon - \delta$ proof (without assuming a priori continuity of the log function) take $k \in (0,1)$ fixed.
Using the triangle inequality we have for all $x \in (0,1)$
$$\begin{align}|x \ln x - k \ln k| &= |x \ln x - x \ln k + x \ln k - k \ln k| \\ &\leqslant |\ln k||x -k| + |x||\ln x - \ln k|... | We just need to show that $g(x)=\ln x$ it's continuos on $(0,1)$, you can easily show $\displaystyle \lim_{x\to a}f(x)=\displaystyle \lim_{x\to a}x=a=f(a)$, then $f(x)$ it's continuous on $\mathbb{R}$, for $g(x)$, let $a>0$ we need to show $\displaystyle \lim_{x\to a}g(x)=g(a)$, in fact:
$$|g(x)-g(a)|=|\ln x-\ln a|... | https://math.stackexchange.com |
11,577 | [
"https://physics.stackexchange.com/questions/11577",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/4186/"
] | I have a circuit whit a AC source a capacitor and a resistance all in series. I find that the difference of potential between the capacitor leads begin to change after some instants as it should. But my question is, if the resistance is very high, say infinite, the capacitor have no tension per se, but it experiences h... | If you choose $R\gg\omega/C$ , then most the voltage will drop off in the resistor. You may now argue that, for any voltage to drop off in a resistor, there needs to be some current in it. And there is, in fact, such a current.
Imagine the condensator plates as two antennas. Yes, pretty strange antennas, but antennas ... | I'd like to expand on leftaroundabout's answer as the question actually was phrased "what is capacitance" and this seems to be the core of the OP's problem (your feeling of the meaning of potential is probably correct though).
The antenna-analogy is a bit vague. You could see the capacitor as two reservoirs of electro... | https://physics.stackexchange.com |
114,437 | [
"https://stats.stackexchange.com/questions/114437",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/55284/"
] | I'm currently doing two multiple linear regressions. Each of them with the same set of predictors (measurements for real estate quality) $X_1,...,X_n$, but with different dependent variables (one of them the purchasing price, the other one the yearly rent) $Y_1$ and $Y_2$.
$Y_1= a_1X_1+a_2X_2+...$<br/>
$Y_2= b_1X_1+b_... | as ShannonC pointed out why not run regression y3=y1/y2 ~ x1,x2,...
however,if that is not possible(e.g. you don't have the original data) you can use taylor expansion to understand how y3 is influenced by x1,x2,...
let's re-state:
$$y_{k,i} = b_{k,0} + \sum_j b_{k,j} x_{j,i}$$
for k=1,2
let's write the taylor exp... | If you're willing to fit the models:
$$
\log Y_1= a_1X_1+a_2X_2+\ldots\\
\log Y_2= b_1X_1+b_2X_2+\ldots
$$
then, noting that $\log c/d = \log c - \log d$, you can interpret the differences of coefficients, $a_j - b_j$, directly for their impact on $\log Y_1/Y_2$. Moreover, if you fit $(\log Y_, \log Y_2)$ as a multivar... | https://stats.stackexchange.com |
366,196 | [
"https://mathoverflow.net/questions/366196",
"https://mathoverflow.net",
"https://mathoverflow.net/users/148974/"
] | Is there any characterization on the set of integers <span class="math-container">$n$</span> such that there is a 3-connected 5-regular simple <span class="math-container">$n$</span>-vertex planar graph?
| That is not true. There are probably shorter answers than the following. Let <span class="math-container">$K$</span> be a field, and denote a separable closure by <span class="math-container">$K^{\text{sep}}$</span>. Let <span class="math-container">$n>1$</span> be an integer.
<B>Definition</B>. A <B>Severi-Brau... | Let <span class="math-container">$X$</span> be an integral regular scheme which is proper over <span class="math-container">$\mathbb{Z}_p$</span>. Assume that the special fibre <span class="math-container">$X_{\mathbb{F}_p}$</span> is irreducible and let <span class="math-container">$k$</span> be the algebraic closure ... | https://mathoverflow.net |
634,411 | [
"https://physics.stackexchange.com/questions/634411",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/298106/"
] | In a recent physics class, I was told that force can change the direction of velocity as well as its magnitude simultaneously. My professor drew a diagram in which a particle is moving along a horizontal line and is acted upon by a force at angle <span class="math-container">$O$</span> with horizontal velocity. He said... | Have a look at Newton's 2nd law, <span class="math-container">$$\sum F=ma. $$</span>
In the perpendicular direction there is no speed. Meaning, when the object is moving horizontally it is not at the same time moving upwards. There is a horizontal speed component, <span class="math-container">$v_x$</span>, but no upwar... | <blockquote>
I want to understand what will be state of motion of this body after [the first] instant.
</blockquote>
That depends on the nature of the force. if the force is acting in a constant direction (e.g. gravity) then after the first instant there is a component of the force parallel to the velocity of the objec... | https://physics.stackexchange.com |
39,939 | [
"https://dba.stackexchange.com/questions/39939",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/-1/"
] | I am trying to improve on a nested MySQL query, using the following table structure (omitting a few dozen not relevant to my question):
<strong>tbl_users</strong>:<br>
<code>ID | email</code> (all subscribers to our email list)
<strong>tbl_mailings</strong>:<br>
<code>ID | mail_title</code> (each marketing email)
<... | Reformulating your query:
<pre><code>SELECT
numopens / numsends AS Open_Rate,
numactions / numsends AS Action_Rate
FROM
( SELECT
( SELECT COUNT(DISTINCT s.userID)
FROM tbl_sends AS s
WHERE s.mailing_id = param.mailing_id
)
AS numsends,
( SE... | I guess joining the three table would do the trick:
<pre><code>SELECT
sub.opens / sub.sends AS open_rate,
sub.actions / sub.sends AS action_rate
FROM
(
SELECT
COUNT(DISTINCT tbl_sends.userID) AS sends,
COUNT(DISTINCT tbl_opens.userID) AS opens,
COUNT(DISTINCT tbl_actions.userID) AS actions
FROM
tbl_se... | https://dba.stackexchange.com |
2,176,170 | [
"https://math.stackexchange.com/questions/2176170",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/350460/"
] | We have the matrix: $\begin{pmatrix}
7-\lambda & 12 & -8\\
-2 & -3-\lambda & 4\\
0 & 0 & 3-\lambda
\end{pmatrix}$
The eigenvalue is $\lambda=3$. Calculate the eigenvector.
<hr>
Insert $\lambda=3$ into the matrix:
$\begin{pmatrix}
4 & 12 & -8\\
... | When calculating the eigenvalues, you may have noticed that $\lambda = 3$ was a double root of the characteristic equation or in other words: it is an eigenvalue with algebraic multiplicity 2.
This means it is possible that you find two linearly independent eigenvectors corresponding to this eigenvalue (that would mea... | you can choose any non zero vector parallel to the plane $x+3y-2z=0$ to be an eigenvector.
This situation arises whenever the characteristic equation has a double root. in this case the characteristic equation is $$(3-\lambda)(\lambda-3)(\lambda-1)=0$$
So any two independent vectors parallel to the plane can be taken... | https://math.stackexchange.com |
2,011,530 | [
"https://math.stackexchange.com/questions/2011530",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/388772/"
] | Since <span class="math-container">$\sqrt{3}$</span> never terminates,there is no finite length ,breadth or height.
By this I mean you never end measuring if you want to achieve extreme precision. Say you keep on measuring to get the correct length with a hypothetical instrument that is super precise.
So is the box vo... | Its all about how you define your units while measuring.
I can always plot √3 on a real number line . Let the length of this plot from 0 be defined as 1 unit. Then volume of the box is 1 unit^3.
This same volume when expressed in the units of centimeters, the volume is
3√3 cm^3.
| If the real numbers are defined,
then the volume is defined
and it is $3^{3/2} = 3\sqrt{3}$.
| https://math.stackexchange.com |
83,506 | [
"https://stats.stackexchange.com/questions/83506",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/32397/"
] | An average of $B$ binomial i.i.d. random variables, each with variance $\sigma^2,$ has variance $\frac{1}{B}\sigma^2.$
If the variables are simply i.d. (identically distributed, but not
necessarily independent) with positive pairwise correlation $\rho$, the variance
of the average is $$\rho\sigma^2 + \frac{1-\rho}{B}... | As a very general rule, whenever $X = (X_1, \ldots, X_B)$ are random variables with given covariances $\sigma_{ij}=\text{Cov}(X_i,X_j),$ then the covariance of any linear combination $\lambda \cdot X = \lambda_1 X_1 + \cdots + \lambda_B X_B$ is given by the matrix $\Sigma = (\sigma_{ij})$ via
$$\text{Cov}(\lambda X, \... | To reformulate the answer of @whuber, if anyone finds it helpful.
Suppose a column vector <span class="math-container">$$X = (x_1, x_2, ..., x_n)^\intercal \in \mathbb{R}^n$$</span> where the random variables <span class="math-container">$x_i$</span> are identically distributed (each with variance <span class="math-co... | https://stats.stackexchange.com |
377,516 | [
"https://stats.stackexchange.com/questions/377516",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/205719/"
] | I've been trying to prove the following:
<span class="math-container">$$\phi'(x)(1-\Phi(x))+2\phi(x)^2>0$$</span>
where <span class="math-container">$\phi$</span> is the standard Normal pdf and <span class="math-container">$\Phi$</span> its cdf. I've tried many simulations and I believe it's true in general, how wou... | <span class="math-container">$$\phi(x) = \frac1{\sqrt{2\pi}}\exp\left( -\frac{x^2}{2}\right)$$</span>
<span class="math-container">$$\phi'(x) = \frac{-x}{\sqrt{2\pi}}\exp\left( -\frac{x^2}{2}\right)=-x\phi(x)$$</span>
Hence the problem is equivalent to
<span class="math-container">$$-x(1-\Phi(x))+2\phi(x) >0$$</... | Consider that for a random variable with any density above <span class="math-container">$x$</span>, <span class="math-container">$E(X|X>x) > x$</span>
For a standard normal variate, you should be able to show that <span class="math-container">$E(X|X>x) = \phi(x)/[1-\Phi(x)]$</span>
(e.g. for the integral in ... | https://stats.stackexchange.com |
62,794 | [
"https://datascience.stackexchange.com/questions/62794",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/67275/"
] | I understand that before feature engineering one has to split the dataset into train and test data, so as to avoid bias in the analysis. I also understand that the machine learning model does not understand data apart from numerical data, thus encoding is required, which is a part of feature engineering. My question is... | This depends somewhat on the model and language (implementation).
First please understand that categorical data is not the same as non-numerical data! Many models can handle categorical data (e.g. regression formats) just fine and some can even handle non-numerical data.
Finally and most important for you <strong>fea... | If you want to ensure that the test data has similar classes to the train set, you can use the stratify option in scikitlearn train test split for Python or the stratify option in Caret for R
| https://datascience.stackexchange.com |
709,669 | [
"https://physics.stackexchange.com/questions/709669",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/236734/"
] | <blockquote>
It is the equivalence principle that provides the bridge between the ideal <span class="math-container">$SR$</span> model and the real world. According to it, we can find at each event a set of local inertial frames (LIFS), which may be small or large depending on
I.) Distribution of nearby masses
ii.) Acc... | The Equivalence Principle (EP) is valid locally: if you perform a <em>local</em> experiment in an inertial frame of reference, you can't say if you're freefalling or not falling, you can't say if you're attracted downwards or accelerated upwards. If you perform a <em>nonlocal</em> experiment, you can say what kind of m... | I've never seen it phrased like that, but I understand it as the following.
Strictly speaking, the frame is defined locally, on a point. So the further you move from that point, the less inertial the frame becomes.
It's reasonable to assume that, if the mass distribution is too dense, the approximation will become bad ... | https://physics.stackexchange.com |
52,601 | [
"https://datascience.stackexchange.com/questions/52601",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/74838/"
] | I have read some paper about using particle swarm optimization. It doesn't look give much different result than K-Means. I tried to use PSO for clustering but the result is pretty much the same with K-Means with some drawbacks like longer execution time and have a lot of different result caused by the random factor.
| K-means makes locally the optimal decision. Most of the time (in particular when this objective works reasonably well at all) this works quite well to find a good local optimum. I doubt that using an approach such as PSO gives you any advantage here - the problem is just too simple, and k-means has the speed advantage,... | You cannot acutally compare the two, because PSO is an optimization algorithm, and K-means is a clustering algorithm. An optimization algorithm tries to find the best solution to a given problem, such as a clustering problem. If the clustering problem is stated like it is done for K-means, then you should find the same... | https://datascience.stackexchange.com |
7,768 | [
"https://stats.stackexchange.com/questions/7768",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/-1/"
] | I've never really found any good text or examples on how to handle 'non-existent' data for inputs to any sort of classifier. I've read a lot on missing data but what can be done about data that cannot or doesn't exist in relation to multivariate inputs. I understand this is a very complex question and will vary dependi... | Instead of assigning special value for non-existent first time runner previous lap time, simply use interaction term for previous lap time with the inverse of first time runner dummy:
$$Y_i=\beta_0+\beta_1 FTR_i+\beta_2 (NFTR_i)\times PLT_i+...$$
here
<ul>
<li>$Y_i$ is your input variable, </li>
<li>$...$ is your ... | For a logistic regression fitted by maximum likelihood, as long as you have both (1) and (2) in the model, then no matter what "default" value that you give new runners for (2), the estimate for (1) will adjust accordingly.
For example, let $X_1$ be the indicator variable for "is a new runner", and $X_2$ be the variab... | https://stats.stackexchange.com |
301,772 | [
"https://physics.stackexchange.com/questions/301772",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/81898/"
] | I have a question about this task:
<blockquote>
Consider a thin spherical shell of dielectric which has a radius R and rotates with an angular welocity <span class="math-container">$\omega$</span>. Suppose that the mass of the shell is negligible.
b) A constant torque <span class="math-container">$N$</span> is applied ... | You are completely correct. The solution is incorrect. Since $\omega$ is a function of time so is $W$. For convenience we can set $k=\frac{4}{9}\pi \mu_0 \sigma^2 R^5$. Then the equation is $W= k\omega^2$ so
$\dot{W}=2k\omega\dot{\omega}=N\omega$
$\dot{\omega}=N/2k$
Solving that gives a different time than in the s... | Well you are right, but the total magnetic energy they calculate is dependent on $\omega$, so will return lower energies if it is applied later as $\omega$ reduces to 0.<br>
At <em>any time</em> the time differential of this energy will equal the work due to friction, which is also dependent on $\omega$.<br>
(I think y... | https://physics.stackexchange.com |
284,459 | [
"https://stats.stackexchange.com/questions/284459",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/162797/"
] | Let's say we have a simple binary classification problem. So for a predictor X we want to predict response Y. Y is binary, so either 0 or 1. Now let's say we use two different classifiers, model1 and model2. While predicting a new data point x_i, model1 predicts with a probability of 0.9 that y_i = 1, while model2 says... | Classifier metrics that compare the predicted probabilities to the true classes go by the name of <strong>proper scoring rules</strong>. The two most popular are the <strong>log-loss</strong>
$$ L = \sum_i y_i \log(p_i) + (1 - y_i) \log(1 - p_i) $$
and the <strong>brier score</strong>
$$ L = \sum_i (y_i - p_i)^2 $$... | It might be the case that one model (say M1 on your case) leads to more extreme predictions compared to the other (M2), meaning that "certainty" (I think this concept is misleading) will be also higher for correctly predicted/classified events. Instead of reporting % of correctly classified events, you could simply com... | https://stats.stackexchange.com |
284 | [
"https://security.stackexchange.com/questions/284",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/3/"
] | Sometimes I have to open files (doc, zip, txt, pdf and some other) downloaded from the Internet (from unproven sites) on my computer.
How could I mitigate the risk and open them in a more or less secure way?
What types of files are more insecure than the other?
| What I can suggest:
<ul>
<li>open files inside VM's if it is possible, or, as @Henri said, use sandbox;</li>
<li>run AV software, at least it adds some security level;</li>
<li>you can use online AV scanner solutions like virustotal.com;</li>
<li>never trust any file from any source - friends can also be compromised, ... | The best way to open such files is by running them inside a sandbox. I would suggest a virtual machine.
Regarding which kind of files are safe, one can make two distinctions. In practice, files like zip, txt, jpg, png, etc are safe. Files like .exe, .msi, .com, .bat, .vbs, etc etc are unsafe. <em>However</em>, there i... | https://security.stackexchange.com |
120,060 | [
"https://cs.stackexchange.com/questions/120060",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/113667/"
] | It should be straight forward to show that there are infinitely many NP-hard problems:
<strong>Proof:</strong>
Take the problem <em>Remove 1 Vertex 3-COL</em> (<span class="math-container">$R1V3COL$</span>) which takes a graph <span class="math-container">$G=(V,E)$</span> as an instance and yields a yes answer iff a v... | As in the comment, consider the collection of problems <span class="math-container">$N$</span>-SAT (Is <span class="math-container">$\phi$</span>, a logical formula in <span class="math-container">$N$</span>-CNF, satisfiable?). Or <span class="math-container">$N$</span>-coloring of graphs, for <span class="math-contain... | For every integer k, take the travelling salesman problem with n > 1 cities where n is a power of k. (Picked the problem that way because all the instances are distinct, so we can say with good conscience that these are distinct problems).
| https://cs.stackexchange.com |
25,475 | [
"https://softwareengineering.stackexchange.com/questions/25475",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/1325/"
] | In many English-based programming languages like C++ and JavaScript, there exist long keywords like <code>function</code>, <code>namespace</code>, <code>exception</code> (in OCaml languages) and such. Do you think shorter keywords like <code>func</code> should replace them? Why or why not? What about replacing them wit... | I accept keywords like <code>func</code> and <code>cons</code> in functional languages, because functional languages tend to be math-like and more symbolic.
But in imperative, object oriented languages, where you are telling the computer how to do something as well as what to do, I prefer that the identifiers and k... | This is a non-issue thanks to the auto-complete capabilities of any text editor worth programming in. You can make a keyword named <code>FunctionNamespaceImplementsInstanceOfSynchronized</code> for all I care.
I'll just hit tab.
One other thing... I don't think I've ever in my programming career come across a keyw... | https://softwareengineering.stackexchange.com |
149,811 | [
"https://math.stackexchange.com/questions/149811",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/31985/"
] | Let $p(z)=2z^4-2z^3+2z^2+7$. I'm trying to determine the number of roots of $p(z)$ that lie inside $\{z\in\mathbb{C}:|z|<2\}$
I used Rouche's theorem and I'm trying to find find $f(z)$ such that $|f(z)-g(z)|<|f(z)|$.
Let $f(z)=z^4$, $g(z)=2z^4-2z^3+2z^2+7$. Let $|z|=2$.
I get
$|f(z)-g(z)|\\=|-z^4+2z^3-2z^2-7... | As in your other question, make sure you are doing proofs in the correct direction.
As to the difference between $2z^4$ and $z^4$: In general, Rouché's theorem is not bidirectional, by which I mean that even if $|f(z)-g(z)|\nless|f(z)|$ for appropriate $z$ it might still be the case that $f$ and $g$ have the same num... | Take $f(z)=z^4, g(z)=z^4-2z^3+2z^2+7$, Then on $|z|=2, |f(z)|>|g(z)|$, so inside $|z|=2$, $f$ and $f+g=p(z)$ has same number of roots.
| https://math.stackexchange.com |
271,176 | [
"https://stats.stackexchange.com/questions/271176",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/155515/"
] | I have a repeated measures design. Each subject was tested in 2 conditions (<strong>RE</strong>, <strong>re</strong>) with different cognitive load levels (<strong>low</strong>, <strong>mid</strong>, <strong>high</strong>). I originally analysed this dataset using a repeated measures ANOVA (mu_N1 ~ lang*load + Error (s... | In general, probably not. The proper procedure is going to depend on the assumptions you are comfortable making about what is independent. In this case, I suspect that you would be comfortable stating that individuals are independent of eachother, but that within individuals things are (at least potentially) related to... | That is a permutation test, not a bootstrap. They are both resampling statistics. They both provide good finite-sample inference that relies on relatively few assumptions.
| https://stats.stackexchange.com |
2,873 | [
"https://robotics.stackexchange.com/questions/2873",
"https://robotics.stackexchange.com",
"https://robotics.stackexchange.com/users/482/"
] | I'm building a robot with differential drive. I've reached the point when I can drive it around on remote control and I'm trying to get the localization working. Now I would like to exactly measure parameters of the robot.
Model of the robot I'm using has two wheels spaced $b$ meters, each wheel has a distance per enc... | <strong>At the foundation of PID control, there is the assumption that the quantity you are measuring (and using to compute your error) has a direct linear relationship with the quantity you are controlling.</strong>
In practice, people frequently bend this rule without things going horribly wrong. In fact this resili... | error = distance to left - distance to right, goal (set point) is 0.
| https://robotics.stackexchange.com |
75,615 | [
"https://electronics.stackexchange.com/questions/75615",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/16051/"
] | I was put in charge of redesigning a motor control circuit which accepts 24-40V DC on its input.
As a side note: As far as i know, the existing circuit has been working fine for several years. The redesign aims to make the production cheaper and replace all obsolete THT parts with SMD and optimizing some other stuff.
... | The data sheets are telling you this: -
<ol>
<li>The device will conduct 1mA somewhere between 47-10% and 47+10% at 25ºC</li>
<li>At 5A the voltage across the device will be no more than 93V (this is a pulse test)</li>
<li>The max impulse current (based on 8/20us pulse) is 500A at 85ºC</li>
<li>At 85ºC ambient watch o... | That varistor is supposed to be used on a supply up to 38V, and you have 44V so it looks like the wrong choice to me. But it won't draw much current (less than 1mA) until the voltage reaches 47V so you are probably ok with the small dissipation that occurs. The next one up, an S10K35 has a rated d.c. voltage of 45V and... | https://electronics.stackexchange.com |
558,325 | [
"https://physics.stackexchange.com/questions/558325",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/267033/"
] | In order to establish an arc in a low voltage, high current suruation such as welding, why do the wires or electrodes need to touch, then pull away to establish an arc?
Why would this arc not be established when the electrodes were brought close to one another initially?
| Electric (arc) welding is performed at relatively low voltage (often less than 10 volts) and high current (150 to 250 amps). This means the electrode needs to brought very close indeed to the workpiece, in order to trigger breakdown and start the arc. In practice, the arc is initially "struck" by physically t... | They don't technically have to touch. They just have to get close enough for the field to exceed the breakdown limit of air (around 3kV/mm).
So if you're doing something at 750V, you just need around...
<span class="math-container">$$d = \frac{V}{E_{break}} = \frac{750\text{V}}{3\text{kV/mm}} = 0.25\text{mm}$$</span>
... | https://physics.stackexchange.com |
320,716 | [
"https://mathoverflow.net/questions/320716",
"https://mathoverflow.net",
"https://mathoverflow.net/users/63864/"
] | Does there exist a <span class="math-container">$C^*$</span>-algebra <span class="math-container">$A$</span> such that the center of <span class="math-container">$A$</span> is <span class="math-container">$0$</span> and <span class="math-container">$A$</span> also has a tracial state?
I know the fact that the center o... | There are lots of AF simple nonunital C<span class="math-container">$^*$</span>-algebras with (finite) traces (these have trivial centre). For example, begin with the usual <span class="math-container">$2^{\infty}$</span> UHF algebra, call it <span class="math-container">$B$</span>, and take an infinite strictly increa... | I think you can obtain an example by modifying the classical Toeplitz algebra.
<span class="math-container">$\newcommand{\H}{\mathbf{H}}$</span>
<span class="math-container">$\newcommand{\bT}{\mathbf{T}}$</span>
<span class="math-container">$\newcommand{\cT}{\mathcal T}$</span>
<span class="math-container">$\newcomma... | https://mathoverflow.net |
91,635 | [
"https://physics.stackexchange.com/questions/91635",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/14233/"
] | Assuming it's possible to vibrate a human at near light speed without harming him, would a few minutes of this from his point of view be much longer from a stationary observer's point of view?
In other words do vibrations work the same as normal movement with regards to time dilation?
So a person could walk into such... | Whether this is possible depends on exactly what you mean by "vibration", but in short, no, it won't work in the way you're probably imagining, and it definitely won't work in the way the currently accepted answer says it will. That answer is completely wrong.
You can repeat the twin-paradox experiment many t... | There's two kinds of vibration that would make this "work": thermal vibration and actual shaking. Actual shaking is out of the question because that would involve pushing and pulling a person back and forth such that their average velocity was near-light speed. The "back and forth" part of this would involve way more a... | https://physics.stackexchange.com |
105,192 | [
"https://mathoverflow.net/questions/105192",
"https://mathoverflow.net",
"https://mathoverflow.net/users/798/"
] | For surfaces there are many statements along the lines of: if two simple closed curves are homotopic, they are isotopic. I'm interested in such questions for families of curves.
More precisely, let $\Sigma$ be a hyperbolic surface, possibly with boundary. We fix an essential simple closed curve $\gamma$ on $\Sigma$. I... | Earlier than Grayson, the determination of the homotopy-types of these spaces was done by Gramain.
There are a few special cases, like the torus and sphere and the non-orientable analogue, the case of null curves. But if they're not null homotopic the components of the embedding space have the homotopy type of $S^1... | If you forget about the parametrization, the "curve shortening flow" isotopes an essential simple closed curve to <em>THE</em> geodesic isotopic to it (this is a celebrated result of Matt Grayson), which I believe is gives a deformation retraction of the unparametrized space to a point. When you throw the parametrizati... | https://mathoverflow.net |
154,373 | [
"https://physics.stackexchange.com/questions/154373",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/66152/"
] | I was always taught that $\frac d {dx} (\ln x) = \frac 1 x$. No derivative had as a result any $dx$ words.
In a physics book I encountered something like this (error discussion) [there might be a little difference, as I don't have the book right now]:
We have $\ln a + \ln b = \ln c$. So after deriving both sides we g... | Regarding your first question, note that
$$\frac{d}{dx}\ln c = \frac{1}{c}\frac{dc}{dx}$$
Thus
$$\frac{1}{a}\frac{da}{dx} + \frac{1}{b}\frac{db}{dx} = \frac{1}{c}\frac{dc}{dx}$$
Then, as physicists often do, 'cancel' the $dx$ on both sides to yield
$$\frac{da}{a} + \frac{db}{b} = \frac{dc}{c}$$
Regarding your se... | The reason you see the $da$, $db$, or $dc$ is precisely because it isn't specified what the derivative is taken with respect to. What happens is the following:
First, they write the differential element of the logarithmic function:
$$d\,\ln f=\frac{df}{f}$$
This is always true no matter what function $f$ is. When we... | https://physics.stackexchange.com |
30,185 | [
"https://mechanics.stackexchange.com/questions/30185",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/14341/"
] | I have just got a few dents repaired using paintless dent removal services. While observing the procedure, I've noticed that the repair guy used a plastic rod-like tool to fix some imperfections by hitting it gently with the wooden grip of the hammer.
This banging on the painted surface, will it cause paint damage in ... | Paintless dent removal will not cause any long term side effects to your paint. The only effects to the paint would be if whatever caused the original dent caused damage to the paint.
Paintless dent repair relies on metal memory and the ability of metal to hold a position. Metal panels can usually take a certain amou... | Paintless dent removal is the way to go and it does not harm your paint. My family has been using snapadent.net for a while and the other day it was my turn. I saw the guy beating up my camaro's fender with a hammer straight to the paint. I was freaking out at first, but the end result was unbelievable!
| https://mechanics.stackexchange.com |
70,001 | [
"https://quant.stackexchange.com/questions/70001",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/61066/"
] | In the company I work for, I use several Python scripts that automate many tasks.
However, this happens via the task scheduler.
I would like to ask you what solutions are you using that make this possible, outside my machine.
Do you use a Virtual Machine?
Are your scripts dockerized on Kubernetes? (sorry, don't underst... | He writes on P. 97 that
<blockquote>
Investors should use expected vol, not implied vol, to calculate
Greeks.
</blockquote>
Expected VOL is NOT realized vol.
He also writes on P.96 that
<blockquote>
(using implied volatility as an estimate of future volatility is
standard market practice for calculating Greeks)
</block... | you find implied volatility using root finding methods, and with this value you get the greeks, obviously it's a model, not the truth, but something usefull to <em>control risks</em>
you should know that underling stochastic process should be considered when creating the model (most time it doesn't reflect, like black ... | https://quant.stackexchange.com |
486,398 | [
"https://electronics.stackexchange.com/questions/486398",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/243707/"
] | I have the following nested struct definition in my <code>.h</code> file:
<pre><code>struct PARAM
{
uint8_t data1;
uint8_t data2[2];
uint8_t data3[2];
};
struct CONFIG
{
uint8_t data4;
struct PARAM par1;
struct PARAM par2;
};
</code></pre>
In my <code>main.c</code> file I declare the followin... | From C99 onwards, C has support for what the standard calls 'compound literals'.
<br/>
You use them like this:
<pre><code>settings[0].par1 = (struct PARAM) {0x01,{0x01,0x01},{0x05,0x06}};
</code></pre>
You can in fact fill the entire settings[0] structure at once:
<pre><code>settings[0] = (struct CONFIG) {
0x01... | In the C language this type of text:
<code>{0x01,{0x01,0x01},{0x05,0x06}};</code>
Is only used when initializing the structure contents as an initializer on the declaration of the variable. It would not be used in an assignment statement.
You would have to type out the fully qualified name for each element of the s... | https://electronics.stackexchange.com |
29,668 | [
"https://dba.stackexchange.com/questions/29668",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/15305/"
] | Alright, I'm trying to make a query for a library that will show which students have never borrowed a book. For this, I have done the following:
<pre><code>SELECT
LEERLINGEN.LLNR,
LEERLINGEN.VOORNAAM,
LEERLINGEN.TUSSENVOEGSEL,
LEERLINGEN.ACHTERNAAM,
LEERLINGEN.KLAS,
COUNT(*) AS AANTAL
FROM
... | You need to use an outer join, otherwise you won't get those students back that did not borrow a book.
Then you need to count() on the students table, not "the group" (which is done when you use <code>(*)</code>)
<pre><code>SELECT
LEERLINGEN.LLNR,
LEERLINGEN.VOORNAAM,
LEERLINGEN.TUSSENVOEGSEL,
LEERLIN... | After translation of table names i think that this will work:
<pre><code>SELECT
LEERLINGEN.LLNR,
LEERLINGEN.VOORNAAM,
LEERLINGEN.TUSSENVOEGSEL,
LEERLINGEN.ACHTERNAAM,
LEERLINGEN.KLAS
FROM
LEERLINGEN
LEFT OUTER JOIN UITLENINGEN ON UITLENINGEN.LLNR = LEERLINGEN.LLNR
WHERE
UITLENINGEN.LLN... | https://dba.stackexchange.com |
201,985 | [
"https://mathoverflow.net/questions/201985",
"https://mathoverflow.net",
"https://mathoverflow.net/users/23661/"
] | I am looking for any known results about the tail sigma-algebra of a branching random walk. To be specific, let $T$ be the nodes of an infinite binary tree rooted at $r \in T$. Let $\{X_t\})_{t \in T}$ be i.i.d. real random variables with distribution $\mu$. For $s \in T$, let $Y_s$ equal to the sum $\sum_{t \in P_s}X_... | Actually, your idea of looking at the generation sums can be made more explicit. Let $s_n$ be the sum of $X_t$ over all vertices $t$ at a level $n\ge 0$, and let $S_n$ be the corresponding sum of $Y_t$ (in particular, $s_0=S_0=X_r$). Then
$$
S_n = 2^n s_0 + 2^{n-1} s_1 + \dots + 2 s_{n-1} + s_n \;,
$$
or
$$
2^{-n} S_n... | It turns out that $\mathcal{F}_\infty$ is never trivial (at least when the distribution of $X_s$ has a second moment). This can be seen by calculating the covariance of the average $Y_s$ at generation $n$ with $Y_r$, and comparing it to the variance of this average.
| https://mathoverflow.net |
429,402 | [
"https://physics.stackexchange.com/questions/429402",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/206971/"
] | I used a 500 watt travel kettle in France last year and it tripped the electrics in my hotel room so I stopped using it. I assumed the current was too high, even though its wattage rating wasn't that high.
However I realised the voltage switch on the kettle was set to 120V and not 240V (the voltage in France).
Would ... | I believe this is a good question and I think the key is understanding what the <strong>voltage switch</strong> on the kettle <strong>does</strong>. Realize that current in the kettle is not bound to the <em>power</em> of the kettle by $P=VI$ rather the actual voltage and current is related by (approximate) Ohm's Law: ... | Suppose that at a voltage of $2V$ the kettle element consists of two parts in <strong>series</strong> each of resistance $R$ ie a total resistance $2R$.
The power dissipated in the element is $\dfrac{(2V)^2}{2R}= \dfrac{2V^2}{R}$
<hr>
If the element is to dissipate the same power when the voltage is halved to $V$... | https://physics.stackexchange.com |
48,219 | [
"https://stats.stackexchange.com/questions/48219",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/11529/"
] | Given Gaussian mixtures $X_1, X_2 \in \mathbb{R}^p$ defined as $$P(X_i = x) = \sum_s \omega^{(s)}_i \mathcal{N}(x; \mu^{(s)}_i, \Sigma_i)$$ where the superscript $(s)$ indexes the $s$-th component of the mixture distribution $X_i$, the covariance of all mixture components are identical, and $\sum_s \omega^{(s)}_i = 1$ ... | Approach (1) might be feasible if the magnitude of the covariances are small compared to the (square of) the magnitude of the mean vectors. If this is the case, then the projection of the $X_i$ onto the unit sphere will subtend a small angle (this ignores some of the data in the tails), and you can ignore the curvatur... | A potentially useful result for circularly symmetric Gaussian kernels in 3-D:
First define:
$$
erf(x)=\int_{0}^{x} \frac{2}{\sqrt{\pi}} e^{-t^2} dt
$$
$$
Q(x)=\int_{0}^{x} \frac{2}{\sqrt{\pi}} t^2 e^{t^2} dt = \frac{1}{2} erf(x)-\frac{x}{\sqrt{\pi}} e^{-x^2}
$$
The following integral can be evaluated in terms of... | https://stats.stackexchange.com |
221,566 | [
"https://mathoverflow.net/questions/221566",
"https://mathoverflow.net",
"https://mathoverflow.net/users/36735/"
] | I am not sure whether this is a suitable question for MO. We know the classical version of Dirichlet's approximation theorem that if $x$ is a real number and $Q>0$ there exist $p,q\in \mathbb{Z}$ with $0<q\le Q$ such that $|x-p/q|<1/qQ$.
I am looking for a version of this theorem where $q$ is only a prime pow... | You cannot do much better than what you observed, because the fractional parts $\{2^n x\}$ are essentially the tails in the binary expansion of $x$, so they can be bounded away from zero (even for rational numbers $x$).
However, Furstenberg (1967) proved that the fractional parts $\{2^m3^n x\}$ are dense in $(0,1)$ fo... | Minor observations:
If $x$ is rational with non-prime power denominator, you certainly can't do much better. If $x = 1/6$, then $|x-p/q| \geq 1/(6q)$ for any prime power $q$.
If $x$ is irrational, then Vinogradov showed that $\{ x q \}$ is equidistributed as $q$ runs over the primes. So, for any $\epsilon>0$, we ... | https://mathoverflow.net |
311,428 | [
"https://electronics.stackexchange.com/questions/311428",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/146165/"
] | i was wondering if it's possible to use a high voltage ceramic capacitor for a 5V ldo, I also will use a 0.33uF 250V ceramic capacitor for the input.
thaanks
| You most certainly can. A ceramic capacitor will actually give you the pretty close to the advertised capacitance if you only apply a small percentage of its rated voltage.
A ceramic capacitor's effective capacitance degrades with voltage so if you were to apply 275V to your .1uF capacitor, you would unlikely get muc... | It will work.
But that is a lot of derating!
It is actually a good idea to use components rating higher than what they are exposed to, it will stress them less, and they will last longer. Now, 2 or 3 times is more usual (e.g. a cap rated 16V on a 5V line is plenty) ... a factor of 50+ might be a little overkill ;)
| https://electronics.stackexchange.com |
19,731 | [
"https://physics.stackexchange.com/questions/19731",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/7187/"
] | I see the phrase 'dynamical variable' in several of my texts and they don't really define it. A google search doesn't yield much either.
I have gathered that a dynamical variable can be represented in terms of position and momentum, and examples are velocity and kinetic energy, but what is the definition?
| In classical mechanics, it is true that the dynamical variables can be represented in terms of position and momentum. The terminology « dynamical variable » in fact comes from classical Hamiltonian mechanics. For example, parts of a mechanical system in Hamiltonian or Lagrangian mechanics.
Quantum Mechanics borrowed... | A dynamical variable is a mathematical variable describing a physical system that depends on time; the dependence of systems in Nature on time is what is referred to as "dynamics".
In various theories, like classical mechanics or quantum mechanics, dynamical variables are functions of $x,p$, or may depend on the spin... | https://physics.stackexchange.com |
565,474 | [
"https://physics.stackexchange.com/questions/565474",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/212603/"
] | Suppose I have a fan blowing air. By Bernoulli's equation, the air pressure in the stream is lower than the air in the surrounding.
Here is my confusion: the moment I turn on the fan, the fan starts pumping extra air into the space in front of it and the air pressure there should actually be <em>higher</em> than in the... | <blockquote>
The moment I turn on the fan, the fan starts pumping extra air into the space in front of it and the air pressure there should actually be higher than in the surrounding.
</blockquote>
You probably mean that the fan is able to push more air where it wasn't and produce an increase in pressure. That is true,... | The air in the moving stream has the same pressure as the surrounding air. Bernoulli only applies along streamlines. In the presence of vorticity adjacent treamlines can have different Bernoulli constants.
| https://physics.stackexchange.com |
129,250 | [
"https://security.stackexchange.com/questions/129250",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/40026/"
] | When a user wants to change the password for an account most web applications present a form where the user needs to enter the new password and the old password. So far I thought that I understood the benefits of this approach. For example, the old password is an additional mitigation to CSRF and it avoids account hija... | My guess would be that for google it allows them to handle all their various different options for TFA(two factor authentication) more easily. Off the top of my head they support 3 or 4 different methods at least. They also allow you to have multiple TFAs enabled at once so you can pick which one to use.
Not saying yo... | It's because people walk away from their computer, leaving them unlocked and logged in.
The second login requirement means that the guy in the chair next to you can't steal your account when you go to get more coffee or if you have to run for your plane and forgot to log off or if someone steals your session
They do ... | https://security.stackexchange.com |
158,234 | [
"https://electronics.stackexchange.com/questions/158234",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/29522/"
] | I have read that current is always the same within a circuit, but as far as I understood voltage is not. Every electronic part I use lowers the voltage more or less, even simple wires do this. So far, so good.
Now I wonder why it does not matter if a resistor comes before or behind an LED with respect to voltage drops... | One thing you need to remember is that voltage is relative. Voltage is a potential difference and it makes no sense to discuss voltage without a 'zero' reference.
In the case of your LED circuit there will be a voltage across the battery, a voltage across the LED, and a voltage across the resistor. If you add up all t... | If you want 3V across a LED it doesn't matter if you apply 13V to the anode and 10V to the cathode. There is still 3V across the LED.
| https://electronics.stackexchange.com |
1,431,506 | [
"https://math.stackexchange.com/questions/1431506",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/218225/"
] | <blockquote>
There are $100$ balls numbered $n_{1}, n_{2}, n_{3}\cdots \cdots n_{100}$ .
They are arranged in all possible ways . How many arrangements
would be there in which $n_{28}$ ball will always be before $n_{29}$ ball
and the two of them will be adjacent to each other ?
</blockquote>
$a.)\ \dfrac{9... | <strong>HINT:</strong>
Let's rephrase the question:
<ul>
<li>There are $27$ balls numbered $n_{1},n_{2},n_{3},\dots,n_{27}$</li>
<li>There are $71$ balls numbered $n_{30},n_{31},n_{32},\dots,n_{100}$</li>
<li>There is $1$ ball numbered $n_{2829}$</li>
</ul>
<blockquote>
Altogether, there are $27+71+1$ balls. How m... | Let suppose that $n_{28}$ is first and $n_{29}$ is second. Then there are $98!$ ways to arrange the other balls. Now, $n_{28}$ can be placed in any of the positions from $1$ to $99$. So the total number of possibilites is $99\cdot 98!=99!$.
| https://math.stackexchange.com |
66,029 | [
"https://dsp.stackexchange.com/questions/66029",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/49503/"
] | I have a code in which two complex signals (I/Q) are created having same frequency but with phase difference. FFT of both signals is taken and atan2 is used to compute phase after which the difference between both phases is calculated.
While changing the input frequency it was observed that for the case where the sign... | Your plot is correct. You are sampling three waves <span class="math-container">$f_1 = 10\textrm{Hz}, f_2 = 30\textrm{Hz}$</span>, and <span class="math-container">$f_3 = 70\textrm{Hz}$</span>, with a sample frequency of <span class="math-container">$F_s = 1.5\times70\textrm{Hz} = 105\textrm{Hz}$</span>. This means tha... | Your plot showing aliasing is applicable to complex signals.
You can emulate this exactly if you use:
<pre><code>y4_samples=10*e^(j*2*pi*f1*Tn)+10*e^(j2*pi*f2*Tn)+10*e^(j*2*pi*f3*Tn);
</code></pre>
Note that real signals such as <span class="math-container">$\cos(2\pi f_1 T_n)$</span> actually consist of two compl... | https://dsp.stackexchange.com |
203,708 | [
"https://physics.stackexchange.com/questions/203708",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/90607/"
] | Hi I want to get to the bottom of some thaughts I have and for that I need an answer to a - I have to admit - highly constructed example.
Lets assume - against all possibility - we find a strange device fixed earth an on alpha centauri and we could communicate nearly instantly with alpha centauri through this device i... | You already seem to have your accepted answer, but your question edits indicate some uncertainty. Let me see if I can offer an easier way to see it.
First off, what is the question? As I read it, your question is essentially,
<blockquote>
Suppose we take special relativity and add, in one privileged reference frame, <s... | Yes, such a device could send messages back in time.
It might appear that it can't because you imagine that there is some "now" and that the device merely connects the now at two distant events.
But the reality is that general relativity is local and says that what clocks measure and what people notice is the length ... | https://physics.stackexchange.com |
319,698 | [
"https://softwareengineering.stackexchange.com/questions/319698",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/230189/"
] | I see a lot of questions revolving around accessing JSON keys that use hyphens (kebab-case), but now I find myself wondering should I just stick to camelCase or snake_case in my keys. I know hyphens also can create complicated mappings when ported between languages. I've seen some JSON deserialize libraries convert tho... | You can use <em>anything</em> as JSON keys, as long as it is valid UTF-8, doesn't contain zero code points, and it would be useful if you could represent the key as a string in the programming language of your choice. I might recommend not to use different Unicode representations of the same string (for example "Ä" wri... | There are plenty of JSON serialization systems that are more than capable of handling mapping between field names that aren't suitable for use in the language they integrate with. In most cases, they aren't hard to use, and require only a little bit of extra effort. In an ideal world, you wouldn't have to, but if you... | https://softwareengineering.stackexchange.com |
33,545 | [
"https://mechanics.stackexchange.com/questions/33545",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/16481/"
] | I haven't replaced my engine air filter ever since buying the car used 4.5 years ago and don't know when if ever the previous owner replaced it. A lot of casual car-owners I ask don't even know about the engine air filter replacement. Gas mileage and power seem likely to decrease with a clogged air filter, but why is n... | There are two major problems incurred by not changing your air filter:
<ol>
<li>As your air filter becomes full of dirt, pollen, <em>crap</em>, what have you, it becomes a restriction on the air inlet. This will <strong>degrade performance</strong> on the vehicle (as you stated). This will cause you more wear and tear... | When the air filter gets more clogged then the negative pressure in the air-intake will increase with the following consequences:
<ul>
<li>Dirt will get forced trough the filter or small gaps and increases wear on the engine.</li>
<li>Through some small leaks in the air line (between the maf and the engine) there will... | https://mechanics.stackexchange.com |
132,407 | [
"https://mathoverflow.net/questions/132407",
"https://mathoverflow.net",
"https://mathoverflow.net/users/33625/"
] | <strong>If $\kappa \rightarrow (\alpha)^r_2$ holds for every $r\in \omega$, then is $\kappa$ an $\alpha$-Erdős cardinal? (or rather, does $\kappa \rightarrow (\alpha)^{<\omega}_2$ hold?)</strong>
$\kappa \rightarrow (\alpha)^r_2$ means that $\forall f: [\kappa]^r \rightarrow 2$, $\exists A\in[\kappa]^\alpha$ such t... | The two properties are not the same.
If $\kappa$ is weakly compact, then $\kappa\rightarrow(\kappa)^r_\lambda$ for any $r<\omega$ and $\lambda<\kappa$. These cardinals are compatible with $V=L$.
A cardinal $\kappa$ for which $\kappa\rightarrow(\kappa)^{<\omega}_2$ is called a Ramsey cardinal, and these card... | Unfortunately $\kappa\rightarrow(\alpha)^r_2$ for every $r$ is much weaker than $\kappa\rightarrow(\alpha)^{<\omega}_2$. In fact, the least $\kappa$ for which $\kappa\rightarrow(\alpha)^r_2$ holds is $\exp_{r-1}(|\alpha|)^+$ (the general Erdos-Rado theorem), and so the least $\kappa$ for which this holds for all $r$... | https://mathoverflow.net |
63,544 | [
"https://datascience.stackexchange.com/questions/63544",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/85747/"
] | I have model like y=mx. Since the adjusted R2 tells you the percentage of variation explained by only the independent variables that actually affect the dependent variable and I have only one independent variable, do I need to consider my adjusted r-square value? Or, r-square is good for this type of model?
| Let's start with the definitions:
<span class="math-container">$$ \text{RMSE} = \sqrt{\frac{1}{N} \sum_{i = 1}^{N} (y_i - \hat y_i)^2}, $$</span>
<span class="math-container">$$ \text{RMSLE} = \sqrt{\frac{1}{N} \sum_{i = 1}^{N} (\log y_i - \log \hat y_i)^2}, $$</span>
where <span class="math-container">$y_i$</span> is ... | RMSE will have a drastic effect of outliers on its values. But in case of RMSLE we can reduce the effect of outliers by many magnitudes & their effect is much less.
RMSLE value will only consider the relative error between Predicted and the actual value neglecting the scale of data. But RMSE value will increase in... | https://datascience.stackexchange.com |
42,681 | [
"https://datascience.stackexchange.com/questions/42681",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/62386/"
] | I'm exploring the <code>https://r4ds.had.co.nz/transform.html#add-new-variables-with-mutate</code> r for data science handbook and don't really understand the min_rank() operator. Doing the exercises it asks to Find the 10 most delayed flights using a ranking function. How do you want to handle ties? Carefully read th... | You can put the results of min_rank in to a new column, e.g.:
<pre><code>f2 <- flights %>%
mutate(rank = min_rank(dep_delay))
</code></pre>
Where showing just the selected rows:
<pre><code>f2 %>%
select(dep_delay, rank)
</code></pre>
You get this:
<pre><code># A tibble: 336,776 x 2
dep_delay ra... | I solved it like this:
<pre><code>mutated = mutate(flights, total_delay = dep_delay + arr_delay)
arrange(mutated, desc(total_delay))
</code></pre>
basically by doing this you create a new colum with the total delay in minutes with which you sort your data by total delay time using arange
| https://datascience.stackexchange.com |
218,356 | [
"https://dba.stackexchange.com/questions/218356",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/156795/"
] | I have 2 tables with the following schemas:
<code>t1(id, type)</code> where id and type are the primary key
<code>t2(type, rate)</code> where type is the primary key
I want to find all id's which appear with all types from t2.
For example given the tables:
<code>t1</code>:
<pre><code> id | type
---------
1 | a... | pg_stat_statements always uses the literal directory "pg_stat_tmp", even if the "stats_temp_directory" parameter has been changed to point elsewhere.
See this note from the source code:
<blockquote>
Location of external query text file. We don't keep it in the core
system's stats_temp_directory. The core syste... | pg_stat_statements is keeping the whole <strong>pg_stat_statements.max</strong> in memory . So please check if you have enough memory for this. All memory is allocated at start , even if you don't need all. When the postgres server stops , all statistics will be saved to disk if you activate this , check <strong>pg_st... | https://dba.stackexchange.com |
2,955,618 | [
"https://math.stackexchange.com/questions/2955618",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/372741/"
] | Haven't got any clue yet, should we prove there is a bijection between <span class="math-container">$B_0$</span> and <span class="math-container">$\mathbb{N}$</span> ?
| Another attempt:
Let <span class="math-container">$B:=$</span>{<span class="math-container">$m/2^n| m,n \in \mathbb{N}$</span>}.
<span class="math-container">$f: \mathbb{N}×\mathbb{N}$</span> <span class="math-container">$\rightarrow$</span> <span class="math-container">$B$</span>,
<span class="math-container">$f:(m... | It's not the most elegant, but you could define a function
<span class="math-container">$$
\mathbb{N}\times\mathbb{N}\to\mathbb{Q}\to B_0
$$</span>
via
<span class="math-container">$$
(m,n)\mapsto\frac{m}{2^n}\mapsto f(m/2^n)
$$</span>
where <span class="math-container">$f$</span> is defined case-wise as <span class="m... | https://math.stackexchange.com |
3,350 | [
"https://scicomp.stackexchange.com/questions/3350",
"https://scicomp.stackexchange.com",
"https://scicomp.stackexchange.com/users/2066/"
] | Recently I am taking advantage of the <code>ode45</code>, as well as <code>ode23s</code>, <code>ode15s</code> solvers in <code>Matlab</code> to find numerical solutions of a system of dynamic equations (differential equations) which are developed by Lagrange method.
However, it is hard for me to get a result even thou... | You seem to think that the matlab integrator is the problem and by this hypothesis you should try a different integrator. But before you do this I think it would be useful to check whether that's actually the case. For example, it may be that your user-provided function hangs. Or that it is just very slow. Or that it p... | You could try solvers written in a different language.
Within Matlab, you can have the code print something (see help odeprint), and then investigate the output. Or add options (help odeset), or shorten the time interval.
Matlab ode solvers don't seem to have a time-limiting option, but you can easily alter the sour... | https://scicomp.stackexchange.com |
116,543 | [
"https://electronics.stackexchange.com/questions/116543",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/37656/"
] | I am currently using two nRF24L01+ (2.4 GHz RF transceivers) together with two AVRs to create a keyless entry system.
My problem is battery lifetime when using a CR2032 battery to power the key.
The key will sleep until a signal is received from the other unit, then wake up and respond.
The RF modules have a power-do... | The short answer is no, there is no such radio module. Note also that the two standby modes available cannot receive anything, they simply provide a faster start-up than is available from sleep mode.
To detect incoming packets you need to be in RX mode, so minimum 12.6mA. Clearly you can't stay in that mode all the ti... | The keyless entry system should send out a preamble/code at regular bursts, which the key wakes up and listens for. The key's RF module should be in power down mode for most of the time, along with the processor.
For example, the keyless entry systems send a 'ping' every 1ms, the key can wake up every 200ms and listen... | https://electronics.stackexchange.com |
90,644 | [
"https://stats.stackexchange.com/questions/90644",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/23984/"
] | Perhaps this is a very basic question, but I didn't find yet a simple solution for this simple problem:
I want to compare two samples (say X and Y) for a continuous variable which is <strong>non-normally</strong> distributed and test if X and Y are significantly different. The sample size of <strong>X</strong> is <st... | I am not a big expert on statistical testing, but the approach you are considering decidedly does <em>not</em> make sense. Imagine that the groups are indeed identical (i.e. null hypothesis is true). Then you will observe p<0.05 in exactly 5% of the cases, and e.g. p<0.01 in 1% of the cases (those would be false ... | Your approach does not make sense. The usual Wilcoxon-test will answer you with high power. Your approach looses this advantage. It may however be reasonable to be afrait of too much power, because even irrelevant differences will show up significant, which would in fact distract the scientist interested in a relevant ... | https://stats.stackexchange.com |
128,803 | [
"https://mathoverflow.net/questions/128803",
"https://mathoverflow.net",
"https://mathoverflow.net/users/20108/"
] | Say that a triple of real numbers $(a,b,c)$ is a realizable triple if there are matrices $A,B\in SL_2(\mathbb{R})$ such that $tr (A)=a$, $tr (B)=b$, and $tr (AB)=c$. Question: what is the shape of the non-realizable set?
This is surely known, but I couldn't find an answer by myself, nor a reference on the www. It's ea... | Let $x, y, z$ be traces of $A, B, AB$ respectively. Define
$$
k(x,y,z)= x^2+y^2 + z^2 -xyz -2.
$$
Then a triple of real traces $(x, y, z)$ is realizable in $SL(2,R)$, unless it is realizable in $SU(2)$, the latter happens if and only if $x, y, z\in [-2,2]$ and $k(x,y,z)\le 2$. See Goldman's paper "Topological component... | Thank you, Misha, for the answer.
Goldman's proof is part of a bigger picture, and it is hard to rip it out of the context. Besides, it looks a bit too sophisticated for such a simple question. Helped by knowing what should happen, I eventually came up with an elementary and self-contained argument, though not partic... | https://mathoverflow.net |
2,417,342 | [
"https://math.stackexchange.com/questions/2417342",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | <blockquote>
Given that $\log_62 = .387$. Find $\log_6 9$.
</blockquote>
I don't know proper method to solve this problem . Please help me.
| Let $\log_62=a$.
Hence,
$$\log_62=\frac{1}{1+\log_23}$$ and
$$\log_69=\frac{2\log_23}{1+\log_23}.$$
Thus, $$\log_23=\frac{1-a}{a}$$ and
$$\log_69=\frac{\frac{2(1-a)}{a}}{1+\frac{1-a}{a}}=2(1-a)=1.226...$$
Now we see a best way:
$$\log_69=\log_6\frac{36}{4}=\log_636-\log_64=2-2\log_62=...$$
| $\log_6(2) = .387 =:a$
$ \rightarrow$:
$6^a = (2×3)^a =2^a3^a = 2;$
$3^a = 2^{1-a}, $
now square both sides:
$9^a = 2^{2(1-a)}.$
Taking $\log_6$ of both sides:
$a\log_6(9) = 2(1-a)\log_6(2).$
Recall: $a = \log_6(2)$:
$\log_6(9) =2 (1-a).$
| https://math.stackexchange.com |
320,128 | [
"https://softwareengineering.stackexchange.com/questions/320128",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/81973/"
] | I have a project structure like this:
<pre><code>My Project
- Other Team's Project
-Third Party Dependency
</code></pre>
<code>My Project</code> requires <code>Other Team's Project</code> to function, and both <code>My Project</code> and <code>Other Team's Project</code> require <code>Third Party Dependency</c... | Imagine what happens if the other team refactors its project to do the same thing <em>without</em> using the third party library and they remove their dependency. Or they switch to another third party library to do the same thing. Will your code still work?
If it does your code only depends on the other project, if no... | This is mostly opinion-based I think, but I'll put in my 2p anyway.
The way I've always done it is by asking myself, <em>what's the minimum amount of knowledge my code <strong>needs</strong> to have to do what its supposed to do?</em>
If my code only uses code from <code>Other team's library</code>, then I will only... | https://softwareengineering.stackexchange.com |
97,645 | [
"https://cs.stackexchange.com/questions/97645",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/93806/"
] | Let's say I have a context free language. It can be recognised by a pushdown automaton. Chances are it can't be parsed with a regular expression, as regular expressions are not as powerful as pushdown automata.
Now, let's put an additional constraint on the language: the maximum recursion amount must be finite.
Becau... | We can take it even further: if we put a limit on the size of the HTML/XML, say 1PB, then there is only a finite number of them, so we can trivially parse them in <span class="math-container">$O(1)$</span> using a giant look-up table. However, that doesn't seem to tell us much about the complexity of parsing HTML/XML i... | <strong>Yes.</strong> If you have a PDA which never makes the stack longer than <span class="math-container">$n$</span>, then there are a finite number of stack contents. Thus the number of (PDA state, stack contents) values is also finite. Thus it's possible to build a finite automaton with that many states to recogn... | https://cs.stackexchange.com |
410,434 | [
"https://math.stackexchange.com/questions/410434",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/69818/"
] | Assume $A$ is a countable dense set in $\mathbb{R}$, and set $B$ has positive (Lebesgue) measure. Prove that $A+B=\{a+b:a\in A, b\in B\}=\mathbb{R}\backslash N$, where $N$ is a set of measure zero.
I haven't come up with a good idea.
Thanks in advance!
| This follows from the following Steinhaus theorem: if $A,B$ are subsets of the real line of positive measure then $A-B$ contains an interval.
| Some more details for the answer by Kena Bel P: $A+B$ is measurable because it's the union of countably many translates of the measurable set $B$. Let $N$ be its complement and suppose, toward a contradiction, that $N$ had positive measure. Then by the cited theorem of Steinhaus, there would be an interval included i... | https://math.stackexchange.com |
9,846 | [
"https://physics.stackexchange.com/questions/9846",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/3044/"
] | Two point charges, -2.5 micro coulombs and 6 micro coulombs, are separated by a distance of 1m (with the -2.5 charge on the left and 6 on the right).
What is the point where the electric field is zero?
This seems exceptionally easy, but I can't figure it out. I can calculate the answer if both charges are negative/po... | Because of symmetry, the point at which the forces cancel must be on the same line as the two charges -- it's effectively a 1-dimensional problem.
If a small positive test charge were <em>between</em> the two charges, it would be "pushed" or "pulled" in the same direction by both charges, so it cannot be there.
If a ... | Following Peter Morgan's eqn, I worked out the answer to be ABOUT (I did not do this on a spreadsheet) (13/7) meters on a straight line away from the smaller charge and opposite the larger charge. That is, the smaller charge is between the zero point and on a straight line from the zero point and the larger charge.
O... | https://physics.stackexchange.com |
416,200 | [
"https://physics.stackexchange.com/questions/416200",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/173904/"
] | In Tensorial form, the definition of the Lie derivative for a covariant and contravariant vector are respectively:
$$\mathcal{L}_U V^\mu=U^\nu\nabla_\nu V^\mu- V^\nu\nabla_\nu U^\mu$$
$$\mathcal{L}_U V_\mu=U^\nu\nabla_\nu V_\mu+ n_\nu\nabla_\mu U^\nu$$
At some point of a calculation, I had to calculate $\mathcal{L}_... | When you take the Lie derivative of a vector, you are looking at how it changes as you move along integral curves. Now if you look at $L_UU$ you are asking how does $U$ change along its integral curves. But the point of an integral curve is that it’s tangent is always $U$. So $U$ does not change as you travel along th... | The first kind of Lie derivative is anticommutative, which implies the desired result. In fact, identifying $V^\mu$ with $V^\mu\nabla_\mu$, the Lie derivative is a commutator.
| https://physics.stackexchange.com |
24,968 | [
"https://chemistry.stackexchange.com/questions/24968",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/4984/"
] | Please consider the following figure of a concentration over time (msec) diagram with variables for an enzyme, a substrate bound enzyme and a product.
<img src="https://i.stack.imgur.com/fTcYo.png" alt="enter image description here">
Recall that $\ce{E + S \leftrightharpoons ES \rightharpoonup E + P}$ and also recall... | Given $\ce{E + S <=>ES -> E + P}$
$\frac{d[\ce{ES}]}{dt}=-\frac{d[\ce{E}]}{dt}= k_1[\ce{E}][\ce{S}]-k_2[\ce{ES}] -k_3[\ce{ES}] $
<blockquote>
is the slope of $[ES]$ equal to $k_2$ ?
</blockquote>
no
<blockquote>
is the slope of $[E]$ equal to $k_1 + k_3$
</blockquote>
no
| <blockquote>
On the plot provided, is the slope of [ES] equal to k2 and is the
slope of [E] equal to k1+k3 thereby giving some insight/explanation
for km which is used to determine the affinity of an enzyme?
</blockquote>
As DavePhD said, no to both counts. First note that in your system, $\ce{E_0=E +ES}$, wher... | https://chemistry.stackexchange.com |
100,604 | [
"https://datascience.stackexchange.com/questions/100604",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/123644/"
] | Why is a MinMax Scaler scaling each column independently? Isn't it losing information if the values are somehow connected?
If the value in column B is always an upper limit for the value in column C, after scaling the value in column B will be smaller than the value in column C because the range is much smaller.
I can'... | When we say two values are 'connected', we are typically talking about correlation (or covariance).
The correlation between variables A and B is conserved across <strong>linear</strong> scalings (which MinMax Scalers perform).
For example, if <code>A = [1,5,10]</code>, and <code>B = [10,50,100]</code>, their Pearson co... | Most models and theory suppose features are independant variables. Moreover some models may take into acccount only biger values, so scaling is important and necssary. Scaling each one independently assures each feature has same range of value, and so the same importance while training your model.
| https://datascience.stackexchange.com |
334,947 | [
"https://mathoverflow.net/questions/334947",
"https://mathoverflow.net",
"https://mathoverflow.net/users/142424/"
] | I have to minimize <span class="math-container">$f(x)=x^4-24x^2$</span> starting on the point <span class="math-container">$x_o=1$</span>. The method converge to <span class="math-container">$x=0$</span>, but i know that the solution is <span class="math-container">$x=+-2\sqrt{3}$</span>. The hessian and the derivate o... | Newton's is not really a "minimizing method". If you're using Newton's method to find a root of <span class="math-container">$f'$</span>, the root you find might be a local minimum, local maximum or neither. To remove the root <span class="math-container">$x=0$</span> from consideration, you might try finding a root o... | A serious Newton minimization algorithm, sometimes called modified Newton algorithm, will employ safeguarding in the form of line search or trust region, to enforce descent across (major) iterations. Such safeguarding is necessary, even for convex problems, to ensure that the algorithm converges to a stationary point. ... | https://mathoverflow.net |
104,783 | [
"https://mathoverflow.net/questions/104783",
"https://mathoverflow.net",
"https://mathoverflow.net/users/25602/"
] | Let $G=(V,E)$ be a (simple) finite graph such that every vertex has degree at least 1. Then it is easy to see that there is a subset $E'$ of $E$ such every vertex in $G'=(V,E')$ still has degree at least 1 and all paths (with no repeating edges) in $G'$ are of (edge-wise) length at most 2. (I just keep removing middle ... | Aaron Meyerowitz suggested to try to reduce the problem to trees and, to me, this seems to work. First we can suppose that $G$ is a connected graph, because we can solve the problem separatly for each component. It is easy to see by Zorn's Lemma, that every connected graph contains a spanning tree, i.e. a subgraph whic... | You should be able to do something similar, at least for graphs with a bijection to the natural numbers.
Using the bijection, take the next unprocessed vertex. If it is of degree one, move on. If it has any
neighbors of degree one, remove all other edges to neighbors of degree not 1, making a disconnected star.
Other... | https://mathoverflow.net |
163,059 | [
"https://physics.stackexchange.com/questions/163059",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/60766/"
] | A lighthouse has two blinking spotlights 0.4 meters apart. The spotlight on the right blinks 5 seconds after the one on the left. How much time elapses between the blinkings of the spotlights as observed by a drone traveling at 0.2c?
To solve the above question, will the 5 seconds be the improper time $(t')$ in the fo... | Eigenvectors exist only for the point spectrum of an operator. For any other point of the spectrum one can only find a sequence of vectors for which $(A-\lambda I)u_n\to0$, where $A$ is said operator, and $\lambda$ is a point in the spectrum which is not an isolated point. So in this case there is a sequence of <em>app... | This doesn't directly answer your question of orthogonality, but may still address your concern.
I need to point out that you seem to be working on a scattering problem and resonant states. Resonant wave functions DO NOT belong to the Hilbert space. They are not even eigenfunctions of the Hamiltonian in an usual se... | https://physics.stackexchange.com |
168,288 | [
"https://chemistry.stackexchange.com/questions/168288",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/125483/"
] | Is there any book for chemistry which has all the reactions in it ? A book of just reactions. I have gone through way too many but each just explains the types of reaction and to get all the reactions collected at once place will take too long. So there any book which has all the reactions listed?
| There is no such heavenly book that contains all the reactions known to humanity; however, two databases are worth mentioning which summarize most of the published chemical reactions. One is Reaxys of Elsevier, and the other is SciFinder.
Reaxys includes not only the recent literature, but the nice thing is that it has... | Organic Chemistry: 100 Must-Know Mechanisms by Roman Valiulin is the closest thing I can think of. Otherwise, pick up Clayden's Organic Chemistry or Bruice's Organic Chemistry and make your own list.
What is this for ? There's also databases of organic transformations for cheminformatics purpouses but nothing is even c... | https://chemistry.stackexchange.com |
52,567 | [
"https://dsp.stackexchange.com/questions/52567",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/38189/"
] | I'm doing an FFT using Python and Numpy on one machine, and C# on another. I'm using some dummy data that mimics how I'll eventually be gathering data from sensors in the C#/UWP application. The two methods yield consistently identical results on the dummy data, which is great. However, the phase at the sine wave's fre... | The Fourier Transform (and the DFT is the discrete form of this with a finite length sequence) is simply a correlation of the input signal with each frequency bin, of the continuous form:
<span class="math-container">$$X(\omega) = \int_{t=0}^Tx(t)e^{-j\omega t}dt$$</span>
(If we consider a finite time duration, howev... | This is a common misconception, that a typical FFT implementation shows phase relative to a sin() function that starts at zero the "left" edge of the FFT aperture.
But the sin() function corresponds to the imaginary part of a complex exponential. And the typical default is for a strictly real FFT result to have a p... | https://dsp.stackexchange.com |
48,306 | [
"https://stats.stackexchange.com/questions/48306",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/19949/"
] | I'm having trouble distinguishing between the concepts of scedasticity and stationarity. As I understand them, heteroscedasticity is differing variabilities in sub-populations and non-stationarity is a changing mean/variance over time.
If this is a correct (albeit simplistic) understanding, is non-stationarity simply... | To give precise definitions, let $X_1, \ldots, X_n$ be real valued random variables.
<em>Stationarity</em> is usually only defined if we think of the index of the variables as <em>time</em>. In this case the sequence of random variables is stationary of $X_1, \ldots, X_{n-1}$ has the same distribution as $X_2, \ldots... | There are 3 degrees of stationary. The weak form requires mean and variance are kept constant. This means that of 3 stationary definitions are <strong>stronger requirements than heteroscedasticity</strong> because heteroscedasticity means constant variance, without reference to the mean.
A process can have heterosce... | https://stats.stackexchange.com |
602,062 | [
"https://physics.stackexchange.com/questions/602062",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/263311/"
] | I want to very simplistically plot the <span class="math-container">$x$</span>-<span class="math-container">$y$</span> position of a vehicle in 2D that can accelerate/brake and drive a constant speed while turning etc.
The basic equations for determining the position for constant speed seem to be
<span class="math-cont... | You know of course that the tidal frequency due to the moon is 12 hours, that is, twice the rotation frequency of Earth. This is because on the side near the Moon the sum of Earth and Moon attraction is increased with respect to average, so the water level rises towards the moon. On the far side the the attraction is d... | When you ask, "What causes tides?", you may hear, "Gravity". A follow up question is: "Which tide is stronger: the Sun or the Moon?",..., "The Moon".
So: Then why does the Earth orbit the Sun and not the Moon?....clearly, the Sun's gravity is stronger (about 250x).
The resolution... | https://physics.stackexchange.com |
18,855 | [
"https://electronics.stackexchange.com/questions/18855",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/1571/"
] | I have several slave devices communicating with a microcontroller master on an I2C bus. My PCB is only about 1" x 1" and the devices are only communicating at 400 kHz. What is the best way to connect all the I2C peripherals to the microcontroller? In a star configuration or in a daisy-chain configuration?
| Propagation delay is not a problem with a 400kHz I2C bus on a 1" by 1" PCB. Lay the traces out in whatever way gives the cleanest routing.
A rule of thumb in EMC design is that you need to consider transmission line effects if your traces are 1/10 the wavelength of your highest significant frequency.
Your highest ... | In your case (1x1 inch, 400 kHz) it doesn't matter. The pullup resistors can be anywhere on each net, and your main issue can be routing simplicity.
The only thing to watch out for is capacitive coupling if you've got other nets with high dV/dt. Just keep somewhat away from those, or for extreme cases, put a deliber... | https://electronics.stackexchange.com |
137,904 | [
"https://electronics.stackexchange.com/questions/137904",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/57933/"
] | I'm designing an op amp circuit to amplify 20mV, 18kHz square wave up to measurable levels (~2-3V). Why is the output signal:
<ul>
<li>Biased at ~400mV</li>
<li>Triangular (high output capacitance? is that a thing?) ?</li>
</ul>
Op amp is chosen to have maximum slew rate (1V/uS).
<img src="https://i.stack.imgur.com/... | Question 1 (biasing): It is no surprise that the output signal contains a dc part because the input signal is centered upon a dc voltage. However, the dc ouput level cannot be verified simply by amplifying the input dc voltage because the form of the output signal has changed.
Question 2 (form): There are two differen... | I would say the output was offset at 1 volt and this is approximately 100 times higher than the offset on the input waveform. Given your amp is configured to have a gain of about 100, there is no surprise here.
The triangulated output is due to the poor gain-bandwidth-product of the op-amp you chose. You want a gain o... | https://electronics.stackexchange.com |
712,844 | [
"https://math.stackexchange.com/questions/712844",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/122781/"
] | Let $(V,\|\cdot\|)$ be a finite-dimensional normed space.
Define $\|T\|_\mathrm{op}=\sup\{\|T(x)\|:\|x\|≦1\}$, for all linear operators on $V$
Define $\Omega$ to be the set of all invertible linear operators on $V$.
With these notations above, below are theorems i'm curious to know what they do <em>mean</em>:
<bloc... | <em>Visualizing</em> such things might prove hard, since we're dealing with vectors in $n^2$ dimensions for some $n$. The theorems are important since they provide, along with the contraction principle, a very good proof of the inverse functions theorem which Rudin gives in full detail.
To prove the first theorem, we ... | Please refer to the exercise 7 on page 155 of Folland's <em>real analysis</em>. You will find more useful information.
Let $X$ be a Banach space.
<ol>
<li>If $T\in L(X,X)$ and $\|I-T\|<1$ where $I$ is the identity operator, then $T$ is invertible; in fact, the series $\sum_0^\infty(I-T)^n$ converges to $(I-T)^{-1}... | https://math.stackexchange.com |
455,619 | [
"https://physics.stackexchange.com/questions/455619",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/220475/"
] | Let's have an ideal conductor with current density <span class="math-container">$\mathbf{J}$</span>. The ideal conductor takes up the entire space, effectively resulting in the entire space <span class="math-container">$\mathbb{R}^3$</span> being permeated with <span class="math-container">$\mathbf J$</span>.
In case ... | Sorry for my poor english !
I think we can go into steady state and forget about the electric field.
The problem is well known for infinite volume charges distributions. In particular for a volume distribution of uniform charge for which "by symmetry" the electric field should be zero, non compatible with the Maxwell... | Congrats! What you've discovered, more or less, is that the case of an infinite uniform <em>current</em> density pervading all space in <em>magnetostatics</em> is the complementary notion to the case of an infinite uniform <em>charge</em> density pervading all space in <em>electrostatics</em>. And it is well-known that... | https://physics.stackexchange.com |
4,419,661 | [
"https://math.stackexchange.com/questions/4419661",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/531015/"
] | I tried using inclusion exclusion where set <span class="math-container">$A$</span> is all the rearrangements where two <span class="math-container">$A$</span>s are consecutive and set <span class="math-container">$B$</span> where two <span class="math-container">$B$</span>s are consecutive. However, I got <span class=... | We wish to find the number of arrangements of AABBBCCDDD in which no two As are adjacent and no two Bs are adjacent.
As you found, the number of distinguishable permutations of AABBBCCDDD is <span class="math-container">$$\binom{10}{2, 3, 2, 3}$$</span>
From these, we must subtract those arrangements with at least one ... | Leaving out the <span class="math-container">$B's$</span> for the moment, there are <span class="math-container">$\dfrac{7!}{3!2!2!} = 210$</span> permutations of <span class="math-container">$AACCDDD$</span>
Of these <span class="math-container">$\dfrac{6!}{3!2!} = 60$</span> will have the <span class="math-container"... | https://math.stackexchange.com |
112,074 | [
"https://mathoverflow.net/questions/112074",
"https://mathoverflow.net",
"https://mathoverflow.net/users/27991/"
] | Given m & n, we have to find out the number of possible matrices of order m*n with the property that A(i,j) can be either 0 or 1 and that no contiguous sub-matrix of both length > 1 & breadth > 1 should have same entries i.e. all of its cells shouldn't be 0 or 1. For example if m = 2 & n = 2, the answer is ... | Let $a_n$ be the number of $2 \times n$ -matrices avoiding constant 2*2-submatrices.
Then
$$a_n = \frac{2^{-n} \left(4 \left(17+4 \sqrt{17}\right)
\left(3+\sqrt{17}\right)^n+\left(\sqrt{17}-17\right)
\left(\sqrt{17}-3\right)^n e^{i \pi n}\right)}{17
\left(3+\sqrt{17}\right)}$$
This should be fairly straigh... | EDIT :
I've edited the argument to make it stronger
Suppose that $m\geq 3$ and $n\geq 5$ so that there is a 3x5 submatrix A. I show that the number of possibilities is zero in this case.
In A, there are at least two rows with at least there $1$'s each (up to relabeling the symbols). Since we cannot have a constant 2x2... | https://mathoverflow.net |
723,247 | [
"https://physics.stackexchange.com/questions/723247",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/230823/"
] | The double-slit experiment requires wave-like interference of possible paths according to the quantum action principle, but it doesn't require entanglement since it's a single-particle phenomenon. Isn't entanglement arguably even more mysterious, since it violates local realism and thus can't be explained by de Brogli... | I think what he was getting at is that we don't yet have a single agreed interpretation of what is 'really' happening at a quantum level, and the two-slits experiment typifies the nature of the conceptual gap we have yet to fill. Entanglement in that context can be considered as 'just' another example of the strange ef... | The concept of entanglement was introduced by Schrödinger.<sup>1</sup> So the idea must have been known to Feynman.
However, entanglement is a consequence of a deeper concept associated with the way quantum system can form superpositions of multiple entities (particles). I guess that Feynman understood that this is the... | https://physics.stackexchange.com |
6,079 | [
"https://dsp.stackexchange.com/questions/6079",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/3332/"
] | When synthesizing and processing audio on modern processors, when would one consider using something other than single precision (32 bit) floating point? Obviously, the audio coming in and going out to the real world is 16/24 bit, so I'm just talking about the precision of the signals (both the audio itself and things ... | IEEE float singles only provide about 24 bits of mantissa. But many DSP/filtering algorithms (IIR biquads with poles/zeros near the unit circle, etc.) require far more than 24 bits of mantissa for intermediate computational products (accumulators, etc.), just to get final results accurate to near 16 or 24 bits. For t... | <blockquote>
the CPU/DSP has hardware floating point support for both single and double precision.
</blockquote>
It really depends on what kind of support you are talking about. On x86, when using the x87 style floating point instructions, you get the full 80-bit internal precision and the same processing time - whe... | https://dsp.stackexchange.com |
15,768 | [
"https://dba.stackexchange.com/questions/15768",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/5328/"
] | There is a field <code>show</code> which is a <code>boolean</code> type in postgesql.
I want to write a sql to update that table, to toggle the value of show. If it's <code>true</code>, it becomes <code>false</code>, if it's <code>false</code>, it becomes <code>true</code>.
Is it possible?
| This will do:
<pre><code>SET show = NOT show
</code></pre>
so a value of
<code>TRUE</code> becomes <code>FALSE</code>,
<code>FALSE</code> becomes <code>TRUE</code>,
<code>UNKNOWN</code> (<code>NULL</code>) stays <code>UNKNOWN</code>.
| The following will turn <code>FALSE</code> or <code>NULL</code> to <code>TRUE</code>, and <code>TRUE</code> to <code>FALSE</code>:
<pre><code>UPDATE tablename SET fieldname = NOT COALESCE( fieldname, 'f' ) WHERE keyvalue = ?;
</code></pre>
| https://dba.stackexchange.com |
478,229 | [
"https://stats.stackexchange.com/questions/478229",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/150856/"
] | Let's say I have a series of previous year's weekly sales for a product <span class="math-container">$\{x_i\}$</span> and the series of sales for the same product this year <span class="math-container">$\{y_i\}$</span>. How can I find out if the increase in sales are statistically significant?
Edit: Context: I have wee... | It sounds like you just want to look at if the change in the total number of sales (or income you could do the same with income) is significant. For this you will need a t test. For each week you should sum the number of sales (or income). So rather than the number of sales for each item, it should just say the number ... | You may want to consider causal impact (CausalImpact Model in R or Python) if you have a time series and you are trying to see if a campaign caused an impact to a group of accounts.
| https://stats.stackexchange.com |
128,976 | [
"https://mathoverflow.net/questions/128976",
"https://mathoverflow.net",
"https://mathoverflow.net/users/24021/"
] | Can you make the category whose objects are pairs of spaces $(X,A)$, and morphisms the obvious diagrams, into a model category? Of course I want this to be done in a meaningful way, that is, agreeing with the adjoint functors $X\mapsto (X,\emptyset)$ and $(X,A)\mapsto X$?
There might be some intuitive reason that it ... | The answer has to be no. There's just no good homotopy theoretic way to talk about subspaces, because up to homotopy every map is an inclusion (via the mapping space construction). So even before you run into the completeness issue Karol raises you have a more fundamental issue of what the homotopy category would be. I... | As Tyler Lawson points out you can use the category of all diagrams on $[1]$. Then the projective and injective model structures are both instances of Reedy model structures. This is discussed in Section 5.2 of Hovey's <em>Model Categories</em> and will work with both Quillen's and Strøm's model structures. (In fact it... | https://mathoverflow.net |
42,423 | [
"https://quant.stackexchange.com/questions/42423",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/35631/"
] | How is the interest and principal payment calculated when mortgage has already missed one payment? Are the new payments calculated off the new balance (non-amortized balance) or the original scheduled balance?
For example: 100k mortgage, 360 terms, 5% nominal rate, monthly payments, country: Mortgage
<pre><code>Perio... | US specific correct answer is (a). Interest is not compounded on missed interested. If you get late, you pay the scheduled payments and get back on track. Ofcourse, late fees await you but there is no interest charged on receiving late interest. Usually, late fees will be way more than that interest.
| In the case of failure to pay a fee, <strong>the problem is</strong> not financial calculation, but <strong>compliance with the obligations of the loan contract</strong>. Therefore, the viable option is the one you designate as 'a)', since the debtor pays the delayed installment, the late payment surcharge and the new ... | https://quant.stackexchange.com |
140,374 | [
"https://softwareengineering.stackexchange.com/questions/140374",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/38762/"
] | Due to the inherent risks associated with using dynamic languages (e.g. Python, Ruby) carelessly, what (if any) standards should be imposed on production code written in these languages?
| Your question contains two contradictory elements:
<ol>
<li>that someone would use the language carelessly, and </li>
<li>that they would follow standards for the production code. </li>
</ol>
Any programmer that is part of a team that has standards for production code should not use a language, dynamic or not, care... | Coding standards should be a simple as possible and hopefully restricted to avoiding language features and idioms which are known to be problematic or broken.
Too many rules in program standards can lead programmers to code sub-optimal and unreadable code because the best solution has been declared "bad" in the standa... | https://softwareengineering.stackexchange.com |
280,143 | [
"https://mathoverflow.net/questions/280143",
"https://mathoverflow.net",
"https://mathoverflow.net/users/42571/"
] | Let $f:A\rightarrow B, g:B\rightarrow C$ be maps in the category CGWH (compactly generated weakly Hausdorff spaces). Do we have a homotopy fibration sequence
$$F(f)\rightarrow F(gf) \rightarrow F(g)$$
consisting of homotopy fibres? (And the dual statement for homotopy cofibres?)
| This is a fundamental and basic property in homotopy theory, as is the dual statement for homotopy cofibers. These appear as Lemmas 1.2.7 and 1.2.5 in the recently published book <em>More concise algebraic topology</em> by May and Ponto. (And in stable model categories these merge and become the octahedral axiom.)
| $\require{AMScd}$
Here are some details. Without loss in generality, we can assume $f$ and $g$ are fibrations. I am assuming that all spaces are based. We may assume that $F_f$ now refers to the actual fiber of $f$ (which is equivalent to its homotopy fiber).
Start with the commutative diagram
$$
\begin{CD}
F_f @>&... | https://mathoverflow.net |
415,191 | [
"https://math.stackexchange.com/questions/415191",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/81556/"
] | A triangle is made up of three points, <span class="math-container">$A, B$</span>, and <span class="math-container">$P$</span>.
<blockquote>
<span class="math-container">$A(-1, 0)$</span>
<span class="math-container">$B(0, 1)$</span>
<span class="math-container">$P$</span> is a point on <span class="math-container">$y^... | By Lagrange Theorem, any element has order $1,5,11$ or $55$.
<strong>Case 1</strong> There is an element of order $55$, then group is cyclic (thus isomorphic to $\mathbb{Z}/55\mathbb{Z})$ , and it is easy to check that there are exactly 10 elements of order 11.
<strong>Case 2</strong> There is no element of order $55... | Let <span class="math-container">$G$</span> be a group of order 55.
Suppose <span class="math-container">$G$</span> contains exactly <span class="math-container">$20$</span> elements of order <span class="math-container">$11$</span>.
Suppose <span class="math-container">$a \in G$</span> and <span class="math-container... | https://math.stackexchange.com |
657,598 | [
"https://physics.stackexchange.com/questions/657598",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/306951/"
] | I've found two different meanings of precision:
Meaning 1 (from Grade 11 Physics textbook,India):
<blockquote>
Precision tells us to what resolution or limit a quantity is measured.
</blockquote>
Meaning 2 (from Grade 11 Chemistry Textbook,India):
<blockquote>
Precision refers to the closeness of various measurements f... | The first definition refers to the precision of a single measurement, whereas the second refers to the precision of a group of measurements.
Suppose I have a digital stopwatch that measures time in seconds with three decimal places. If I make one measurement of the period of a pendulum I will get a value that has a pre... | Not at all. Those definitions of precision do not have opposite meanings, just different. The first one refers to the number of digits a given measured quantity can have (its resolution). For example, the speed of light in vacuum is measured to be:
<span class="math-container">$$ c = 299\,792\,458 \rm\, m/s $$</span>
W... | https://physics.stackexchange.com |
425,853 | [
"https://electronics.stackexchange.com/questions/425853",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/105450/"
] | Proportional valves are drived by current because we need a steady force and since resistance changes with temperature if they were driven by voltage this force wouldn't be steady.
Ok, this is what I read. What I don't understand is how can a device be driven only by current and not by voltage if voltage is the cause ... | A voltage regulator allows the load to take the current it wants, while forcing the terminal voltage to remain constant.
A current regulator allows the load to determine the terminal voltage it wants, while forcing the current to remain constant.
Either can be implemented with a mainly voltage-output stage (like an e... | "Controlled by current" does not mean that no voltage is required (although that might be true in some specialized situations), it means that current is what determines the action of the valve. So you should adjust the voltage in order to maintain a certain current if you want the valve "position" to be relatively stab... | https://electronics.stackexchange.com |
998,667 | [
"https://math.stackexchange.com/questions/998667",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/146131/"
] | Decompose the following polynomial:
$A=x^4(y^2+z^2) + y^4(z^2+x^2) + z^4(x^2+y^2) +2x^2 y^2 z^2 $
(by the way I'm not quite sure about the tags and the title feel free to edit them if you wish)
| $$
A = x^4(y^2+z^2) + x^2(y^4+2y^2 z^2 +z^4) +y^2z^2(y^2+z^2) \\
= (y^2+z^2)\left(x^4+(y^2+z^2)x^2 +y^2z^2\right) \\
=(y^2+z^2)(x^2+y^2)(z^2+x^2)
$$
| Firstly, let $x^2=a$, let $y^2=b$ and let $z^2=c$ then we have $a^2b+a^2c+b^2a+b^2c+c^2a+c^2b+2abc$. I'm afraid that I just recognise this, just like how I recognise the difference of two squares, as:
$$(a+b)(b+c)(a+c)$$
| https://math.stackexchange.com |
331,179 | [
"https://softwareengineering.stackexchange.com/questions/331179",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/246307/"
] | I couldn't find an answer, so let's try here...
I thought a bit about security and came to the conclusion, that the used hash algorithm should be as slow as possible, to slow potential attackers.
(Slow means like ~1.5s).
I don't want to use multiple hash function or the same just a few times again, I just want a slow o... | These hashes are sometimes called "password hashes", because they are designed to protect against exactly the threat model you mention: someone getting a hold of a copy of your password database and brute-forcing it. A subset is also known as "password-based key-derivation functions (PBKDF)".
Scrypt is a relatively ne... | This is what functons such as scrypt, bcrypt, and pbkdf2 are designed for. They are resource intensive and combine multiple iterations together. The number of iterations is a parameter that can be adjusted to make the algorithm as slow as you want.
| https://softwareengineering.stackexchange.com |
75,978 | [
"https://mathoverflow.net/questions/75978",
"https://mathoverflow.net",
"https://mathoverflow.net/users/10271/"
] | Let $M$ and $N$ be two real square matrices of size $p+q$. The matrix $M$ is nonsingular. The matrix $N$ has the following block structure, where $A$ is a $q{\times}p$ matrix. $N = \left(\begin{array}{cc} \text{O}_{p,p} & \text{O}_{p,q} \\ A & \text{Id}_{q} \end{array}\right)$. I would like to conclude that the... | Take $p=q=1$, let $M = \begin{pmatrix} 0 & 1 \newline 1 & 0 \end{pmatrix}$ and let $N = \begin{pmatrix} 0 & 0 \newline 0 & 1 \end{pmatrix}$. Then the algebraic multiplicity of the zero eigenvalue in $MN = \begin{pmatrix} 0 & 1 \newline 0 & 0 \end{pmatrix}$ is $2$.
| It's not true. Consider the case ($p = 2,\ q=1$) $N = \pmatrix{0 & 0 & 0\cr 0 & 0 & 0\cr
a & b & 1\cr}$. The characteristic polynomial of $MN$ is ${t}^{3}- \left( am_{{13}}+bm_{{23}}+m_{{33}} \right) {t}^{2}$, and $a m_{13} + b m_{23} + m_{33}$ could easily be 0, resulting in an algebraic mu... | https://mathoverflow.net |
341,484 | [
"https://softwareengineering.stackexchange.com/questions/341484",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/132665/"
] | Progressive enhancement usually refers to web content, and the idea of having a page that is readable before it's fully functional.
I'm suddenly preoccupied with the idea that I can't progressively enhance my code. I want to be able to add methods to my values. I might have a string property. If I want to enhance it w... | You can add new methods to all classes which you control, and if you don't control a class, you can provide a subclass which support the new methods, and this will be transparent to any caller.
So the issue you describe is only an issue if you want to extend a class which you <em>don't</em> control and which is sealed... | Procedural languages deal with this very simply. When you want more functionality, you just add an <code>isValid(String)</code> function. The callers that didn't know about this function don't use it. The <code>isValid</code> function doesn't have to be physically located together with other <code>String</code> func... | https://softwareengineering.stackexchange.com |
44,407 | [
"https://physics.stackexchange.com/questions/44407",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/5841/"
] | I am trying to design a system for a project and I need some input on if this is right or not.
I have $10 \frac{kg}{s}$ of fluid flowing through a pipe. It breaks up into $10$ 1 inch diameter pipes. I am giving a picture below. Does this mean that the feed pipe, the one supplying the $10 \frac{kg}{s}$, should be just ... | I assume your objective is to minimize resistance to flow, in which case the dominating influence would be the length of the 1-inch pipes and the shape of their entry points where they enter the plenum.
If you want the fluid velocity at the entrance to the plenum to be the same as it is in the 1-inch pipes, then the p... | To conserve momentum of the fluid, it's the cross sectional area that should be conserved. The area is in a square relationship to the diameter, so simply multiplying the diameter of the small pipes by 10 won't be correct.
Here's how I figure the calculation should go:
$A_{in} = A_{out}$
$\pi(D_{in}/2)^2 = \pi(D_{o... | https://physics.stackexchange.com |
48,372 | [
"https://dba.stackexchange.com/questions/48372",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/27259/"
] | I have a VM with IP address 192.168.0.192 running postgreSQL.
If I specify
<pre><code>listen_addresses = '*'
</code></pre>
then I can connect from another VM at 192.168.0.191 and from localhost.
But I can't seem to <strong>use a list to tell</strong> postgreSQL to use those two addresses. If I change listen_addres... | Yes, <code>listen_addresses</code> can be set to a list of addresses <em>on the local host</em> to bind to for listening.
In your example:
<blockquote>
listen_addresses = '192.168.0.191, localhost'
</blockquote>
If the local machine has IP <code>192.168.0.192</code>, you should specify that IP, not the remote host... | I've found that instead of using <code>localhost</code> it needs to be <code>127.0.0.1</code> if you're specifying any other addresses as well.
So in my case of listening on the Docker host IP address as well as localhost, but not the external IP, this doesn't work (I get a connection refused from inside my Docker con... | https://dba.stackexchange.com |
527,799 | [
"https://math.stackexchange.com/questions/527799",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/38011/"
] | In Hatcher's Algebraic Topology section 1.3, Cayley complexes are explained. The book states that we get a Cayley complex out of a Cayley graph by attaching a 2-cell to each loop. There is an example showing the Cayley complex for $\mathbb{Z}\times\mathbb{Z}$ (the fundamental group of the torus). We attach one 2-cell t... | This is essentially the same answer as user32240 but I will try to explain it differently. Hatcher's description is a bit sloppy. The correct thing to say is that if $R$ is the set of defining relators for $G$, then each element $r\in R$ labels a loop based at every vertex of the Cayley graph. To each of these based ... | The Cayley complex of $\mathbb{Z}/n\mathbb{Z}$ contains $n$ vertices and $n$ edges connected in a circle. So if you want to attach a disk, you have $n$ choices because $(123),(231),(312)$ gives three different attaching disks. For example in the $\mathbb{R}\mathbb{P}^{2}$ case, you glue a disk along the relation $(12)$... | https://math.stackexchange.com |
416,464 | [
"https://electronics.stackexchange.com/questions/416464",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/89348/"
] | I have run some verilog simulations in questa simulator and while viewing the waveforms i see that it would have been easier for me to debug the signals had there been some enums defined for them (To look at strings instead of bit streams).
Is there a way i can add enum definition for signals without re-running the si... | You want a user defined radix. For example
<pre><code>radix define States {
11'b00000000001 "IDLE",
11'b00000000010 "CTRL",
11'b00000000100 "WT_WD_1",
11'b00000001000 "WT_WD_2",
11'b00000010000 "WT_BLK_1",
11'b00000100000 "WT_BLK_2",
11'b00001000000 "WT_BLK_3",
11'b00010000000 "WT_BLK_4",
11'b00100000000 "WT_BLK_5",
1... | Search on "virtual signals" in the documentation. you can split/join signals into a bus and create names for the new bus. Then if you save the waveform, it will still be there when you load it next time.
| https://electronics.stackexchange.com |
372,780 | [
"https://softwareengineering.stackexchange.com/questions/372780",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/308444/"
] | Let us say I am working on a project with maybe 100 nuget packages installed and 50 of these packages require an update.
I would ike to get a clear overview on which package updates cause any problems. I was thinking of something like this but any other ideas are welcome:
<ol>
<li>Foreach nuget package installed</li... | Package authors should be respecting compatibility with the proper use of the package version. What you're intending to do sounds like it would be a lot of work if at all possible. Your project using the NuGet packages shouldn't "assume" that the latest and greatest version of the packages is going to make <em>your pro... | I would start small and simple.
<ol>
<li>Create a branch with CI</li>
<li>Update all packages</li>
<li>Run CI with all tests and stuff</li>
<li>If everything passes, merge branch into master</li>
<li>If something fails, update the packages manually</li>
</ol>
From my experience with NuGet, backwards compatibility is ... | https://softwareengineering.stackexchange.com |
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