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
values |
|---|---|---|---|---|---|
45,653 | [
"https://physics.stackexchange.com/questions/45653",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/16458/"
] | Given Newton's third law, why is there motion at all? Should not all forces even themselves out, so nothing moves at all?
When I push a table using my finger, the table applies the same force onto my finger like my finger does on the table just with an opposing direction, nothing happens except that I feel the opposing... | I think it's a great question, and enjoyed it very much when I grappled with it myself.
Here's a picture of some of the forces in this scenario.$^\dagger$ The ones that are the same colour as each other are pairs of equal magnitude, opposite direction forces from Newton's third law. (W and R are of equal magnitude in... | I had similar problem in understanding the 3rd law. I found the answer myself while sitting in my study chair which has wheels!
sitting in the chair, I folded my legs up so that they are not in touch with ground. Now I pushed the wall with my hands. Of course, wall didn't move but my chair and I moved backward! why?... | https://physics.stackexchange.com |
430,980 | [
"https://stats.stackexchange.com/questions/430980",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/21006/"
] | I'm trying to figure out why adding a cubic term in the model doesn't guarantee a perfect multicollinearity. If <span class="math-container">$X$</span> is known, then <span class="math-container">$X^3$</span> is known in both magnitude and sign and vice versa. It may not be the case between <span class="math-container"... | Multicollinearity refers to the situation in which the regressor matrix <span class="math-container">$Z$</span> does not have full column rank <span class="math-container">$k$</span>.
This is the case if it is possible to <em>linearly</em> combine the columns <span class="math-container">$z_1,\ldots,z_k$</span> into t... | You often will get multicollinnearity issue with cubes but not the <em>perfect</em> kind. In your case a perfect multicollinnearity can be defined as: <span class="math-container">$\alpha x+x^3=c$</span>.
This is not true <em>by definition</em>. However, when <span class="math-container">$x<<1$</span> you get <s... | https://stats.stackexchange.com |
4,972 | [
"https://stats.stackexchange.com/questions/4972",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/2164/"
] | I am a novice in statistics so please correct me if I am doing something fundamentally wrong. After wrestling for a long time with R in trying to fit my data to a good distribution, I figured out that it fits the Cauchy distribution with the following parameters:
<pre><code> location scale
37.029894 18.6... | First off, your response variable is discrete. The Cauchy distribution is continuous. Second, your response variable is non-negative. The Cauchy distribution with the parameters you specified puts about 1/5 of its mass on negative values. Whatever you have been reading about the QQ norm plot is false. Points falling cl... | Agree with HairyBeast (+1) that Cauchy is not appropriate here (it's symmetric for one thing) and that negative binomial might well be better.
Disagree about QQ-plot though. You can do a QQ-plot for any distribution, not just normal. What you say about interpretation of a QQ-plot is correct, but note that 2 of your po... | https://stats.stackexchange.com |
66,874 | [
"https://chemistry.stackexchange.com/questions/66874",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/29385/"
] | I know that depending on the amino-acid-composition of the peptide the distance to the isoelectric point determines the charge. Is there a universal point at which all proteins are definitely negatively charged?
| If functional group carbon is part of principal chain then metamers are also positional isomers. In case of polyvalent functional group like sulphoxide($\ce{>S=O}$), secondary amine ($\ce{-NH-}$) etc. structure formed by change in structure of alkyl groups will be metamers only. For example, diethyl amine ($\ce{Et2... | Actually metamerism comes under positional isomerism. It's like this "metamerism is a subset of positional isomerism". When a compound has a functional group as a part of its chain then we call positional isomerism as metamerism to specify that the functional group is a part of the chain.
| https://chemistry.stackexchange.com |
153,673 | [
"https://mathoverflow.net/questions/153673",
"https://mathoverflow.net",
"https://mathoverflow.net/users/4054/"
] | One of the standard conjectures in algebraic geometry is that an operator $\Lambda$ on the cohomology algebra of a projective variety is algebraic. To my lying eyes it looks like there are two definitions of the operator $\Lambda$ out there and that the two don't agree.
On the algebraic side, let $X$ be a projective v... | In addition to Kleiman's "Algebraic cycles and the Weil conjectures" mentioned by abx, I can highly recommend reading the first article in the Motives proceedings (Jannsen/Kleiman/Serre) by Kleiman: "The standard conjectures".
It introduces several $\Lambda$ and $\star$ operators, and shows that when one is algebraic,... | I wonder where you have found this definition of $\Lambda_a $. My standard (!) reference for the standard conjectures is Kleiman's "Algebraic cycles and the Weil conjectures", in "10 exposés sur la cohomologie des schémas". Here they define 2 closely related operators, named $\Lambda $ and ${}^c\Lambda$; both are defin... | https://mathoverflow.net |
106,288 | [
"https://electronics.stackexchange.com/questions/106288",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/28592/"
] | I have an application where I'll be receiving an audio stream, much like using headphones, into my processor. I would like to run FFT and various DSP algorithms on the sampled audio and then spit the audio back out as if the device wasn't there. Basically doing DSP in real-time in between samples I guess. The processor... | FFT needs to sample at at least twice the frequency of your signal, and should not introduce any loss. Loss is introduced on compression or encoding. Your input and outputs should be able to match the frequency, amplitude, and frequencies of the signal you are sampling.
Your biggest challenge is processing the DSP alg... | Unfortunately, you will have to accept losses in your signal if you try to reconstruct them from the samples. Unless you make certain strong assumptions about the audio signal the Nyquist-Shannon sampling theorem doesn't help. A general raw audio signal will not satisfy these conditions.
If you take \$K\$ samples at... | https://electronics.stackexchange.com |
11,134 | [
"https://dba.stackexchange.com/questions/11134",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/6023/"
] | i have a table with 12 months+ data in it.
I backup the table then delete all rows keeping only rows inserted in the last month.
But the table size won't shrink, nor the underlying physical file.
<pre><code>dbcc showcontig ('mytable') with tableresults;
ObjectName ObjectId IndexName IndexId Level Pages Rows... | Try <code>ALTER INDEX ALL ON mytable REBUILD</code>
With the index size value being so high, this points to a non-clustered index. Clustered index size is very small usually.
You can check with the <code>all_indexes</code> parameter when using the DBCC SHOWCONTIG command:
<pre><code>dbcc showcontig ('Risk') with all_in... | You can shrink that database files through the Enterprise Manager. Right click on the database, All Tasks->Shrink Database
| https://dba.stackexchange.com |
22,658 | [
"https://dsp.stackexchange.com/questions/22658",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/14872/"
] | We know that Fourier transform $F(\omega)$ of function $f(t)$ is summation from $-\infty$ to $+\infty$ product of $f(t)$ and $e^{-j \omega t}$:
$$ F(\omega) = \int\limits_{-\infty}^{+\infty} f(t) \ e^{-j \omega t} \ dt $$
Here, what does the exponential term mean?
| It's a complex exponential that rotates forever on the complex plane unit circle:
<span class="math-container">$$e^{-j\omega t} = \cos(\omega t) - j \sin(\omega t).$$</span>
You can think of Fourier transform as calculating correlation between <span class="math-container">$f(t)$</span> and a complex exponential of each... | If you don't like thinking about imaginary numbers, complex numbers and functions, you can alternatively think of the complex exponential in the FT as just shorthand for mashing together both a sinewave and a cosine wave (of the same frequency) into a single function that requires less chalk on the chalkboard to write.... | https://dsp.stackexchange.com |
232,021 | [
"https://security.stackexchange.com/questions/232021",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/235148/"
] | I have a mobile application and the backend is hosted on a cloud provider. I would like to ask for feedback on encrypting all REST API calls that will be used to communicate with the server, if we should or we shouldn't do it.
<em>Adding details:</em>
< Certificate pinning is in place >
for example instead of hav... | Considering that TLS is in place with a solid configuration (i.e. certificate pinning), which I find no reason not to, you'd need to work out the business risk you're trying to mitigate by encrypting this information.
Ask yourself, what would you gain by doing this? What type of attack vectors are you mitigating?
How... | One of the points of TLS is to solve this problem - a secure way to transport data across untrusted networks, preferably using ephemeral keys for the actual encryption portion. Doing this yourself is possible but is coming dangerously close to "rolling your own" as you will have to solve problems like key storage, etc.... | https://security.stackexchange.com |
208,228 | [
"https://physics.stackexchange.com/questions/208228",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/-1/"
] | I am having some trouble reconciling the Minkowski (4-dimensional) and the pre-Minkowski (3+1-dimensional) approach to special relativity. Let me describe (how I interpret) the Lorentz transformations in these two approaches.
<strong>3+1 view</strong>
There exist two reference frames, $S(t,x,y,z)$ and $S'(t',x',y',z'... | This is merely the equivalence between passive and active transformations.
In your "pre-Minkowskian" case, you can consider the $S$ and $S'$ frames to actually be the same in every way, and it is the velocity vector that is being transformed. Since all the basis vectors are the same, there is no way to say that $S$ a... | I think the fact that $\vec{e}_{x'} = \vec{e}_x \cosh\chi + \vec{e}_t \sinh\chi \neq \vec{e}_x$ can be interpreted as a direct expression of relativity of simultaneity.
Based on our non-relativistic experience, when we say that $S$ observes $S'$ moving in the x direction, while $S'$ observes $S$ moving in the x' dire... | https://physics.stackexchange.com |
14,125 | [
"https://quant.stackexchange.com/questions/14125",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/1976/"
] | I'm working on a client memo explaining several approaches to equity hedging, and I'm looking for a not-too-technical term for a hedging strategy where I try to keep options near the money, as to have a quickly reacting hedge, expensive, but drastically reduced drawdown (hopefully).
Of course, the opposite would simpl... | These are all examples on Ito Formula in its general form (with quadratic variations):
<img src="https://i.stack.imgur.com/i2WOF.jpg" alt="enter image description here">
<img src="https://i.stack.imgur.com/WsfuN.jpg" alt="enter image description here">
<img src="https://i.stack.imgur.com/HDeBI.jpg" alt="enter image ... | I thought this was an interesting example to add. It concerns a "ratio model" of habit (as opposed to a "difference" model of habit). See, for example, Abel (1990, American Economic Review). Let
$$
x_t = \lambda \int_{-\infty}^t e^{-\lambda(t-s)} c_s ds.
$$
(For context, $x_t$ is a log habit index that is given by a ge... | https://quant.stackexchange.com |
2,051 | [
"https://math.stackexchange.com/questions/2051",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | Give an example of sequence of Measurable functions defined on some measurable subset $E$ of $\mathbb{R}$ such that $f_{n} \to f$ pointwise almost everywhere on $E$ but $$\int\limits_{E} f \ dm \not\leq \lim_{n} \inf \int\limits_{E} f_{n} \ dm$$
| Since Fatou’s Lemma <em>holds</em> for non-negative measurable functions, I suppose you are looking for an example involving not necessarily non-negative functions.
Take, for instance, $f_n(x)=-1_{n\leq x\leq n+1}$ (just $-1$ times the indicator function on the interval $[n,n+1]$). Then $f=\lim\limits_{n\to\infty}\inf... | For $E=[0,1]$ set: $f_n(x)=n- n^2x$ for $x\in [0,\frac1n]$ $f_n(x)=0$ else.
| https://math.stackexchange.com |
261,945 | [
"https://softwareengineering.stackexchange.com/questions/261945",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/148561/"
] | How are Creational patterns useful? Most of the material I came across explains the use case to "solve common problems associated with object creation, improve flexibility".
I can think of just two similar cases where these might help:
<ol>
<li>Encapsulating object creation code into a factory like class, so that al... | <strong>Why use creational patterns (in Java)?</strong>
The main reason for creational patterns is <em>to separate</em> the <em>creation</em> of an <em>instance</em> from its <em>consumption</em>.
That seems in the first place rather abstract, but has easy to see consequences.
1) You hide creational logic from the <... | The Singleton is classified as a creational pattern, so that's one obvious use: ensuring that everybody always communicates with the same instance. (The benefits of this are controversial, but I didn't say it was a <em>good</em> use. Just a rather obvious use.)
Factory methods spare an API user from knowing (and maint... | https://softwareengineering.stackexchange.com |
2,273,224 | [
"https://math.stackexchange.com/questions/2273224",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/324777/"
] | On the one hand, we may define $\mathcal{O}_{\mathbb{P}^{n}}(l)$ as the invertible sheaf with trivializing cover $\{D(X_i): i\in \{0,...,n\}\}$ and transition functions $\left(\frac{X_i}{X_j}\right)^l$.
On the other hand, we may define it as the sheaf of modules induced by the graded module $K[X_0,...,X_n](l)$.
I w... | I have to say I find it pretty confusing me too, especially when I was reading Hartshorne.
Let's compute global section of the sheaf $\mathcal O_{P^n}(l)$. Let $s$ be such a section, i.e a family of regular map $s_i : U_i \to \mathbb C$ such that $\phi_{ij} s_i = s_j$, where $\phi_{ij} = (\frac{x_i}{x_j})^l$. (Usuall... | For any sheaf of $\mathcal{O}_{\mathbb{P}^n}$ modules $\mathcal{F}$ on $\mathbb{P}^n$, let $\Gamma_*(\mathcal{F})= \bigoplus_{n\in\mathbb{Z}} \Gamma(\mathcal{F}\otimes\mathcal{O}(n))$. Now, let $\mathcal{F}$ be a quasicoherent sheaf of $\mathcal{O}_{\mathbb{P}^n}$ modules. Then there is a natural isomorphism $\beta:\wi... | https://math.stackexchange.com |
312,772 | [
"https://mathoverflow.net/questions/312772",
"https://mathoverflow.net",
"https://mathoverflow.net/users/38832/"
] | Let <span class="math-container">$\pi:X \to \mbox{Spec}(\mathbb{C}[t]/(t^2))$</span> be a smooth, projective morphism and <span class="math-container">$L$</span> be an invertible sheaf on <span class="math-container">$X$</span>. Denote by <span class="math-container">$L_0$</span> the restriction of <span class="math-co... | This follows easily from the theory of modules over <span class="math-container">$R = \mathbb C[t]/(t^2)$</span>. Indeed, we have a short exact sequence
<span class="math-container">$$0 \to H^0(\mathscr L_0) \to H^0(\mathscr L) \to H^0(\mathscr L_0) \to 0,$$</span>
induced by the observation that <span class="math-cont... | <span class="math-container">$\newcommand{\C}{\mathbb{C}}\newcommand{\D}{\C[t]/t^2}\newcommand{\im}{\mathrm{im}\,}$</span>A module <span class="math-container">$M$</span> over <span class="math-container">$\D$</span> is flat if and only if the inclusion <span class="math-container">$(t)\subset \D$</span> remains inject... | https://mathoverflow.net |
721,972 | [
"https://physics.stackexchange.com/questions/721972",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/342500/"
] | I'm new to this Q&A site, so sorry if my formulas don't look great!
I'm trying to create a function that shows the time on a clock B that's been thrown up from and caught in position A, with respect to the mass of the object it's on (the function would be TB(m)). According to a clock A in position A, clock B has be... | I think it is a bit messier than you would like. You cannot ignore the "Special Relativity part" because the time dilation caused by your launch speed will be of a similar order of magnitude to the gravitational effects you consider in your question.
You need to adopt a more "holistic" approach and ... | <blockquote>
I'm trying to create a function that shows the time on a clock B that's been thrown up from and caught in position A, with respect to the mass of the object it's on (the function would be TB(m)). According to a clock A in position A, clock B has been in the air for 10s, but due to GR, the time shown on clo... | https://physics.stackexchange.com |
681,762 | [
"https://physics.stackexchange.com/questions/681762",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/321540/"
] | Let's take for example hydroelectricity produced at a dam and not consumed by any household or industry... Will the electricity need to be stored? What will happen if it is not stored? Does it flow like water and get wasted back to the earth? What happens to it?
| In the power system, electricity must be consumed at the moment when it is produced. Otherwise, voltage and frequency deviations occur which could lead to a <em>power outage</em>. Imagine the transmission line is a capacitor: the voltage on the line is then defined as
<span class="math-container">$$v = \frac{1}{C} \int... | It is not practical to store the actual electricity. It can be stored, for example, in a battery as chemical energy, and then recovered at a later date as electrical energy. But this expensive and, in general, the electrical output power of a hydroelectric plant will be adjusted to closely match the load requirements. ... | https://physics.stackexchange.com |
2,376,421 | [
"https://math.stackexchange.com/questions/2376421",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/445310/"
] | I got till $(\sec x )(2\sec^2 x - 1) > 0$, but am not sure, how to proceed from here because I don't think I can simply divide out the $\sec x$ without changing the inequality.
I thought of splitting into two cases $\sec x > 0$ or $\sec x < 0$, but the answers I am getting are out of domain.
Any help will b... | Let $u = \sec(x)$. Then, you're trying to solve $u(2u^2-1)\geq 0$, which has solutions $$-\frac{1}{\sqrt{2}}\leq u\leq 0\text{ or } u\geq \frac{1}{\sqrt{2}}$$ As either $\sec(x)\leq -1 < -\frac{1}{\sqrt{2}}$ or $\sec(x)\geq 1 > \frac{1}{\sqrt{2}}$, the above inequality will be satisfied exactly when $\sec(x)$ is ... | $$\left(\frac{\sin{x}}{\cos^2x}\right)'=\frac{1+\sin^2x}{\cos^3x},$$ which gives $\left[0,\frac{\pi}{2}\right)$; $\left(\frac{3\pi}{2},2\pi\right]$.
If indeed, you want values of $x$ then $0<x<\frac{\pi}{2}$; $\frac{3\pi}{2}<x<2\pi$
| https://math.stackexchange.com |
206,395 | [
"https://physics.stackexchange.com/questions/206395",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/71328/"
] | I've read something about fluid interface and know that liquids tend to reduce their surface but my question is that why do they tend to reduce their surface?
| The average <em>vector</em> momentum of an electron bound to an atom is exactly zero. (Otherwise, the electron would leave the atom!)
The average <em>magnitude</em> of the momentum can't be zero, because of the uncertainty principle. So Feynman is using the approximation $\vec p = \vec 0 + \Delta p \hat p$, where th... | This might be a bit helpful.
The uncertainty in the momentum of an electron in an atom is defined as: <span class="math-container">$(\Delta P )^{2}= \langle P^{2}\rangle-\langle P\rangle^{2}$</span>. An electron bounded by the nucleus, has an average momentum of zero, which means (<span class="math-container">$\Delta... | https://physics.stackexchange.com |
199,615 | [
"https://dba.stackexchange.com/questions/199615",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/143246/"
] | We have Distributed AG across two windows clusters.
<ul>
<li>Clus01 - Has an AG set up with no DB inside it (AG1) </li>
<li>Clus02 - Has an AG set up with no DB inside it (AG2) </li>
<li>Clus01 - Has a DistAG set up with AG1 (primary) and AG2 (secondary)</li>
</ul>
We
<ul>
<li>Joined Clus02 to the DistAG</li>
<li>A... | <blockquote>
Is there anyone that knows how I can refresh an environment with DistAG without having to remove the whole DistAG and set it up again?
</blockquote>
Removing and re-creating is going to be your best bet if this is done with many databases... though I'd argue the need for a database in a Distributed Availai... | You can use the below statement to remove the database from "DistAG".
Execute the below statement on primary node of "DistAG".
<pre><code>ALTER DATABASE Genesis SET HADR OFF;
</code></pre>
Once you execute the above statement, the database state will turn to restoring mode, and then you can drop the database.
| https://dba.stackexchange.com |
12,823 | [
"https://security.stackexchange.com/questions/12823",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/8503/"
] | I was just implementing a new feature on the registration form of my site that checks if an email address is still available to register an account with. I am wondering if it is secure to show a user that a certain email address is no longer available.
Is this feature a security risk related to the privacy of my membe... | It does leak information on what email addresses have already been used, but this is only really an issue if an attacker can rapidly check through possible email addresses.
The simple solution is:
<ul>
<li>only allow a small number of attempts or requests from a particular IP in a session</li>
</ul>
This way a valid... | A way to implement the availability check without giving any feedback to potential attackers is to always send an email to the address used for the registration. What the content of the email could look like:
<ul>
<li>"An account for a@b.c [with username xy] was just registered at $site. Click here to confirm."</li>
<... | https://security.stackexchange.com |
140,238 | [
"https://cs.stackexchange.com/questions/140238",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/135413/"
] | How many languages can a Regular Expression describe is it only one or infinite?
I have tried to google it but i haven't found any answer?
I know that a Regular Expression describes a Regular Language?
| If you look at the definition of regular expressions (they are usually defined in parallel to the language denoted) you see there is only one language denoted by a given expression. That language could be empty, finite or infinite, but that is another question.
| Every regular expression describes one language --- the language of words matched by the regular expression, or, in other words, the language of words accepted by the finite automaton corresponding to the regular expression.
| https://cs.stackexchange.com |
762,625 | [
"https://math.stackexchange.com/questions/762625",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/142999/"
] | Can you give any idea regarding the evaluation of the following limit?
$\lim_{n\to \infty}\left(1 - \frac {1}{n^2}\right)^n$
We know that $\lim_{n\to \infty}\left(1 - \frac {1}{n}\right)^n = e^{-1}$, but how do I use that here?
| Here is a hint:
$\left(1 - \dfrac{1}{n^2}\right) = \left(1 - \dfrac{1}{n}\right)\left(1 + \dfrac{1}{n}\right)$
Also use the fact that $(ab)^n = a^n b^n$.
| Note that
$$\lim_{n\to\infty}\left(1-\frac{1}{n^2}\right)^{n^2}=\frac{1}{e}.$$
It follows that for large enough $n$, we have
$$\frac{1}{10}\lt\left(1-\frac{1}{n^2}\right)^{n^2}\lt 1.$$
Thus if $a_n$ is our expression, then for large enough $n$ we have
$$\left(\frac{1}{10}\right)^{1/n}\lt a_n\lt 1.$$
Now Squeeze.
| https://math.stackexchange.com |
417,621 | [
"https://mathoverflow.net/questions/417621",
"https://mathoverflow.net",
"https://mathoverflow.net/users/475366/"
] | Let <span class="math-container">$K \subset S^3$</span> be a slice knot. Then it bounds a smooth embedded disk <span class="math-container">$D \subset B^4$</span>. Let <span class="math-container">$S^3_{p/q}(K)$</span> denote a <span class="math-container">$3$</span>-manifold obtained by <span class="math-container">$p... | Yes, the generalisation is also true. This must be written somewhere, but I don't know where (any help from other users?), and finding such a statement is often hard.
So, here's the idea instead. Turn the surgery into an integral surgery, i.e. do 0-surgery on <span class="math-container">$K$</span> and <span class="mat... | There is a more general result that this follows from. This is in Gordon's paper cited above; see Theorem 4. In any event, you don't really need the handle arguments in Marco's answer.
Here is a sketch of Gordon's construction, in slightly different language. Of <span class="math-container">$K$</span> and <span class="... | https://mathoverflow.net |
54,554 | [
"https://security.stackexchange.com/questions/54554",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/36548/"
] | I was thinking about how security on my Linux server can be increased and came up with this question: If most intrusions are performed by gaining remote access to machines, would it be possible and sensible to restrict admin user or maybe even root login by a hardware device? Meaning that it wouldn't be possible to log... | In addition, I would add that most attackers don't directly log in via the console or ssh, but instead use bugs in other software in order to break out into a root shell or elevate their privileges. Adding 2-factor authentication or other mechanisms will definitely improve your security posture, but must be viewed as p... | This is called two-factor authentication. It already exists, though it may not be trivial to implement in all cases.
| https://security.stackexchange.com |
504,660 | [
"https://electronics.stackexchange.com/questions/504660",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/254897/"
] | I know, there's an overwhelming amount of information on the Internet about what eddy currents are, but on the fundamental front, I simply fail to grasp how currents could circulate/or even exist in a piece of metal. I always thought that in order for current to flow, conductors must be in different potentials. A solid... | <blockquote>
I can't imagine how current would flow within a block of conductor metal when all of the metal is a block.
</blockquote>
Imagine a loop of wire, surrounding a region of changing magnetic field. This loop of wire will have a voltage induced around it, and a current will flow through it.
If that region i... | <blockquote>
I always thought that in order for current to flow, conductors must be in different potentials.
</blockquote>
This rule depends on a couple of simplifying assumptions that taken together are called the "lumped circuit approximation".
One of these assumptions is that there are no significant changing m... | https://electronics.stackexchange.com |
41,717 | [
"https://datascience.stackexchange.com/questions/41717",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/63254/"
] | Working on a school project where we have to match some users based on common interests. Assuming I have a list of inputs like this:
<ul>
<li>Name Interest<strong>1</strong> Interest<strong>2</strong> Interest<strong>3</strong> Interest<strong>4</strong> Interest<strong>5</strong></li>
<li>Name Interest<strong>4</stro... | Yes, it will impact because when you change the loss function, the numerical value of the loss function will change. So, this will change gradient values of the parameters during the back propagation. Therefore, change in loss function will impact the parameters.
| If you change the objective function, the optimal solution to the objective function is likely to change though sometimes they might ended up to be the same.
If the optimal solution remains the same, we wouldn't have to have differen objective function. MSE pays more attention to large error and MAE tend to give spars... | https://datascience.stackexchange.com |
251,357 | [
"https://dba.stackexchange.com/questions/251357",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/193257/"
] | I have 3 tables <code>Student</code>, <code>Teacher</code> & <code>User</code>.
Student:
<pre><code>CREATE TABLE Student( id INT NOT NULL PRIMARY KEY,name VARCHAR(50) NOT NULL);
INSERT INTO [dbo].[Student]([id],[name]) VALUES(4,'Ram'),(5,'Raman');
</code></pre>
Teacher:
<pre><code>CREATE TABLE Teacher( id INT N... | By defining a foreign key (FK) you are saying that the value I place in this table (the "child" or "referencing" table) must have a matching value already in the other table (the "parent" or "referenced" table). By defining User.user_id as a foreign key referencing both Student.id and also Teacher.id you are saying tha... | Something I would be worried about with this approach is what will keep me from having a Student_id of 27 for Kevin, and a Teacher_ID of 27 for Mr. Jones? If you absolutely want to avoid this you could use a single base table for Person, then everyone get's their own number.
In keeping the current plan, I would pull t... | https://dba.stackexchange.com |
8,685 | [
"https://dsp.stackexchange.com/questions/8685",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/709/"
] | I'm learning DSP slowly and trying to wrap my head around some terminology:
<ul>
<li><strong>Question 1</strong>: Suppose I have the following filter difference equation:
$$y[n] = 2 x[n] + 4 x[n-2] + 6 x[n-3] + 8 x[n-4]$$
There are 4 coefficients on the right-hand side. Are the "number of taps" also 4? Is the "filte... | OK, I'll try to answer your questions:
Q1: the number of taps is not equal the to the filter order. In your example the filter length is 5, i.e. the filter extends over 5 input samples [$x(n), x(n-1), x(n-2), x(n-3), x(n-4)$]. The number of taps is the same as the filter length. In your case you have one tap equal to ... | <ul>
<li>Question 1: The number of taps = number of coefficient s = Length of
filter in case of FIR filter. The order of the filter is equal to
Length of filter-1.</li>
<li>Question 2: $n$ should be set to 9 if you are using FIR filter.</li>
<li>Question 3: This is an IIR filter since you have a feed back in it.
Try to... | https://dsp.stackexchange.com |
35,946 | [
"https://softwareengineering.stackexchange.com/questions/35946",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/12929/"
] | Is it a code smell to call public method in private method of the same object instance?
| No not bad smell. This might be needed, why do you suspect it to be wrong ? A method at an atomic level is an independent entity that does a task. As long as it does a task anyone who has access to it can call it to get the task done.
| Code smell? Yes, not a really bad one, but a good indicator that the class may have too many responsibilities.
Take it as a sign that the class may need to be broken up into different objects, private methods shouldn't really need to call public methods of the same object, certainly in a clean OO design.
Of course, o... | https://softwareengineering.stackexchange.com |
10,826 | [
"https://math.stackexchange.com/questions/10826",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/3317/"
] | I have $n$ random elements from 0 to $k$. What is the probability that there is some element which appears more than $n/2$ times (or $n/2$ times).
| This is related to the problem of finding the integral of a normal distribution, in short this means that there is no simple formula that can solve your problem, you will have to do it as a sum of probabilities for a number of simpler cases.
The probability of a specific element appearing exactly $m$ times in a group ... | If we interpret the $n$ random elements from $0$ to $k$ to mean that each different selection is equally likely, then this question does have a very nice closed form answer: see (1) below.
To be clear, say we are selecting two objects from the set $ \lbrace 0,1 \rbrace $ then by each selection being equally likely we ... | https://math.stackexchange.com |
19,891 | [
"https://stats.stackexchange.com/questions/19891",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/6096/"
] | I am doing a simple marketing project that has the following types of variables:
<ul>
<li>X1 - continuous (e.g. income)</li>
<li>X2 - categorical (e.g. gender)</li>
<li>Y - continuous (e.g. number of a product type purchased such as tubs of ice-cream) </li>
</ul>
I am interested in the relationship between income (X1... | In general, you should not base your model selection solely on statistical significance. Substantive meaning is more important.
In this particular case, you can graph the predicted values for males and females, with the x-axis being income and the y-axis the number of items bought, and a line for each gender.
@gung m... | Your results suggest that there is no interaction--you simply have a main effect of X1. You could say something like, "The number of tubs of ice-cream people buy is related to their income. For instance, if person A's income is one unit higher than person B's income, person A typically buys $\beta_1$ more tubs of ice... | https://stats.stackexchange.com |
170,013 | [
"https://physics.stackexchange.com/questions/170013",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/23756/"
] | I am not a physicist (math/comp-sci) but I understand that Angular Momentum is supposed to be conserved. I find this confusing because there seems to be many simple, common cases where a restrained, revolving body, when released seems to lose it's angular momentum.
For example, let's say that a hammer thrower is revo... | <blockquote>
Now, after the hammer is released, the thrower still has her same angular momentum (and has to slow herself down), but the hammer no longer seems to have any.
</blockquote>
Even though the hammer isn't rotating around the axis, it still has the same angular momentum it had at release <em>with respect to... | Let us see a similar example: two people on skates going with some velocity towards each other both a bit on left off their common center, and in the moment of the closest approach, they just catch each other by right arms and they start to rotate.
In fact they have (as one system) the same angular momentum all the ti... | https://physics.stackexchange.com |
329,674 | [
"https://softwareengineering.stackexchange.com/questions/329674",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/11571/"
] | I create page to import records so users can import records using a CSV file.
I am already using Sanitization to remove unexpected characters for specific field of the record, Do I have to validate each record in csv file before insert it into Database then display the errors to user or just using sanitization only ca... | If you don't know an application or a feature in an application well enough to basically use it, you need to learn this first, that is an obvious prerequisite of any testing. There are two ways to do this: RTFM, or let someone explain it to you (both ways not mutual exclusive). The person who explains it to you might b... | In my sense, the best way to get an application tested, is to put someone who doesn't know the project at all; especially how it works. Here some points for why:
<ul>
<li>If the project has a user guide/manual, just following that in order to help you to do all of the tasks asked. This will show that the manual is/n't... | https://softwareengineering.stackexchange.com |
47,519 | [
"https://physics.stackexchange.com/questions/47519",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/17148/"
] | The example I'm trying to understand is:
$ \hat{S}_{x} \begin{pmatrix}
\frac{1}{\sqrt{2}}\\
\frac{1}{\sqrt{2}}
\end{pmatrix} = 1/2 \begin{pmatrix}
\frac{1}{\sqrt{2}}\\
\frac{1}{\sqrt{2}}
\end{pmatrix} $
My interpretation of this is that the vector shows you the probabilities of a particle being spin up or spin down i... | Your equation says that your "vector" is an an eigenvector of your operator, i.e., that the x-projection of the spin is certain an equal to 1/2. As well it says that probabilities to find certain z-projections are equal to 1/2.
This "vector" is not an eigenvector $\begin{pmatrix}
0\\
1
\end{pmatrix}$ or $\begin{pmatri... | $$ \frac{1}{\sqrt{2}} \begin{pmatrix}
1\\
1
\end{pmatrix} $$ is the eigenvector of $\hat{S}_x=\frac{\hbar}{2}\sigma_x$ with eigenvalue $+\frac{\hbar}{2}$. $$ \frac{1}{\sqrt{2}} \begin{pmatrix}
1\\
-1
\end{pmatrix} $$ is the eigenvector of $\hat{S}_x=\frac{\hbar}{2}\sigma_x$ with eigenvalue $-\frac{\hbar}{2}$.
So ... | https://physics.stackexchange.com |
918,859 | [
"https://math.stackexchange.com/questions/918859",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/135279/"
] | $\large\zeta_7\left(\zeta_3\right)^5$ where $\large\zeta_n=\cos{\frac{2\pi}{n}}+i\sin{\frac{2\pi}{n}}$
I am having trouble getting a final answer that makes sense to me. Here is what I tried:
$\large\zeta_7\left(\zeta_3\right)^5=\left(\large\zeta_{21}\right)^{3}\left(\zeta_{21}\right)^{35}=\left(\zeta_{21}\right)^{38... | Euler's formula makes this straightforward: $$\zeta_n = \cos \frac{2\pi}{n} + i \sin \frac{2\pi}{n} = e^{2\pi i/n}.$$ Consequently, $$\zeta_7 \zeta_3^5 = e^{2 \pi i/7} (e^{2 \pi i/3})^5 = e^{2\pi i/7 + 10 \pi i/3} = e^{76\pi i/21} = e^{34 \pi i/21} e^{2\pi i} = \cos \frac{34\pi}{21} + i \sin \frac{34\pi}{21},$$ whe... | For this you need to know that $$(A \text{ cis } B) \cdot (C \text{ cis } D) = (AC) \text{ cis } (B + D)$$
And that
$$(A \text{ cis } B)^n = \left(A^n \text{ cis } Bn\right)$$
Your problem is
$$\large\zeta_7\left(\zeta_3\right)^5$$
$$\left(1 \text{ cis } \frac{2\pi}{7}\right)\left(1 \text{ cis } \frac{2\pi}{3}\righ... | https://math.stackexchange.com |
10,454 | [
"https://bioinformatics.stackexchange.com/questions/10454",
"https://bioinformatics.stackexchange.com",
"https://bioinformatics.stackexchange.com/users/4424/"
] | In <code>f1</code> below I am trying to split <code>$1</code> based on each line and create a bed file. If the line is a snp then the <code>:</code> is spilt and the text is <code>$1</code> the last digit is <code>$2</code> -1 and <code>$3</code> and the letter to the left of the <code>></code> is <code>$4</code> an... | The following Python script seems to do the job.
<pre class="lang-py prettyprint-override"><code>#!/usr/bin/env python3
from __future__ import print_function
import re
import sys
for line in sys.stdin:
if '_' in line:
match = re.search(r'(\S+):(\d+)_(\d+)del([ACGT]+)ins([ACGT]+)', line)
print(mat... | Here is an <code>R</code> solution, would probably be slower than the suggested <code>Python</code> solution though:
<pre><code>bed <- readLines("bed.txt")
res <- data.frame()
for(i in bed) {
temp <- strsplit(i, ":") %>% unlist()
# case N>N
if(grepl(">", temp[[2]])) {
begin <- str_... | https://bioinformatics.stackexchange.com |
252,469 | [
"https://softwareengineering.stackexchange.com/questions/252469",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/145355/"
] | I'm creating an MVC application in VS2012 and I reverse-engineered the models from existing Oracle tables. The tables are part of a MASSIVE database. Besides the main tables that I need to insert, update and delete from I also (on a whim) selected a few tables and views that my application only needs to read from. Now ... | Assuming the CLI is a wrapper that parses the command-line arguments, and passes them to f() and does something with the results, the unit test for the CLI would be that it properly parses the command-line arguments and marshals the function arguments for f(), and that the results are properly 'used' (however that is d... | <blockquote>
How should I test the CLI?
</blockquote>
In general, I leave testing the user interface to the user or some stand-in/advocate for the user (QA people). If there are utility functions to say... parse command line arguments, then I would unit test those.
If the wrapping function is not a user interface, ... | https://softwareengineering.stackexchange.com |
39,877 | [
"https://quant.stackexchange.com/questions/39877",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/34304/"
] | I am new to forex trading and using an Oanda free practice account to play with it. However, I'm not understanding a few things.
<ol>
<li>When I buy or sell a currency pair, am I buying/selling a currency or am I buying a security to represent my trade?</li>
<li>When I open and close positions on Oanda, they appear t... | The interpolation of the implied vol surface is no easy task unfortunately and it is subject of extensive research.
This is because you want the vol surface to have some nice characteristics, e.g.: be smooth, non arbitrable, etc.
Two approaches exist:
<ul>
<li>Assuming a parametric form for the volatility surface an... | I would say the best solution will be curve fitting, which is definitely not an easy solution.
Depends on whether you are fitting interest rates or stocks/commodities. You might try <strong>SABR</strong> for the former and <strong>SVI</strong> for the letter. There are lots of papers about them that you should easily ... | https://quant.stackexchange.com |
172,959 | [
"https://mathoverflow.net/questions/172959",
"https://mathoverflow.net",
"https://mathoverflow.net/users/21815/"
] | Let $k$ be a finitely generated field, $\ell$ a prime different from the characteristic of $k$, $S$ a $k$-variety, and $\mathcal{V}$ a lisse $\ell$-adic sheaf on $S$. Fix an algebraic closure $\bar{k}$ of $k$. Let $\bar{S}$ (resp. $\bar{\mathcal{V}}$) denote the pull-back of $S$ (resp. $\mathcal{V}$) to $\bar{k}$.
Eve... | I don't really know how to answer the question in a meaningful way, but let me make a few remarks about the parallel situation for Hodge theory. Suppose that
$S$ is a smooth complex algebraic variety, and that $V$ is a polarizable variation of Hodge structure over $S$ (e.g. $V= R^if_*\mathbb{Q}$ for a smooth projectiv... | If $V_n$ is a locally constant sheaf of $\mathbb{Z}/\ell^n\mathbb{Z}$-modules of finite rank over $S$ in the etale toplogy, then it is representable by a scheme $X_n$ which is affine over $S$. (Etale locally, it is representable by Spec of the product of finitely many copies of the structure sheaf with itself. Then app... | https://mathoverflow.net |
2,583,428 | [
"https://math.stackexchange.com/questions/2583428",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/257047/"
] | <blockquote>
Check to see if the function $f$ which satisfies the following relation is one-to-one or not:<br>
$$\forall x,y\in \Bbb R: f(x+f(y))=f(x)+y$$
</blockquote>
Please judge my solution:<br>
I set $y=0$ to get: $f(x+f(0))=f(x)$, now I deduce that this function is one to one if $f(0)=0$, otherwise, for ex... | If $f:\mathbb R\to \mathbb R$ satisfies $f(x+f(y))=f(x)+y$, $\forall x,y\in\mathbb R$, then I'll prove that $f$ is injective.
Let $P(x,y)$ be the statement $f(x+f(y))=f(x)+y$.
Let $f(0)=c$.
$P(-c,0)\implies c=f(-c)$
$P(-c,-c)\implies c=0$
$P(0,y)\implies f(f(y))=y$, $\forall y\in\mathbb R$
If $f(a)=f(b)$, then $f... | This is incorrect; rather, the conclusion is the converse of what you said. If $f(0)\neq 0$, then you are correct that $f$ will not be one-to-one. However, if $f(0)=0$, then you cannot conclude that $f$ is one-to-one (all you know is that one possible approach to showing it is not one-to-one fails).
| https://math.stackexchange.com |
41,778 | [
"https://electronics.stackexchange.com/questions/41778",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/2824/"
] | Yesterday I when I came home I found my soldering iron was still switched on from few hours before. I have one of those big aluminium holders you get with expensive soldering stations. I was wondering if it's possible to mount a sensor where the red blob is in the picture.
<img src="https://i.stack.imgur.com/dSziG.pn... | I would mount a microswitch like this one
<img src="https://i.stack.imgur.com/BcOMC.jpg" alt="enter image description here">
under the iron, so that it rests on the long lever. There are microswitches in all kinds of flavors, but the long lever has the advantage that it doesn't need much force (read: weight) to activ... | Steven's answer is the simplest solution you can get, but just for the record you could also use a PTC or NTC resistor mounted near where the tip of the iron would be. That would make the detector "invisible" and it's also a cheap solution.
| https://electronics.stackexchange.com |
3,996,696 | [
"https://math.stackexchange.com/questions/3996696",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/775857/"
] | <blockquote>
Prove that
<span class="math-container">$$\int_0^{\infty } \frac{\tan ^{-1}(a x)-\tan ^{-1}(b x)}{x} \, dx=\frac{1}{2} \pi \log \left(\frac{a}{b}\right);\;0<a<b$$</span>
</blockquote>
I do not have a clue on how to solve this. In class I have only been taught how to find converging diverging and I d... | Note
<span class="math-container">\begin{align}
&\int_0^\infty \frac{\tan^{-1}ax- \tan^{-1}
bx}x dx \\= &\int_0^\infty \int_b^a \frac1{1+x^2t^2}dtdx
=\frac\pi2 \int_b^a\frac 1tdt = \frac\pi2 \ln \frac ab
\end{align}</span>
| Defne <span class="math-container">$$F(a,b)=\int_{0}^{\infty}\left(\frac{\tan^{-1}(ax)-\tan^{-1}(bx)}{x}\right)dx$$</span>
use the formula:
<span class="math-container">$$g(\alpha)=\int_{0}^{\infty}f(\alpha,t)dt \Longrightarrow g^{\prime}(\alpha)=\int_{0}^{\infty}\frac{\partial f}{\partial\alpha}(\alpha,t)dt$$</span>
t... | https://math.stackexchange.com |
327,302 | [
"https://electronics.stackexchange.com/questions/327302",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/161814/"
] | If I have an 11.6 Ah 48 V battery, how many kWh does it take to fully charge it from empty? I calculated 556.8 Wh using the formula P = I * V (11.6 * 48 / 1000), which works out to be 0.5568 kWh. Is that correct? Do you have to factor in the voltage and/or current of the charger at all?
| Start by computing just the energy that will be stored in the battery. (11.6 Ah)(48 V) = 557 Wh = 0.557 kWh.
<b>However</b>, there are various losses along the way that are hard to quantify.
The Ah rating of the battery is what it can supposedly deliver. It's what comes out, not what you have to put in to get that ... | What you have calculated is the amount of energy that can be taken from the battery. That energy also needs to be stored in the battery by the charger, but there are losses in the battery and in the charger that need to supplied by the source during charging. There are also losses in the battery during use, so a batter... | https://electronics.stackexchange.com |
371,661 | [
"https://physics.stackexchange.com/questions/371661",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/55421/"
] | Suppose I want to make an experiment to measure the magnitude of the orbital angular momentum of an electron. When I take the average of my measurements, which out of the values below does it correspond to:
\begin{equation}\sqrt{\left\langle L^2 \right\rangle}~\text{vs.}~\left\langle \sqrt{L^2} \right\rangle~\text{vs.... | I am going to start out by talking about linear momentum rather than angular momentum, and then explain why I have done this later.
If I make a series of measurements $k_1,k_2,k_3...$ and want to take their average magnitude, then which of the above expressions I calculate correspond quite simply to the order in whic... | I will be working from your comment that states you are interested in a general operator. Thus, I will interpret that your question is that, if you have a vectorial operator, how to compute the magnitude of its angular momentum. For this, you actually thought in 3 possible operators: <span class="math-container">$L^2$<... | https://physics.stackexchange.com |
34,573 | [
"https://biology.stackexchange.com/questions/34573",
"https://biology.stackexchange.com",
"https://biology.stackexchange.com/users/15933/"
] | When searching for a plasmid on Addgene what does it mean if it is said to be an "empty backbone"? Why would one want an empty backbone?
| If you look on a very basic plasmid map, you usually see three different features: An origin of replication (often abbreviated ori or origin) which is necessary for the replication of the plasmid, a resistance gene (not necessary but useful to select for) and and multi cloning site (abbreviated MCS) which contains a nu... | cause you want to put your own stuff in without worrying about other people's junk interfering with your stuff.
Something like why would you want an empty hard drive and not one which is almost full with stuff from somebody you don't know.
| https://biology.stackexchange.com |
466,733 | [
"https://physics.stackexchange.com/questions/466733",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/215725/"
] | When we derive the expression of canonical ensemble, we write the Hamiltonian of the whole system which contains our system and a heat reservior in the form,
<span class="math-container">$$
H_{\text{whole}} = H_1(\boldsymbol x) + H_2(\boldsymbol y)
$$</span>
<span class="math-container">$\boldsymbol x, \boldsymbol y$</... | The magnitude of interaction energy of macroscopic systems is, in case of systems describable by the Boltzmann distribution, negligibly small compared to energy of the interaction systems. This is because the intermolecular forces decay fast with distance, so only those molecules near the interface contribute.
That is... | The canonical ensemble is in thermal equilibrium with a heat bath at a fixed temperature.
| https://physics.stackexchange.com |
231,354 | [
"https://physics.stackexchange.com/questions/231354",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/24860/"
] | Is the Planck temperature (<span class="math-container">$1.416×10^{32}$</span> Kelvin), the hottest possible temperature that can ever be reached, with absolute zero as it's opposite analog?
All I know is a particle with that temperature would have energy high enough, so that its gravitational force would be comparab... | The Planck temperature isn't the hottest possible temperature in the same sense that zero Kelvin is a theoretical minimum. It is simply the temperature at which it's black body radiation is of the order of the Planck length.
The Planck length is the length scale at which it is theorised that quantum-gravitational effe... | We need to think about what it means for one system to be "hotter" than another, or to have a temperature $T$. Thermodynamics defines the temperature of systems in thermal equilibrium. If an energy level $i$ of energy $U_i$ has degeneracy $g_i$, its occupation level $\propto g_i \exp \left( -\beta U_i \right) $ for som... | https://physics.stackexchange.com |
105,786 | [
"https://softwareengineering.stackexchange.com/questions/105786",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/5511/"
] | I have multiple applications some that use data from the same sources.
Is it best practice (or what are the pros/cons) to:
<ul>
<li>leave the data in databases shared by multiple applications
<ol>
<li>saves space as only one database is needed</li>
<li>complicates indexing as different applications have different que... | Space is cheap these days, so I'd advise to use one database per application.
Sharing one database for amongst multiple applications has some serious <strong>disadvantages</strong>:
<ul>
<li>The more applications use the same database, the more likely it is
that you <strong>hit performance bottlenecks</strong> and t... | I had a similar situation once. My problem was to build 3 applications, one for inventory management, one for procurement management, and one for managing users, i.e. employees. My recommendation is not to break databases physically per application, or join them physically per application. Rather, IMHO logical separati... | https://softwareengineering.stackexchange.com |
722,380 | [
"https://physics.stackexchange.com/questions/722380",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/253697/"
] | Suppose I had a rod of diameter <span class="math-container">$d$</span> composed of some material with tensile strength <span class="math-container">$T$</span>. If I then exterted a pulling force <span class="math-container">$F$</span> on the ends of the bar, how do I compute the force <span class="math-container">$F$... | I'm not sure whether the previous answers address your interest in the molecular understanding of the change . The stress tensor tells us that pressure is momentum flux. The kinetic flux hasn't changed, but the intermolecular force term has. The force varies very quickly with distance and the molecules are a tiny bit c... | By placing your finger in the water, you are exerting a force and the water responds to that force by rising higher in the glass. That force adds to the apparent weight of the water/glass system. If you were to mark the place where the water rises on the glass and then remove your finger the weight would return to th... | https://physics.stackexchange.com |
99,119 | [
"https://mathoverflow.net/questions/99119",
"https://mathoverflow.net",
"https://mathoverflow.net/users/7206/"
] | Several days ago a friend asked me the following:
<blockquote>
We know that in $\mathcal P(\mathbb N)$ we can find a family of size continuum that every [distinct] two intersect in a finite set. Can we do that with $\mathcal P(\mathbb R)$, that is a family of size $2^{\frak c}$ many subsets of real numbers that the ... | The answer to your first question (with finite intersections) is negative.
Indeed, if $X$ is an infinite set and $I$ has cardinality greater than that of $X^{\aleph_0}$ then $X$ can't contain $I$ distinct subsets with pairwise finite intersection. This answers your question since $c^{\aleph_0}=c$.
Indeed, let $(A_i)_... | It is consistent with ZFC that a set of size $\aleph_1$ does not have $2^{\aleph_1}$ subsets, each of size $\aleph_1$, with all pairwise intersections countable. This is an old result of Jim Baumgartner; see "Almost-disjoint sets, the dense-set problem, and the partition calculus", Ann. Math. Logic 9(1976), 401-439, p... | https://mathoverflow.net |
90,105 | [
"https://softwareengineering.stackexchange.com/questions/90105",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/30438/"
] | If we only work with one branch in Subversion, should we even bother? Can't we just work on the trunk to speed things up?
This is how we develop with Subversion:
<ul>
<li>There is a trunk</li>
<li>We make a new development branch</li>
<li>We develop a new feature on that branch</li>
<li>When the feature is done, it i... | IMHO working directly on the trunk is fine if you can commit in small increments and you have continuous integration in place, so you can ensure (to a reasonable extent) that your commits don't break existing functionality. We do that too in our current project (in fact I haven't worked in any project using task-specif... | What you are describing with your feature development is parallel development (simultaneous development targeting different product releases) and it does require branches to handle it properly. You could have one branch either for each release or for each feature if you often have to recompose the features that will ma... | https://softwareengineering.stackexchange.com |
126,846 | [
"https://softwareengineering.stackexchange.com/questions/126846",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/24256/"
] | I'm doing the same. When there is something "to-do" in my code, I write <code>//TODO ...</code>. But I'm curious to know when this started and if there is a reason for writing "to-dos" in all capital letters?
| Also, it's great to have a way to easily search for code sections you glossed over and want to get back to.
You can do a case-sensitive search for "TODO" to immediately find what you skipped before.
"todo" (lower-case) could potentially be part of a larger word/function/variable, but "TODO" (upper-case) is probably no... | I just stands out and is easy to catch... nothing complicated!
| https://softwareengineering.stackexchange.com |
185,000 | [
"https://dba.stackexchange.com/questions/185000",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/133895/"
] | I have these words in my <code>words</code> table in a MySQL db:
<pre><code>kaki
koku
kao
kakeru
</code></pre>
How can I get all rows that contains ONLY 'ka','ki','ku','ke','ko'?
I want to get <code>kaki</code> and <code>koku</code> but not <code>kao</code> and <code>kakeru</code>, because although <code>kao</cod... | Try some <code>REPLACE()</code> tricks:
<pre><code>SELECT *
FROM #words
WHERE REPLACE(
REPLACE(
REPLACE(
REPLACE(
REPLACE(
word
,'ka','')
,'ke','')
,'ki','')
,'ko','')
,'ku','')
= '';
</code></pre>
| <pre><code>WHERE word REGEXP "^(k[aeiou])+$"
</code></pre>
will check for a pair of (k+vowel) letter pairs.
<ul>
<li><code>k</code> matches just <code>k</code>.</li>
<li><code>[aeiou]</code> matches any of the letters in the brackets.</li>
<li><code>^</code> and <code>$</code> "anchor" both ends -- no other text on e... | https://dba.stackexchange.com |
593,767 | [
"https://physics.stackexchange.com/questions/593767",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/259324/"
] | I don't understand why it is necessary to state the superposition principle. Reading Griffiths' Introduction to electrodynamics, the superposition principle is described as experimental fact that is not implied by Coulomb's law. Well, it is clear to me that non-linear phenomena can exist, but as far as know, Couloumb's... | Superposition principle and using vectors for modeling forces are entirely independent things.
<strong>Using vectors for modeling forces</strong>
In Classical Mechanics, forces either are defined through the Second Law (<span class="math-container">$m \mathbf{a} = \mathbf{F}$</span>), or are introduced as <em>primitiv... | You're always going to use vectors for forces, because forces have magnitude and direction, so nothing else really models them well. Even if the superposition principle was not valid we would still have to use vectors. We would have to have to use some law <span class="math-container">$\vec{F} = \vec{F}(\vec{r},Q_1, \... | https://physics.stackexchange.com |
53,257 | [
"https://electronics.stackexchange.com/questions/53257",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/10660/"
] | I have a project that calls for an 18VDC power supply. I've found one that meets specs online for about $40.00, and I've found an 18VAC power supply that meets specs (apart from being AC) at my local science & surplus store for $3.99.
So I was thinking about this over the weekend, and it seems to me that I could ... | Assuming the 18VAC is regulated enough for you, you might want to add a smoothing capacitor on the output side but that's about it.
There's a fair chance the AC one is just a transformer so unregulated, whereas a DC one will likely be switch-mode and reasonably well controlled. Depends on your requirements.
| You can obviously build an AC to DC converter circuit. It does involve a bit more than a few diodes. Four diodes in a bridge circuit can turn the AC into pulsating DC. It will require a bulk storage capacitor to smooth this out into a DC voltage. You would apply this capacitor downwind from the diode bridge. Do note th... | https://electronics.stackexchange.com |
100,157 | [
"https://mathoverflow.net/questions/100157",
"https://mathoverflow.net",
"https://mathoverflow.net/users/18263/"
] | Let $(C,\partial)$ and $(C',\partial')$ be chain complexes of $R$-modules where $R$ is a (commutative) ring. Let $F$ and $F'$ be finite filtrations of $C$ and $C'$ respectively, i.e., $$\varnothing = F_0C \subset F_1C \subset \ldots \subset F_nC = C$$ and similarly for $F'$. There exist spectral sequences associated to... | Say your two filtered chain complexes are concentrated in degree zero. Then the spectral sequences degerate, and your questions become: If you have two filtered abelian groups and an isomorphism between the associated graded modules, can you deduce that the abelian groups are isomorphic? The answer is no; you can tak... | What can be said is that if $C,C'$ have degree-wise finite filtrations and if $f: C \to C'$ is a filtration preserving chain map such that there is $r$ such that $E^r(f): E^r_{ij}(C) \to E^r_{ij}(C')$ is an isomorphism for all $i,j$ then $f$ is a quasi-isomorphm. This can be found in Brown: Cohomology of Groups, VII, P... | https://mathoverflow.net |
441,900 | [
"https://physics.stackexchange.com/questions/441900",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/195021/"
] | The explanation I've read on why a free electron can't emit a photon goes like this:
<em>Let there be a free electron of mass <span class="math-container">$m$</span> moving with constant velocity <strong><span class="math-container">$v$</span></strong>. We may enter a new reference frame where the electron isn't movin... | The mass of the electron is fixed. If you want to use some of it, then the electron must decay into another, lighter particle.
There are not very many particles that are lighter than the electron. Basically, there are the neutrinos (which are quite light, but have nonzero mass) and there are the massless gauge bosons ... | The rest mass of an elementary particle is a fundamental property of the particle and cannot be changed. So an electron cannot change its rest mass without stopping being an electron i.e. decaying into some other particle. Since the rest mass is the mass in the electron's rest frame, and in your example we are working ... | https://physics.stackexchange.com |
40,312 | [
"https://dsp.stackexchange.com/questions/40312",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/22596/"
] | Consider a simple linear, time invariant system of the form:
$y_k = cy_{k-1} + (1-c)x_k$
The impulse response of this system can be computed by either dimpulse or by applying lfilter to a vector composed of a one followed by zeros:
<pre><code>import scipy.signal as sp_signal
import numpy as np
Ts = 1
c = 0.9
A = [1... | In lfilter the transfer function is described in decreasing powers of z, as shown below copied from the python doc for signal.lfilter:
<pre><code> -1 -M
b[0] + b[1]z + ... + b[M] z
Y(z) = -------------------------------- X(z)
-1 -N
a[... | I don't think it's because of differences between lfilter and dimpulse transfert function definitions. You must take care of the power of z in num and den definitions and get the same results in both cases.
Please just consider these equalities :
$$H(z)=\frac{1-0.9\cdot z^{-1}}{0.1} = \frac{z\cdot(1-0.9z^{-1})}{z\cdo... | https://dsp.stackexchange.com |
110,388 | [
"https://softwareengineering.stackexchange.com/questions/110388",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/12539/"
] | I am having a memory leak when running a test suite due to singleton objects are created and not cleared after each test case. And these singleton objects are holding references to heavy objects.
So I don't know if there is a better solution than clearing the singleton objects in the <code>tearDown()</code> methods in... | typically, this means the singleton has outgrown its original intention, and should be made a standard object (read: not a singleton).
testability is one major reason. pains updating programs to use multiple instances are another. yet another: it's often a difficult bottleneck at execution. the singleton pattern is of... | <blockquote>
So I don't know if there is a better solution than clearing the
singleton objects in the tearDown() methods in each test class!!
</blockquote>
Get rid of the singletons. Singletons are hard to test anyway (global state). Use dependency injection instead. Mock heavy objects with lightweight ones if you... | https://softwareengineering.stackexchange.com |
79,852 | [
"https://physics.stackexchange.com/questions/79852",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/22928/"
] | I've had a lot of difficulty conceptually understanding the physics of how a car turns on an unbanked curve, so I'm hoping you could help me out. When a car is moving in uniform circular motion, we know that <span class="math-container">$|\vec{a}| = \frac{v^2}{r}$</span>, and the direction of acceleration is towards th... | I had fun trying to make this as intuitive as possible. I hope I've succeeded without doing the physics of the situation much injustice.
When a car is driving straight ahead, the plane in which the wheels are rotating is aligned with the direction of movement. Another way of saying this is that the rotation axis is pe... | The key IMO is that the "effective friction" of a wheel that is free to rotate is very different in different directions. Moving inline with the wheel the effective friction is low because the wheel can rotate. Moving perpendicular to the wheel the effective friction is simply the friction between wheel surface and gro... | https://physics.stackexchange.com |
112,645 | [
"https://security.stackexchange.com/questions/112645",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/56343/"
] | There are tons of questions, answers, articles, papers, cartoons etc. on the subject of how to choose a password in such a way that it's difficult to guess or crack with a dictionary-based attack.
I don't understand why a system would allow me to try that kind of attack in the first place, so I'm wondering what it is ... | Depends what you're defending against, really.
<ol>
<li>Trying to log into a web application, attempts should indeed be restricted in some way. IP rate restriction is a good start, username level rate restriction is better, combining the two is even better. However, imagine that I have a long list of usernames and pass... | The system is to blame for a breach in the system. If they allow millions of logins per second, and an unauthorized user gained access then IMO that's on them. If the system decides to not encrypt persons private data (looking at you OPM) then this blame is on the system. Using outdated operating systems and softwar... | https://security.stackexchange.com |
121,824 | [
"https://math.stackexchange.com/questions/121824",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/13643/"
] | I would like to show that the function:
$$f:(x,y)\mapsto \frac{x\sin(y)-y\sin(x)}{x^2+y^2}$$
is a $C^1$-function.
$$ \frac{\partial f}{\partial x}(x,y)=\frac{\sin(y)-y\cos(x)}{x^2+y^2}+\frac{2x(y\sin(x)-x\sin(y))}{(x^2+y^2)^2}$$
$$ \frac{\partial f}{\partial y}(x,y)=-\frac{\partial f}{\partial y}(y,x)=... $$
So I... | Because of the Taylor expansion $\sin(x) = x - {x^3 /6} + ...$, if $x$ and $y$ are small enough you can write $\sin(x) = x + E(x)$ and $\sin(y) = y + E(y)$, where $|E(x)| < |x|^3$ and $|E(y)| < |y|^3$. So you have
$${2x(y\sin(x) - x\sin(y)) \over (x^2 + y^2)^2} = {2x(yx + yE(x) - xy - xE(y)) \over (x^2 + y^2)^2}$... | A partial answer to at least show that $f$ is differentiable at $(0,0)$. Then it remains to show that its derivative is continuous. I didn't check if that also follows from this bound directly.
Combine $\tan(x)\geq x$ and $\sin(x) \leq x$ for $x \in [0,\pi/2)$ to get
$$
\cos(x) \leq \frac{\sin(x)}{x} \leq 1
$$
f... | https://math.stackexchange.com |
1,834,174 | [
"https://math.stackexchange.com/questions/1834174",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/217910/"
] | <strong>Question</strong>
<blockquote>
Proposition 2.11 (Exercise 10) - The following implications are valid if and only if the measure is complete:
a.) If $f$ is measurable and $f = g$ $\mu$-a.e., then $g$ is measurable.
b.) If $f_n$ is measurable for $n\in \mathbb{N}$ and $f_n\rightarrow f$ $\mu$-a.e., t... | Your proof is essentially correct. The proof of the last part, that is (b. <span class="math-container">$\Rightarrow$</span> <span class="math-container">$\mu$</span> is complete), is very similar to the proof that (a. <span class="math-container">$\Rightarrow$</span> <span class="math-container">$\mu$</span> is comp... | A small correction:
There may exist a little error in your proof on a. <span class="math-container">$\Rightarrow$</span> <span class="math-container">$\mu$</span> is complete.
The Reason is that if <span class="math-container">$A \subset E $</span> and <span class="math-container">$\mu(E) = 0$</span> then <span class... | https://math.stackexchange.com |
294,413 | [
"https://electronics.stackexchange.com/questions/294413",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/140330/"
] | I have a project for one of my classes and it is to design a BJT multistage amplifier. I can use any combination of NPN or PNP and any type bias that I choose, but I do have a few constraints which are given by the professor which are to use a set input voltage, audio frequency, and power output.
I know how to perform ... | If you have analog signals crossing the boundary, and that is your "analog ground" in the sense that it is the reference for all of your analog signals, then I think the high side switch is less likely to give you problems. The low side switch will introduce a potentially nasty voltage between the two references due to... | In this situation, I would use an N-channel MOSFET on the ground. Alternatively, as previously mentioned use a P-channel MOSFET on VCC. Either way works, as long as you are keen on ensuring there are no sneak paths. <br> <br>
Why?
<br>
Using an NPN will result in voltage drops Vbe and Vce. Using an N-channel results i... | https://electronics.stackexchange.com |
50,537 | [
"https://stats.stackexchange.com/questions/50537",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/21114/"
] | I'm reading a paper where author discards several variables due to high correlation to other variables before doing PCA. The total number of variables is around 20.
Does this give any benefits? It looks like an overhead to me as PCA should handle this automatically.
| This expounds upon the insightful hint provided in a comment by @ttnphns.
<strong>Adjoining nearly correlated variables increases the contribution of their common underlying factor to the PCA.</strong> We can see this geometrically. Consider these data in the XY plane, shown as a point cloud:
<img src="https://i.st... | I will further <strong>illustrate</strong> the same process and idea as @whuber did, but with the loading plots, - because loadings are the essense of PCA results.
Here is three 3 analyses. In the first, we have two variables, $X_1$ and $X_2$ (in this example, they do not correlate). In the second, we added $X_3$ whic... | https://stats.stackexchange.com |
288,001 | [
"https://softwareengineering.stackexchange.com/questions/288001",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/39431/"
] | I'm building a matchmaking queueing service for a 5v5 team-based game.
I have a large set of users that are grouped into unique lobbies comprised of 1-5 users each. Each user has an integer based rank from 1-16. These ranks will eventually be converted to a trueskill rating, but for now, we're sticking with integers ... | First thing is to know the partitions of the number 5 (the team size you want to assembly). There are algorithms to generate the partitions of a number but since the number is small and fixed you don't need to worry about it. The partitions of 5 are: {5}, {4,1}, {3,2}, {3,1,1}, {2,2,1), {2,1,1,1}, and {1,1,1,1,1}.
Mak... | <ol>
<li>Calculate the total rank of all players in a single lobby.</li>
<li>Match with other lobby based on that. (+- 1.5 range pretty much)</li>
</ol>
Isn't it as simple as this?
| https://softwareengineering.stackexchange.com |
141,127 | [
"https://electronics.stackexchange.com/questions/141127",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/36992/"
] | <img src="https://i.stack.imgur.com/ToPGk.png" alt="enter image description here">
The book says that the for loop 2, the KVL is \$-j3 I_{1} + (12+j6)I_{2}=0\$ , but the loop 2 leaves the dot in its coil. Isn't it supposed to be \$-j3I_{1} + (12-j6)I_{2} =0\$ ?
| <img src="https://i.stack.imgur.com/X8lpP.jpg" alt="enter image description here">
<strong>The dot only determines the polarity of the mutual voltage, it won't affect other directions in the calculation.</strong> In figure (b), it shows the equivalent circuit take into the mutual voltage. Because the direction of \$I_... | equation on loop 1 is
j(5-4)i1-j(4)i2=12<0
equation on loop 2 is
(12+6j)i2-3ji1=0
now equating we get
i1=(2-4j)i2
applyng this on loop 1 equation we get
i2=2.91<14.03
and
i1=13.001<-49
hence solved
| https://electronics.stackexchange.com |
71,493 | [
"https://dsp.stackexchange.com/questions/71493",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/39618/"
] | I know that complex exponential functions are eigensignals to LTI systems. Do these include real exponential functions? E.g. <span class="math-container">$2^t, e^t, ...$</span>
Thanks for the help!
| Mathematically (and theoretically), there is no need for the exponential function to be a complex sinusoid. The math is unchanged. The problem is that practical LTI systems are not boundless nor are they acausal. So setting aside those problems, every LTI system has input/output relationship described by the convolu... | Complex exponentials are eigenfunctions of LTI systems because they are eigenfunctions of the convolution operator:
<span class="math-container">$$\begin{align}e^{j\omega_0t}\star h(t)&=\int_{-\infty}^{\infty}h(\tau)e^{j\omega_0(t-\tau)}d\tau\\&=e^{j\omega_0t}\int_{-\infty}^{\infty}h(\tau)e^{-j\omega_0\tau}d\ta... | https://dsp.stackexchange.com |
123,413 | [
"https://mathoverflow.net/questions/123413",
"https://mathoverflow.net",
"https://mathoverflow.net/users/31887/"
] | I am trying to understand the relationship between the simplicial path space and loop space with the path space of a topological space, and the loop space of a topological space.
I have understood that the simplicial path space of a simplicial object $A$ is homotopy equivalent to the constant simplicial object $A_0$ b... | I'll refer to my ancient book "Simplicial objects in algebraic topology". It is best to restrict to Kan complexes $K$ with a single vertex. In 23.3 and 23.4, it is shown that the path projection $PK \to K$ is a particularly nice kind of simplicial bundle provided that its
fiber $L(K)$ is a simplicial group, which usu... | Have you considered the sequence Omega X --> PX --> X
in the two categories where Omega denotes based loop space
and P based path space
| https://mathoverflow.net |
14,181 | [
"https://physics.stackexchange.com/questions/14181",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/74/"
] | For simple potentials like square wells and harmonic oscillators, one can explicitly calculate the product $\Delta x \Delta p$ for stationary states. When you do this, it turns out that higher energy levels have higher values of
$\Delta x \Delta p$.
Is this true for all time-independent potentials?
Certainly, it... | The answer is no, and a counterexample is the following plateau potential:
$V(x) = x^2 \ \ \ \ \; \mathrm{for}\ \ \ \ x\ge -A$
$V(x) = A^2 \ \ \ \ \mathrm{for}\ \ \ \ -A-k \le x < -A$
$V(x) = \infty\ \ \; \ \ \mathrm{for}\ \ \ \ x <-A-k$
A is imagined to be a huge constant, and k is a large constant, but ... | I will give a counter-example. Consider the potential below, $V$ is considered to be very large but not infinity. The ground state of the system should be approximate as the ground state of an infinite potential well. The uncertainty is greater than $\hbar/2$.
While when the particle is confined in the parabolic poten... | https://physics.stackexchange.com |
42,177 | [
"https://electronics.stackexchange.com/questions/42177",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/5542/"
] | Example:<br>
<img src="https://i.stack.imgur.com/Iu1kW.png" alt="Name?">
Does this drawing has a technical name?
I see that people usually call these drawings "PCB". But, PCB is physical object, physical implementation of circuit, isn't it? Is these kind of drawings really called "PCB", or do they have a special name... | I would also call the drawing you show a "PCB layout", as it obviously isn't the PCB object. But like Wouter already indicated the definition of a PCB contains a lot more information that the copper pattern shown. First, this is just one layer, there could be a second copper pattern on the top/bottom side (for PTH the ... | I would call it a "[PCB] layout". It's still somewhat a general term, but applies to your picture better than just "PCB".
| https://electronics.stackexchange.com |
35,381 | [
"https://electronics.stackexchange.com/questions/35381",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/8534/"
] | I have a device that is controlled by connecting a resistor between two of its connectors. The resistor can take any value between 0 and X, where X is an odd value. (E.G. X=1523ohm)
Does anybody have an idea how I can realise a circuit whose resistance can be varied manually between 0 and X? There must not be the poss... | Like gbarry says. The next higher value you find in the Digikey offering is 2 kΩ, so you would need a 6386 Ω resistor in parallel. That is between one end and the wiper, not across the full resistance. 6386 Ω is also an odd value, so you may want to compose that using different standard values as well... | Choose a variable resistor--probably the next value up from 1523 ohms would be best. Then, calculate a resistor in parallel with the maximum setting that gives you 1523 ohms. Now you are at 1523 at max, and can adjust it down to zero.
| https://electronics.stackexchange.com |
35,278 | [
"https://security.stackexchange.com/questions/35278",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/18104/"
] | I'm trying to hack my own <strong>WPA2</strong> network for learning purposes.
I have the <code>*.cap</code> file generated by <code>aircrack-ng</code> tools after a WPA handshake.
I've tested by including my own password and a bunch of incorrect passwords on a wordlist and <code>aircrack-ng</code> crack successfull... | I'd look at OCLHashcat, as it let's you brute force with specific character sets and doesn't need to generate the list beforehand.
I'm not sure what you mean by "incrementally" however, if you mean stopping and starting OHC will let you do that.
In terms of cracking WPA2, you've got 3 options.
<ol>
<li>Dictionary... | <pre><code>john --stdout --incremental:all | aircrack-ng -e ESSID -w - /home/user/input_wpa.cap
</code></pre>
Edit the john conf file to change from 8 to 10
| https://security.stackexchange.com |
388,467 | [
"https://electronics.stackexchange.com/questions/388467",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/194798/"
] | I'm doing research for the best antenna for a 2.4 GHz connection from our device to a local router. The problem is that our device will have varying loads connected to the other pins of the micro controller.
My question is will the varying loads on other pins of a micro controller will effect the impedance matching f... | Each pin on a MCU has an individual driver. If one pin drives a matching antenna network, any other pins are driving something else independently. Of course this assumes that the power rail for the MCU is solidly driven by power supply and properly bypassed, so any cross-talk over (presumably) common power rail is mini... | No, it should not have any influence, to my knowledge. Output impedance (that your antenna has to match) is a property of the power output stage inside the microcontroller.
| https://electronics.stackexchange.com |
294,262 | [
"https://physics.stackexchange.com/questions/294262",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/128561/"
] | I've given this Hamiltonian with one degree of freedom:
$$
H(x,p)=\frac{p^2}{2}+\frac{\omega_0^2x^2}{2}+\lambda\left(\frac{p^2}{2}+\frac{\omega_{0}^2x^2}{2}\right)^2
$$
I need to find the general solution for the motion of this particle $(x(t),p(t))$. It's required to solve the equations of motion, not only find them.
... | I want simplification, so I'll take $\omega_0 = 1$. You can rescale $x$ and always get this kind of simplification, if $\omega_0$ is a constant. The Hamiltonian then becomes
$$H(x,p) = \frac{p^2}{2} + \frac{q^2}{2} + \lambda \left( \frac{p^2}{2} + \frac{q^2}{2} \right)^2 \, .$$
Hamilton's equations are given by
\beg... | what if same question is asked to find equation of motion.
i know equations of Hamilton. i am having hard time in finding meaning full solutions. i think my apporoch is not good.
can you suggest similar other example?
thanks
| https://physics.stackexchange.com |
108,380 | [
"https://softwareengineering.stackexchange.com/questions/108380",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/34856/"
] | In programming, various terms like abstraction are mentioned quite heavily. What, however, is composition? What is a basic but effective example of composition at play to result in good quality code (good quality meaning easy to understand, short and concise, etc).
| In the <em>functional programming</em> sense, a composition is:
if a function f takes a foo and returns a bar, and
if a function g takes a bar and returns a blah,
then there is a function h which takes a foo and returns a blah, such that:
h(foo) = g(f(foo))
That is, if f is "take the length of a string" and ... | Composition is simply when you have multiple objects compose one more complex object. Example:
<pre><code>class Person {
string FirstName { get; set; }
string LastName { get; set; }
}
</code></pre>
Person is composed of a FirstName and LastName
| https://softwareengineering.stackexchange.com |
3,897,383 | [
"https://math.stackexchange.com/questions/3897383",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/809359/"
] | I was trying to solve the following exercise:
<blockquote>
Show that <span class="math-container">$(\arcsin (x))'=\frac{1}{\sqrt{1-x^2}}$</span> for all <span class="math-container">$x\in (-1,1)$</span>.
</blockquote>
So far I got that <span class="math-container">$(\arcsin (x))'=\frac{1}{\cos(\arcsin (x))}$</span>, bu... | Assuming that <span class="math-container">$\Re(\lambda )>0$</span>, we have
<span class="math-container">$$\int_0^\infty (a t-t \log (t)+1)\,e^{-\lambda t}=\frac{\lambda +\log (\lambda )+(a+\gamma -1)}{\lambda ^2}$$</span> which is exact and gives your expression.
| I think splitting it would do the work. The formula is just the question of calculating something like:<br>
<span class="math-container">$ \int_{0}^{\infty} (1- t\log t +at) e^{-\lambda t}dt$</span><br>
After splitting, each term is .<br>
i) <span class="math-container">$\int_{0}^{\infty} e^{-\lambda t} dt= \frac{1}{\l... | https://math.stackexchange.com |
12,920 | [
"https://quant.stackexchange.com/questions/12920",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/8231/"
] | When calculating CAGR for intervals shorter than a year (or intervals that are longer than, but not integer years in length), should you use the 252 trading days or the 365.25 calendar days?
The formula I am using follows:
CAGR = ( Current Value / Initial Value ) ^ (1 / (Days passed / Days in the year)) - 1
| You can use both standards, but when you apply or compare this rate the standards must be equal, and it should be noted which convention you used.
Note that 300/365 yeardays would in percentage be equal to 205/250 tradingdays, so its really just a convention that would make no difference in actual time.
| It depends on what makes more economic sense:
<ol>
<li>If you are calculating CAGR for FX (which is traded effectively 24/7) strategy returns for instance, it would seem fair to use 365.25 calendar days.</li>
<li>If you are calculating CAGR for internal reporting of trading strategy returns on a product with 5 market ... | https://quant.stackexchange.com |
253,310 | [
"https://stats.stackexchange.com/questions/253310",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/91058/"
] | Let's say I have a database to track my customers.
On any given day, a customer is active or non-active, and I can discover this in real-time by querying the database. So, right now, I can easily find out that I have X active customers.
What I can't do is discover how many customers I had on July 5, 2014, for example... | You could call it a time series or a time-series variable (though most people neglect the hyphen). You could also use the more general term of longitudinal data; time-series data entail one type of longitudinal data. (If the measurements were taken at only two points, this would hardly constitute a time series and wo... | It is called a time series observation. This is a bit of an oxymoron or a contradiction in terms because it is never truly observed although it is called an observation. It is a collection of transactions .. your customer data (which can be observed) but it itself is never truly observed. We observe transactions and t... | https://stats.stackexchange.com |
59,173 | [
"https://softwareengineering.stackexchange.com/questions/59173",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/18957/"
] | Who wants to work in a fast-paced environment? Not me! I want a civilized environment where people have a sense of balance. Higher quality work gets done that way and work life isn't full of stress and anguish.
| It's code for "We change our minds a lot about what we want from the software, and if we hire you we don't want you to complain about it. In fact, we expect you to put in a lot of overtime to implement our latest whim decision because we're <em>fast paced</em>. You've been warned."
In programmer-speak, it means "we h... | <strong>Because most job ads aren't written by software engineers.</strong>
It's the exact same reason so many require the "ability to multi-task". Every respectable software developer knows that the human brain cannot multi-task. Or have 10 years experience in Windows Server 2008.
| https://softwareengineering.stackexchange.com |
62,851 | [
"https://physics.stackexchange.com/questions/62851",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/22964/"
] | Suppose a man leaves from Earth to a star which is 1000 light years away. He accelerates to a velocity such that the entire trip lasts a year, from the reference frame of the rocket.
Now lets pretend the person in the rocket wants to have a transmission of the radio to him.
Due to time dilation, when a day passes on ... | You are <em>not quite</em> correct (see edits). Except I wouldn't say that "in reality it takes just slightly over 1000 years" - the rocket frame is no less real than the Earth's frame. As far as the actual numbers go: at one gee acceleration it takes about a year in the rocket frame to accelerate, and a year again to ... | This is not correct. From the rocket ship's point of view, time on earth is passing more slowly, so the frequency of radio transmissions received is going down. You say "Due to dilation when a day passes on earth only a few seconds pass on the rocket ship" but that is the view that earth has. The rocket ship has the op... | https://physics.stackexchange.com |
537,629 | [
"https://math.stackexchange.com/questions/537629",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/94404/"
] | I'm at a loss. My guess is no, but I'm new to doing these problems. It can not be solved with cross multiplication but there are other ways of solving these problems I'm sure. Thank you for any help.
| No. There is no way to manipulate this ODE so that the method of separation of variables can be used. This is a first-order linear differential equation since it has the form ${dy\over dx}+P(x)y=Q(x)$. We must use an integrating factor $I(x)=e^{\int P(x)dx}$ in order to solve this ODE. $${dy\over dx}-y=x$$
Let $I(x)=e... | It is not separable. A separable differential equation has the form $dy/dx = f(x)g(y)$. But if $x+y = f(x)g(y)$ for all $x$ and $y$ then we would have
\begin{align*}
(x+x)(y+y) &= f(x)g(x)f(y)g(y)\\
&= f(x)g(y)f(y)g(x) =(x+y)(y+x)
\end{align*}
for all $x$ and $y$, which you can see is false by taking $x=0$ an... | https://math.stackexchange.com |
129,855 | [
"https://security.stackexchange.com/questions/129855",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/77994/"
] | The queries below show SQL injection in appscans for MySQL.
Is there a way to do a proof of concept or otherwise to how can they be exploited?
<pre><code>INSERT INTO cscart_sessions (session_id, expiry) values('6432e5ecf6c81730aa70c2b05415fe6d OR ' '','1468236649')
SELECT session_id FROM cscart_sessions WHERE session... | If you're directly using the SesssionId in your query, without escaping the SQL special characters then <strong>YES</strong> the Queries are vulnerable to SQL Injection.
For example,
Someone could send request to the server with following cookie :
<pre><code>csid=a'; DROP TABLE cscart_sessions;
</code></pre>
And y... | Depending on whether or not the code that makes these queries is properly sanitizing inputs, you may or may not have a vulnerability.
Another factor would be whether the construction of the queries is parameterized or simple uses concatenation.
As commenters have pointed out, there is no way to tell for sure based on... | https://security.stackexchange.com |
650,127 | [
"https://physics.stackexchange.com/questions/650127",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/292099/"
] | If in the quantum world reality is just based on observation then why atoms and molecules are real?
I mean, it is said that when a quantum particle is not observed it's neither spinning up nor down (if I understood that correctly), and from what I understand everything has its quantum particle, and so, <strong>why mole... | Prahar gave a good answer to your first two questions so let me answer the third. If you let <span class="math-container">$\epsilon, \chi_i, \chi_j, \chi_k$</span> carry spinor indices <span class="math-container">$\alpha, \beta, \gamma, \delta$</span> respectively, then
<span class="math-container">\begin{equation}
(\... | For the first identity, <span class="math-container">$\sigma^2$</span> is the second Pauli matrix, NOT <span class="math-container">$\sigma^\mu \sigma_\mu$</span>.
The second identity is very easy to prove
<span class="math-container">\begin{align}
\epsilon^T \sigma^2 \chi &= \epsilon^a (\sigma^2)_{ab} \chi^b \\
&a... | https://physics.stackexchange.com |
300,150 | [
"https://math.stackexchange.com/questions/300150",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/34083/"
] | $$\int\frac{dx}{(z^{2}+x^{2})^{3/2}}$$
I arrived at this after a substitution $t=x/z$:
$$\frac{1}{z^{2}}\int\frac{dt}{(1+t^{2})^{3/2}}$$
but now stuck with that 3/2 in the exponent.
| Let us substitute $x = z \tan(t)$. Differentiating we get
$ dx = \frac{z\, dt}{\cos^2(t)}$. Now,
$$ \int \frac{z\,dt}{\cos^2(t) z^3 (1+\tan^2(t))^{3/2}}=$$
$$ \frac{1}{z^2} \int \frac{dt}{\cos^2(t) (\frac{1}{\cos^2(t)})^{3/2}}=$$
$$ \frac{1}{z^2} \int \cos(t) \,dt= \frac{1}{z^2} \sin(t) + C = $$
$$ \frac{1}{z^2} \frac... | You can also try a hyperbolic substitution:
$$\frac{x}{z}=\sinh u\Longrightarrow dx=z\cosh u$$
$$\int\frac{dx}{(z^2+x^2)^{3/2}}=\frac{1}{z^3}\int z\cosh u\; du\frac{1}{(1+\sinh^2u)^{3/2}}=\frac{1}{z^2}\int\frac{du}{\cosh^2 u}=$$
$$=\frac{2}{z^2}\int\frac{e^udu}{e^{2u}+1}=\frac{2}{z^2}\arctan e^u+C\ldots$$
| https://math.stackexchange.com |
299,470 | [
"https://mathoverflow.net/questions/299470",
"https://mathoverflow.net",
"https://mathoverflow.net/users/36688/"
] | I begin my question with a multilinear question then I will consider two local smooth analogies:
Assume that <span class="math-container">$\alpha$</span> is a real valued symmetric <span class="math-container">$k$</span>-tensor, that is a <span class="math-container">$k$</span>-linear map <span class="math-cont... | Any <span class="math-container">$k$</span>-linear symmetric map <span class="math-container">$\alpha:(\mathbb R^n)^k \to \mathbb R$</span> is the polarization of a homogeneous polynomial <span class="math-container">$\bar \alpha: x\mapsto \alpha(x,x,x\dots,x)$</span> of degree <span class="math-container">$k$</span> o... | To your first question: no, even if we take out the orthonormal condition, by a dimension count argument (with one nontrivial exception).
Assume there is generally. The $\lambda_i$ can be removed by scaling the $x_i$ (possibly up to sign or degeneracy, neither of which affect dimension), so we can think about just the... | https://mathoverflow.net |
369,111 | [
"https://stats.stackexchange.com/questions/369111",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/221929/"
] | Suppose that we have integer random variables <span class="math-container">$X>0$</span> and <span class="math-container">$Y>0$</span> and constant number <span class="math-container">$a$</span>. We have: <span class="math-container">$X+Y < a$</span>. Can we say that the covariance of these random variables is ... | Make use of the standard covariance properties:
<span class="math-container">$Cov(aX, bY) = a\cdot b\cdot Cov(X, Y)\\
Cov(X+c, Y) = Cov(X, Y)\\
Cov(X+Y, Z) = Cov(X, Z) + Cov(Y, Z)\\
Cov(X, X) = Var(X)$</span>
where <span class="math-container">$a$</span>, <span class="math-container">$b$</span> and <span class="math-... | I give you an answer to calculate the coefficients using the inverse of the Covariance Matrix, which is also referred to as the Anti-Image Covariance Matrix
In simple linear regression: <span class="math-container">$Y=\beta_0+\beta_1X$</span>
you can write <span class="math-container">$\beta_1=\frac{cov(x,y)}{var(x)}$... | https://stats.stackexchange.com |
98,647 | [
"https://dba.stackexchange.com/questions/98647",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/48747/"
] | My database has 2 partitions files: PRIMARY, DATABASE_ARCHIVED. A lot of tables have a column "archived". When archived is 1 row is in the DATABASE_ARCHIVED partition. When archived is 0 row is in the PRIMARY partition.
To improve performance I started to modify requests in the application by adding <code>archive=0</c... | No, if you are talking about partitioned tables, sql uses the where clause to filter rows. if your query doesn't have the where clause it will return all rows.
if you can't test your changes you probably shouldn't be making them.
| As far as I know, even you have 2 database files to take advantage from partitioning you must create partitions (this feature is available only in <code>Eneterprise Edition</code> of SQL Server). Each partition will be in different file. No Archived will be in <code>PRIMARY</code> file. Others will be in <code>DATABASE... | https://dba.stackexchange.com |
174,089 | [
"https://electronics.stackexchange.com/questions/174089",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/53052/"
] | I'm still a junior engineer and wondering how to perform eye diagram measurement of a CLK using an Oscilliscope?
My second question: Is the eye diagrams just using to measure CLKs, or one may also measure Data using eye diagram?
My third question: what are the benifits in the verification when using eye diagrams inste... | Eye diagrams are not really useful when looking at a clock signal because to make any sense of it requires triggering on the rising or falling edge. Once that is accomplished it won't look like an eye diagram: -
<img src="https://i.stack.imgur.com/tlrtB.png" alt="enter image description here">
It will look like this:... | An eye diagram is not useful to observe a clock signal because the clock is periodic. It is most useful for observing data signals because it allows the results of many data bits to be displayed on one diagram. It is very useful in showing how different bit patterns affect the ability of a data receiver to correctly ... | https://electronics.stackexchange.com |
236,675 | [
"https://dba.stackexchange.com/questions/236675",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/2259/"
] | Suppose I have a stored procedure that is duplicated, with some modifications, in several databases. And I want to reference the database in which the stored procedure is stored, even if it is executed in another database.
Is there a way to retrieve the full path (..) or otherwise retrieve the database in which the s... | <blockquote>
I want to reference the database in which the stored procedure is stored, even if it is executed [from] another database.
</blockquote>
Just use one-part or two-part names in the stored procedure, and it will reference objects in the database containing the stored procedure. In particular,
For static ... | Here is a quick example that I put together showing common functions used to get close to what you are looking for.
<pre><code>/** Create a procedure in master to demonstrate
DB_NAME()
OBJECT_SCHEMA_NAME()
OBJECT_NAME()
@@PROCID
**/
USE [master]
GO
CREATE OR ALTER PROCEDURE dbo.uspTestMe
AS
BEGIN
... | https://dba.stackexchange.com |
176,349 | [
"https://dba.stackexchange.com/questions/176349",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/126818/"
] | my question is really simple, and does not need any particular documentation.
When I run <code>expdp</code> command like this using Oracle Database 11g Release 11.2.0.4.0 - 64bit:
<pre><code>expdp system/******* directory=DATA_PUMP_DIR dumpfile=SYSAUX.dmp logfile=sysaux.log TABLESPACES=SYSAUX
</code></pre>
I see that... | Depends on what tables you try to export. Built-in tables are not exported. User-created tables are exported.
<pre><code>SQL> create table system.t1 tablespace sysaux as select * from dba_objects;
Table created.
SQL> select count(*) from dba_segments where tablespace_name = 'SYSAUX';
COUNT(*)
----------
... | Actually SYS.AUD$ table CAN be exported but only using old export command exp instead of expdp.
<pre><code>exp \"/ as sysdba\" file=aud_table.dmp tables=sys.aud$;
</code></pre>
Since 11g or 10 (not sure) AUD$ table is prohibited to be exported.
The most convenient way is what @Balazs Papp suggested, to create table ... | https://dba.stackexchange.com |
437,643 | [
"https://mathoverflow.net/questions/437643",
"https://mathoverflow.net",
"https://mathoverflow.net/users/95347/"
] | Suppose there exists a transitive model of <span class="math-container">$\sf ZFC$</span>. Is it the case that every consistent theory that extends <span class="math-container">$\sf ZF$</span> must have a model that is an element of the minimal transitive model of <span class="math-container">$\sf ZFC$</span>?
| The answer is no, because by the Gödel-Rosser theorem, there are continuum many consistent completions of ZF, but the minimal transitive model of ZFC is countable, and so has only countably many theories. So some of the consistent extensions are not realized in that model.
Another argument is simply this: the theory of... | As Joel said, most of the consistent extensions <span class="math-container">$T$</span> of ZFC are not in the minimal transitive model <span class="math-container">$M$</span> of ZFC and therefore don't have models in <span class="math-container">$M$</span>. It seems worth noting that this is the <strong>only</strong> r... | https://mathoverflow.net |
197,803 | [
"https://stats.stackexchange.com/questions/197803",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/105444/"
] | I'm just a bit confused that when you convert an ARMA process to either a pure AR or MA representation then is it always an AR or MA infinite process that we obtain? For instance an ARMA(1,1), does it have an MA and AR infinite representation?
| Lets restrict attention to $ARMA(1,1)$ processes for the moment. I.e., we consider a process
\begin{align}
y_t - ay_{t-1} = y_t(1-aL) = e_t(1-bL) = e_t - be_{t-1}
\end{align}
Where $e_t\overset{iid}{\sim}WN(0,\sigma^2)$ and $a,b \in \mathbb{R}$. Here, $WN$ means 'White noise', i.e. we leave the distribution unspecified... | Any ARMA(p,q) process has an AR or MA infinite process, provided the stationary condition holds. The main purpose of this conversion is in theoretical studies. For example, it is much much easier to find the variance of an ARMA(p,q) process by a conversion to an MA($\infty$) process.
| https://stats.stackexchange.com |
15,913 | [
"https://electronics.stackexchange.com/questions/15913",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/4245/"
] | I need to get a heat gun so I can do some SMT work. What wattage / heat range should I be looking for?
Obviously it needs to get hot enough to melt the solder (preferably lead-free) yet not fry the components. I have seen everything from about 1400W up to 3400W with max temperature ranges anywhere from 350 to 650°C
... | What you are asking for is specific to soldering. It's not called a "heat gun", but a "hot air soldering station".
At a minimum, it must have a way to set the output air temperature over the normal soldering range. This should be calibrated somehow, like a degF or degC reading, not just a warm/hot dial. Having air ... | Power is not as important as temperature control.
You need it to be able to stabilize air temperature at 250-300-350C.
Power might be from 200W and up, as long as it is temperature controlled.
Also ability to control air speed is nice (too much air will blow stuff away).
| https://electronics.stackexchange.com |
3,425,790 | [
"https://math.stackexchange.com/questions/3425790",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/371243/"
] | I have this sketch of a proof, but I am unsure about it.
Problem:
Let <span class="math-container">$f,g \in L^1(\mathbb{R}^n)$</span>, where they have compact support. Show that <span class="math-container">$f * g (x) = \int f(x-y)g(y) \mathrm{d} y$</span> has compact support.
For <span class="math-container">$\int... | Your idea is good but needs some finishing touches.
Indeed, for <span class="math-container">$f* g(x)$</span> to be non-zero, you have to be able to find some <span class="math-container">$y\in \mathbb{R}$</span> such that <span class="math-container">$f(x-y)\neq 0$</span> and <span class="math-container">$g(y)\neq 0$... | We can choose <span class="math-container">$r>0$</span> such that the supports of <span class="math-container">$f,g$</span> are both contained in <span class="math-container">$B(0,r).$</span> The support of <span class="math-container">$f(x-y)$</span> is then contained in <span class="math-container">$B(x,r).$</span... | https://math.stackexchange.com |
308,986 | [
"https://electronics.stackexchange.com/questions/308986",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/140421/"
] | Take this example:
<pre><code>port(clk : in std_logic;
areset_n : in std_logic;
ena : in std_logic;
load : in std_logic;
load_val : in unsigned(cnt_width-1 downto 0) := (others => '0');
counter_value : out unsigned(cnt_width-1 downto 0) := (others => '0') ... | Do not use initial values in synthesizable VHDL i.e. VHDL which creates a logic circuit in your target FPGA/CPLD/ASIC. Use a reset term, controlled by the reset input you have.
This makes your design far more portable other devices and lets you drop in other IP more easily. In my experience, the vast majority of IP you... | Tt is possible to initialize registers, wires can not be initialized because they are simple connections to registers (which, most of the time, need to be initialized).
When RAM-based FPGA configures (e.g. Altera Cyclone series), it initializes registers to 0 (to my knowledge), thus it is a task of designer to ensure ... | https://electronics.stackexchange.com |
8,853 | [
"https://mathoverflow.net/questions/8853",
"https://mathoverflow.net",
"https://mathoverflow.net/users/-1/"
] | In logic modules, theorems like Soundness and completeness of first order logic are proved. Later, Godel's incompleteness theorem is proved. May I ask what are assumed at the metalevel to prove such statements? It seems to me that whatever is assumed at the metalevel should not be more than whatever is being formulated... | It depends on what you're trying to prove, and for what purpose you are proving these metatheorems.
So, the notion of "more" you're appealing to in asking about the metalevel is not completely well-defined. One common definition of the strength of a logic is "proof-theoretic strength", which basically amounts to the ... | Here is some more information for the first question. I think that to prove the meta theorems in mathematics, in particular, soundness, completeness, incompleteness, heuristic logic does not exhaust the meta principles being used. Some meta principles relating to heuristic set theory or heuristic category theory must t... | https://mathoverflow.net |
676,994 | [
"https://physics.stackexchange.com/questions/676994",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/297949/"
] | I was reading about the gaseous state when this question struck my mind: What made us assume that, at every point inside the container, a gas exerts equal pressure? When one brings a barometer, is it true it measures the same pressure at every point inside? Is this applicable to both ideal and real gases?
| An imbalance of pressure would itself cause an internal flow in the gas. So if the gas has reached equilibrium the pressure must be the same everywhere.
The above is for a gas in ordinary circumstances, without any applied field such as a gravitational field. If there is such a field then the gas flows until the pressu... | It depends on the resolution of your measuring device.
A gas contains on the order of <span class="math-container">$10^{22}$</span> molecules zipping about. The pressure on a wall of the container is due to the tiny force applied by these molecules when they collide with the wall. If you could take a snapshot of each o... | https://physics.stackexchange.com |
68,694 | [
"https://softwareengineering.stackexchange.com/questions/68694",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/23071/"
] | Programmers, like myself, often is too worried about if their code is really going to work, rather than how to communicate with the users. Scared of throwing an exception at the user, we test with different scenarios in different environment like development, staging and production.
But when it comes to deliver a sim... | I think you might be generalizing about what typical programmers do. Most programs I use and the ones our team creates have useful exception messages for users.
To create useful messages, you have to put yourself in the user's shoes which can be difficult, but should be something you are doing whenever you are writ... | You should focus on being a good programmer and use resource/string files for messages like that. The proper business group can then update those strings and turn the message into something more user-friendly.
| https://softwareengineering.stackexchange.com |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.