qid int64 1 4.65M | metadata listlengths 3 3 | prompt stringlengths 31 25.8k | chosen stringlengths 17 28.2k | rejected stringlengths 19 40.5k | domain stringclasses 28
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24,166 | [
"https://electronics.stackexchange.com/questions/24166",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/1870/"
] | I've just gotten through replacing capacitors on a trio of dead LCD screens (nothing's blown up yet, so far) - they either had one or two capacitors on their inverter circuit SLIGHTLY bloated, and not quite leaky. I ended up replacing all capacitors of the same brand/'colour', even the ones that looked fine, in case.
... | Charge thru a resistor to the working voltage. Choose a resistor so RC (where R is the resistance, C is the capacitance, and RC is the time constant) is workably large. The final voltage should equal the applied voltage - IR, where I is the leakage current. The rate of charge will give you C ( if I is large you will... | Use a DMM with capacitance measuring. If you don't have one, try charging and discharging the cap through a resistor and measuring the voltage curve.
| https://electronics.stackexchange.com |
191,790 | [
"https://softwareengineering.stackexchange.com/questions/191790",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/35897/"
] | I'm creating a model where one element can contain other elements. I'm currently writing the interface to the model, and I'm not sure which is the best of the two options:
<strong>Option 1</strong>
<pre><code>interface Container {
List<Element> getElements(); // returns an unmodifiable list.
void addElement... | It's actually a question of "Who manages the list".
In Option 1, you provide the container with the elements, and keep it its responsibility to manage them: The order, maybe reject some elements, etc.
With Option 2, the container is a dumb container. It gets the list from someone else, and has no say as to what's in ... | I generally prefer <strong>Option 3</strong>:
<pre><code>interface Container {
MutableList<Element> getElements();
}
</code></pre>
This is the pattern used by most API's that are written in the .NET framework. It keeps the API as simple as possible -- particularly in .NET where you have property getters as in... | https://softwareengineering.stackexchange.com |
99,266 | [
"https://physics.stackexchange.com/questions/99266",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/36806/"
] | A rigid air tight container of volume 1.5 m$^3$ is filled with a gas of density 8 kg/m$^3$. The container has a density meter that allows us to watch the changes in density in the gas in the container. Inside the container there is an rubber ball (example, spherical balloon) containing a different gas. We don't know ... | <ol>
<li>In your 5th "Let" statement "$\frac {d\rho_c}{ dt} = 0.1kg/m^3/sec$ the rate pressure changes in the large container", "pressure" should be "density" and there is a sign error.</li>
<li>Your equations 1-4 are correct, but it is important to determine what is known and what is unknown and analyze how many indep... | We start from point (5) above where things started to go wrong:
$\frac {dV_c}{ dt}= - \frac {dV_b}{ dt} \tag{5}$
$\frac {dV_c} {dt} = m_c / (\frac {d\rho_c}{dt} ) = - \frac {m_c } {0.1} \tag{6}$
$\begin{align} \int_{V_c,0}^{V_c,1} {dV_c} = - \frac {m_c } {0.1} \int_0^t dt \tag{7} \end{align}$
$\begin{align} V... | https://physics.stackexchange.com |
134,608 | [
"https://security.stackexchange.com/questions/134608",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/11178/"
] | <h1>Background</h1>
<em>The Windows 10 update model</em>
Windows 10 is used mainly on PCs.
Microsoft sells Windows 10 licenses. You buy a license, and then you can install Windows 10 on your PC. Then you can get security updates directly from Microsoft. Microsoft promises that they will continue to provide security ... | <blockquote>
Why doesn't Google distribute Android firmware images with lots of included drivers, ...
</blockquote>
This means that Google would need to get access to the drivers in the first place and not only by binary but also by source. Only this way they could keep the drivers working when major changes to the ... | From an InfoSec perspective, I can think of one major reason:
<h1>Vulnerable Code</h1>
If stock Android were to contain all drivers needed, that would mean shipping with a lot of 3rd-party drivers from a lot of vendors. Disregarding file-size issues, with all the different vendors and their various quality levels, th... | https://security.stackexchange.com |
12,701 | [
"https://robotics.stackexchange.com/questions/12701",
"https://robotics.stackexchange.com",
"https://robotics.stackexchange.com/users/17373/"
] | I hope you can help me and this is the right forum to ask.
In the process of building and programming my own Quadcopter, I encountered the term Euler angles. I took some time to understand them and then wondered why they are used in multicopter systems.
In my understanding Euler angles are used to rotate a point or v... | Euler angle can be used to describe the orientation of the quadcopter relative to the local level surface and relative to a azimuth reference (typically true north). The roll ($\phi$) and pitch ($\theta$) angles describe the tilt of the quadrotor relative to the local vertical; i.e., relative to the gravity vector. Th... | Your own equations make a lot of assumptions about the problem you are trying to solve. They are sufficient to solve your problem, but will not work for certain conditions (z = 0 for instance).
Euler angles are similar. A way describe orientation (we typically use orientation to mean all angles, not just pitch and ro... | https://robotics.stackexchange.com |
548,756 | [
"https://physics.stackexchange.com/questions/548756",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/237691/"
] | I am reading about solid state physics, and I think I got right the concept of crystal lattice. We first define a Bravais lattice as the set of vectors spanned by <span class="math-container">$\{\vec{a}_1,\vec{a}_2,\vec{a}_3 \}$</span> over the integers, i.e. vector of the form <span class="math-container">$$R=n_1\vec{... | Your understanding that the reciprocal lattice is only defined for a Bravais lattice is correct. When you have a so-called <em>lattice with a basis</em>, you would calculate the reciprocal lattice primitive vectors using only the primitive vectors of the underlying Bravais lattice as:
<span class="math-container">\begi... | The reciprocal lattice definitely takes into account the basis for the Bravais Lattice. Take for example a 3-dimensional lattice with basis <span class="math-container">$\{ \vec{a}_1, \vec{a}_2, \vec{a}_3 \}$</span>. The reciprocal basis is then
<img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/a14700b... | https://physics.stackexchange.com |
615,999 | [
"https://physics.stackexchange.com/questions/615999",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/233550/"
] | A question from JEE Advanced 2k19 goes like this:
<blockquote>
Consider a spherical gaseous cloud of mass density <span class="math-container">$\rho(r)$</span> in free space where <span class="math-container">$r$</span> is the radial distance from its center. The gaseous cloud is made of particles of equal mass <span c... | "<span class="math-container">$\rho(r)$</span> is constant in time" does not imply that <span class="math-container">$\rho(r)$</span> is a constant function. What it means is that for any given <span class="math-container">$r_0$</span>, <span class="math-container">$\rho(r_0)$</span> does not change with time... | You write;"The total mass of the gas cloud should act as if all of it is concentrated at the center, right?" No, this is wrong.
In every sphere the gravitational force at some radius R is only dependent of the mass in r<R, the masses outside of r i.e, r>R do not contribute to the gravity at R.
So you ... | https://physics.stackexchange.com |
319,461 | [
"https://mathoverflow.net/questions/319461",
"https://mathoverflow.net",
"https://mathoverflow.net/users/8628/"
] | Is there a countable connected space <span class="math-container">$(X,\tau)$</span> such that for all <span class="math-container">$x\in X$</span> the space <span class="math-container">$X\setminus\{x\}$</span> is not connected any more with the induced subspace topology?
| Let <span class="math-container">$\mathbb{R}$</span> be with its usual topology, and let <span class="math-container">$f:\mathbb{R}\to \mathbb{Z}$</span> defined by:
<span class="math-container">$$
f(x)=\left\{\begin{matrix}
2k & x=2k,\text{ where } k\in\mathbb{Z}\\
2k+1 & 2k<x<2k+2,\text{ where } k\in\m... | The other answer describes the "Khalimsky line". It is not <span class="math-container">$T_1$</span>, but it is possible to obtain Hausdorff examples by starting with a countable connected Hausdorff space <span class="math-container">$X$</span>, blowing up its points into more copies of <span class="math-container">$X... | https://mathoverflow.net |
1,447,217 | [
"https://math.stackexchange.com/questions/1447217",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/5775/"
] | $$\lim _{x\to 1}\left(\frac{1-\sqrt[3]{4-3x}}{x-1}\right)$$
So
$$\frac{1-\sqrt[3]{4-3x}}{x-1} \cdot \frac{1+\sqrt[3]{4-3x}}{1+\sqrt[3]{4-3x}}$$
Then
$$\frac{1-(4-3x)}{(x-1)(1+\sqrt[3]{4-3x})}$$
That's
$$\frac{3\cdot \color{red}{(x-1)}}{\color{red}{(x-1)}(1+\sqrt[3]{4-3x})}$$
Finally
$$\frac{3}{(1+\sqrt[3]{4-3x}... | the problem is that you used
(a-b)(a+b) to simplify a cubic root instead of a simple root
| You mixed up square roots and cube roots:
$$(1-\sqrt[2]{a})*(1+\sqrt[2]{a})=1-a$$
$$(1-\sqrt[3]{a})*(1+\sqrt[3]{a})≠1-a$$
| https://math.stackexchange.com |
373,486 | [
"https://softwareengineering.stackexchange.com/questions/373486",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/303640/"
] | So I don't know if this is good or bad code design so I thought I better ask.
I frequently create methods that do data processing involving classes and I often do a lot of checks in the methods to make sure I don't get null references or other errors before hand.
For a very basic example:
<pre><code>// fields and pr... | The problem with your basic example isn't the null check, <strong>it's the silent fail</strong>.
Null pointer/reference errors, more often than not, are programmer errors. Programmer errors are often best dealt with by failing immediately and loudly. You have three general ways to deal with the problem in this situati... | Since <code>_someEntity</code> can be modified at any stage, then it makes sense to test it every time that <code>SetName</code> is called. After all, it could have changed since the last time that method was called. But be aware that the code in <code>SetName</code> isn't thread-safe, so you can perform that check in ... | https://softwareengineering.stackexchange.com |
41,417 | [
"https://mechanics.stackexchange.com/questions/41417",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/19042/"
] | I had the gearbox mount break in my Mk2 Citroen Saxo (one of three mounts holding the engine, gearbox etc up). The engine fell down due to the lack of mount, and the gearbox hit the floor (and cracked).
I will, of course, be replacing the gearbox and the gearbox mount, but should I replace the other engine mounts? I a... | In the same way that you don't technically "need" to replace tyres or shock absorbers in pairs, I would suggest it best practice to replace the other mounts. If one has worn to the point of failure, it will be presumably the same age as the others. Plus the rather dramatic failure plus the extra stress placed on the ... | You don't NEED to, but be aware that the other mounts tend to wear more quickly after 1 has been replaced. If you don't replace the others, keep an eye on them.
| https://mechanics.stackexchange.com |
603,859 | [
"https://math.stackexchange.com/questions/603859",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/26874/"
] | $$\lim_{x\to \infty}\frac{x^{2011}+2010^x}{-x^{2010}+2011^x}$$
I'm not sure where to even start with this one. One idea I had was that perhaps it could be "split" up as:
$$\lim_{x\to \infty}\frac{x^{2011}}{-x^{2010}+2011^x} + \lim_{x\to \infty}\frac{2010^x}{-x^{2010}+2011^x}$$
But solving this requires knowledge of ... | Here's one approach: divide the top and bottom by $2011^x$ to get
$$
\lim_{x\to \infty}\frac{x^{2011}+2010^x}{-x^{2010}+2011^x}
=
\lim_{x\to \infty}\frac{\frac{x^{2011}}{2011^x}+\left(\frac{2010}{2011}\right)^x}{-\frac{x^{2010}}{2011^x} + 1}
$$
Where can you go from there?
Also, for the record: for any positive numb... | We could begin by evaluating, for $\;x>0\;$
$$x^{2010}<2011^x\iff2010\log x<x\log2011$$
and the second inequality is clear for $
\;x\;$ big enough, so exponential rules over polynomial, and thus
$$\frac{x^{2010}}{-x^{2010}+2011^x}\xrightarrow[x\to\infty]{}0\\{}$$
$$\frac{2010^x}{-x^{2010}+2011^x}\xrightar... | https://math.stackexchange.com |
6,949 | [
"https://security.stackexchange.com/questions/6949",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/4730/"
] | I find that truly random diceware passphrase, more often than not, either contain a word that is easily misspelled or has an order that is illogical. I think there are three ways to make a diceware passphrase more memorable:
<ol>
<li>Throw out passphrases until you get one you can remember</li>
<li>Throw out individua... | <ol>
<li><em>Throw out passphrases until you get one you can remember</em> -
If you look at 16 passphrases, and keep the one you like best, then you've reduced your entropy by at most 4 bits (log<SUB>2</SUB> 16 = 4). The intuition is similar to what you gave for rearranging: the worst case is that there is logically ... | Entropy is about probabilities, so there are averages everywhere.
For method 1, if you throw away on average <em>n-1</em> passwords and keep the <em>n</em>-th, then entropy is divided by <em>n</em> (in bits, you use <em>log<sub>2</sub> n</em> bits). This assumes that the attacker can accurately model what is a "hard t... | https://security.stackexchange.com |
3,359 | [
"https://physics.stackexchange.com/questions/3359",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/1466/"
] | I did one module of physics for my GCSE one year ago which taught me about transverse EM waves & the EM spectrum, but since then, I do not understand how a wave would move in 3D space. Can someone show me some animation or something? I can understand it in 2D space (ie on a graph) but not 3D. I also read somewhere ... | Ell, I sympathise. You look at the graphs showing an oscillating electromagnetic wave and misread the transverse axes as spatial. So you think an electromagnetic wave is like the wave you can flick along a skipping rope. No it isn't.
The best way to think about it is that at <em>each point in space</em> where the wave... | Well most texts tell you that light is an EM wave and will give you an image of a sine wave and then they start to define things like frequency, amplitude etc., leaving you to think that EM waves are like that zig-zag thing on the graph. But actually in nature what oscillate are the fields. The E-field, i.e. a region a... | https://physics.stackexchange.com |
1,139,483 | [
"https://math.stackexchange.com/questions/1139483",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/199011/"
] | The sequence is $-1,2,-3,4,-5,...$ I understand that it will oscillate between arbitrarily large positive and negative values as $n \rightarrow \infty $.
How could I formalize that argument?
| Using @Sujaan Kunalan suggestion,
There are subsequences
$a_{2j} = 2j(-1)^{2j} = 2j \rightarrow +\infty$ when $n=2j \rightarrow \infty$
and
$a_{2j+1} = (2j+1) \cdot (-1)^{2j+1} = -(2j+1) \rightarrow -\infty$ when $n=2j+1 \rightarrow \infty$
The original sequence cannot converge and this could be concluded in two... | $|a_n-a_{n+1}|=2n+1\geq 1$. So the sequence isn't <em>Cauchy.</em>
Then you might also want to show the limit can't be $+\infty$ or $-\infty$.
| https://math.stackexchange.com |
260,900 | [
"https://physics.stackexchange.com/questions/260900",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/117789/"
] | Why does an ideal gas show only translatory motion and not vibratory motion?
| An ideal gas particle is considered to be a non-interacting point mass i.e. a zero-dimensional object. As such it has no internal degrees of freedom - it cannot rotate or vibrate. The only degrees of freedom it has are the three translational ones.
| By definition an ideal gas is a gas composed by non-interacting point particles moving randomly. It is not meant to represent a real gas, but a simplification of a gas with more complex interactions, such as vibrations and rotations. For example, most real gases at room temperature can be approximated by an ideal gas, ... | https://physics.stackexchange.com |
2,063,858 | [
"https://math.stackexchange.com/questions/2063858",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/297749/"
] | I'm trying to show
$$\int_0^1 f(x)\;dx=0$$ where $f(x)$ is $1$ on $\frac{1}{n}$, $n\in\mathbb{N}^+$, and zero everywhere else. Here is my attempt:
Proceed by induction. Consider the integral sum $\int_{1}^1 f(x)\;dx$. Then this is just zero. Now suppose $\int_{\frac{1}{n}}^1f(x)\;dx=0$. Then $\int_{\frac{1}{n+1}}^1f(... | As you suspect, using series and (Riemann) integrals is a touchy subject. Let me suggest a safer and shorter alternative.
You have correctly proven that for all $n \in \Bbb N \setminus \{0\}$
$$\int \limits _{\frac 1 n} ^1 f(x) \ \Bbb d x = 0 .$$
Keeping in mind that $f \le 1$ and that $\int _a ^b f \le \int _a ^b g... | Consider the function defined by
$$f_n(x)=\begin{cases}
1 & 0 \le x < \frac{1}{n}\\
f(x) & \frac{1}{n} \le x \le 1
\end{cases}$$
You have for all $x \in [0,1]$
$$0 \le f(x) \le f_n(x)$$
$f_n$ is a step function and
$$\int_0^1 f_n(x) \ dx =\frac{1}{n}$$ which enables to conclude that $f$ is Riemann integ... | https://math.stackexchange.com |
275,783 | [
"https://softwareengineering.stackexchange.com/questions/275783",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/170706/"
] | This is just not related to asp.net mvc but rather related all the mvc frameworks that support code first approach. One of the advantages I often see on asp.net mvc and tutorial blogs is that it supports code first approach. I'm work as a .net consultant and on the various projects( old and new) I have never seen anyon... | I always use code first and would never think about data first unless I have to integrate with an existing database. Why?
<ol>
<li>Instead of starting by figuring out what database looks like, I start by figuring out what my application does.</li>
<li>I don't have to reconcile my up-front database design with the func... | If a team has:
<ul>
<li>A DBA,</li>
<li>Or a developer who is even slightly knowledgeable about databases,</li>
</ul>
then the team will be inclined to avoid code-first approach. This is the same as using WYSIWYG editors when your team knows HTML: while the job <em>can</em> be done with a WYSIWYG, you still need to d... | https://softwareengineering.stackexchange.com |
215,310 | [
"https://security.stackexchange.com/questions/215310",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/199648/"
] | Let's say you're creating a banking web application with mobile apps. A bank user has permissions "transfer money" and "view balances." When a user logs in, we can create a session token (typical random string, stored in the database on the server and in a cookie or device storage on the client) with "view balances" pe... | Why not just store multiple expiration times server side? You already need server-side expiration time if you have a token that is not signed. Just add a separate expiration time for each capability and check that one on action.
For performamce reasons, you may or may not want a separate expiration time for the entir... | I have come across similar approach in complex applications which are microservice based. If different teams handle these services, having different session ID's tied to different capabilities can be a way forward. However it is not an elegant solution. The ideal approach should be to use the session for identification... | https://security.stackexchange.com |
137,001 | [
"https://mathoverflow.net/questions/137001",
"https://mathoverflow.net",
"https://mathoverflow.net/users/19593/"
] | Does every contractively complemented subspace of $c_0(I)$ is isometric to $c_0(J)$ for some $J\subseteq I$?
May be someone has a counterexample?
| Yes, it is true, but I don't know where it is proved in the literature. The reason I say yes is that what you want is a corollary of a unpublished almost isometric theorem that Zippin proved in the 1970s! Here is either an outline of a proof or complete nonsense: Let $P$ be a contractive projection on $c_0(I)$. Then ... | As you may know already, every contractively complemented subspace of $\ell_1(I)$ is isometric to $\ell_1(J)$ for some $J \subset I$ --- see for example Semadeni, Banach Spaces of Continuous Functions, 1971, p. 488. This is just the dual of your desired proposition. If $X$ is a contractively complemented subspace of ... | https://mathoverflow.net |
21,621 | [
"https://dsp.stackexchange.com/questions/21621",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/13742/"
] | I am trying to answer the following question:
Is the system described by equation:
$$y[n]=0.5y[n-1]+x[n]-0.5x[n-1]$$
an <strong>IIR</strong> filter? My answer is yes.
Thank you
| This is the <strong>FIR</strong> filter, although it looks like an IIR. If you calculate the coefficients you get finite impulse response:
$h=[1]$
This happens due to zero-pole cancellation:
$Y(z)-0.5Y(z)z^{-1}=X(z)-0.5X(z)z^{-1}$
$H(z)=\dfrac{Y(z)}{X(z)}=\dfrac{1-0.5z^{-1}}{1-0.5z^{-1}}=1 $
Yes, it can be tricky.... | Jojek's answer is of course correct. I would just like to add some more information because much too often have I seen the terms "IIR" and "recursive" confused. The following implications always hold:
$$\begin{align}\text{IIR}& \Longrightarrow\text{recursive}\\
\text{non-recursive}&\Longrightarrow\text{FIR}\en... | https://dsp.stackexchange.com |
84,739 | [
"https://dsp.stackexchange.com/questions/84739",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/64689/"
] | What does convolving an image with the filter [1 -2 1] do? I see that it's a form of edge detection but it's also very similar to the results I get when convolving with the derivative kernel [1 -1] so I don't really see the difference between them
| Both are high pass filters. Filtering with [1 -2 1] is the same as filtering with [1 -1] twice. So the longer kernel is a steeper high pass filter and emphasize the edges more.
| Convolution with the <code>[1 -1]</code> kernel (A) is a derivative filter, which computes the finite difference approximation to the first derivative.
The <code>[1 -2 1]</code> kernel (B) is the result of convolving kernel A with itself. Thus, convolving an image with kernel A twice is the same as convolving with kern... | https://dsp.stackexchange.com |
158,137 | [
"https://electronics.stackexchange.com/questions/158137",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/43242/"
] | i would love to start learning AVR programing for Atmel's chips. Currently i'm using only Arduinos with their standard API and im really happy with that, however i would like to learn more and go even deeper. I believe it would be usefull to have a basic knowledge of how things work.
I found some videos on YouTube an... | The next logical step in your learning is not to jump straight into low level programming of raw chips, but to do low level programming of the Arduino.
Just because the Arduino provides a high level API it doesn't mean that you have to use it.
You can program an Arduino as if it were a plain Atmel chip - access the r... | Please mind that your 'Question' is pretty open.
The only real question I spotted in it was: "Should I start with AVR or PIC".
I think that, with the knowledge of Arduino, it's better to go with AVR.
But then again, this can vary on opinion.
You might want to check some tutorials on how to do the (important) basics.
M... | https://electronics.stackexchange.com |
424,587 | [
"https://physics.stackexchange.com/questions/424587",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/204799/"
] | In Peskin & Schroeder Ch. 2, p. 14, in proving that the NRQM propagation amplitude for a free particle is nonzero everywhere, they move from
\begin{equation}
U(t)~=~ \frac{1}{(2\pi)^3} \int d^3p \hspace{2pt} e^{-i(\mathbf{p}^2/2m)t} \cdot e^{i\mathbf{p}\cdot(\mathbf{x} - \mathbf{x_0})}
\end{equation}
to the end res... | You seem to think that
\begin{equation}
\frac{1}{(2\pi)^2i\Delta x} \int_{0}^{\infty}dp \hspace{2pt}p\hspace{2pt}e^{-i(\mathbf{p}^2/2m)t} \left( e^{-ip\Delta x} - e^{ip\Delta x} \right) = 0
\end{equation}
probably because the exponential functions kinda look like they cancel, but this is not so.
Note that
$$ e^{-i... | Note the integral of an arbitrary Gaussian function,
\begin{equation}
\int_{-\infty}^{\infty} e^{-ax^2 + bx} dx = \sqrt{\frac{\pi}{a}} ~e^{\frac{b^2}{4a}}
\end{equation}
| https://physics.stackexchange.com |
75,588 | [
"https://electronics.stackexchange.com/questions/75588",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/23393/"
] | I often read/hear that "my device is burnt" or "... component is burnt due to over voltage" or ".. ESD (board) is burnt when I hold it" etc.
I wonder why "burn" is used to describe it? Is something really burns or is it just a term to describe that the component/device cannot be used anymore as something burns?
| In English, "burnt" is often used loosely to indicate a part got damaged by something excessive done to it, not necessarily actual combustion. It can also be said that a part is "toast", "got fried", "smoked", and a few others. These expressions are not meant to be taken literally.
In most cases, damage to electroni... | Although "burnt" can definitely be used with there's just a soft failure (ESD or something that is invisible or under a plastic application), the device can CERTAINLY burn. As in, crater inside, explode and set fire to things.
| https://electronics.stackexchange.com |
293,767 | [
"https://physics.stackexchange.com/questions/293767",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/106747/"
] | Consider a cell having terminal voltage $V'$ and EMF, $V$ having internal resistance, $r$ and current, $I$ flows through the circuit in which an external resistance, $R$ is connected.
Potential drop across internal resistance= Ir
$$V= V' + Ir = IR + Ir$$
$$V=I(R+r)$$
Now if the cell is in an open circuit then $I=0$... | <em>Note that: The electromotive force (emf) is the potential difference of a source when no current is flowing.</em> EMF has nothing to do with whether the terminals are connected to form a circuit or not. EMF is an inherent characteristic of a cell due to certain chemical reactions necessary for the driving of electr... | You noted that I is 0 for open circuit but didn't consider the resistance which is really high.
So going further.
V=I(r+R)
For R>>r
V=IR
The current is 0 when reistance is infinite. However they are limits so lets consider I slightly greater than 0 and V slightly lesser than infinity. Simply, I here is given by I=V/R ... | https://physics.stackexchange.com |
295,935 | [
"https://physics.stackexchange.com/questions/295935",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/111390/"
] | My Textbook wrote the Kinetic Energy while teaching Hamiltonian like this:
(in Cylindrical coordinates)
$$T=\frac{m}{2}[(\dot{\rho})^2 +(\rho \dot{\phi} )^2+ (\dot{z})^2]$$
I know to find velocity in Cartesian coordinates
$$position = x+y+z$$
$$velocity = \dot{x}+\dot{y}+\dot{z}$$
But in this case, even though the c... | Imagine an object which it a fixed distance $\rho$ from the origin in the xy-plane (z=0).
The angle between the x-axis and the line joining the origin to the object is $\phi$.
Now change the angle to $\phi + \Delta \phi$ in a time $\Delta t$
Using your notation the <strong>angular</strong> speed $\phi^* = \frac {\... | Kinetic energy is always $\frac{1}{2} m \dot{\vec{r}}^2$. What is important here is the vector sign for the position vector $\vec{r}$. In cartesian coordinates, $\vec{r}=x\vec{e_x}+y\vec{e_y}+z\vec{e_z}$ and therefore $\dot{\vec{r}}=\dot{x}\vec{e_x}+\dot{y}\vec{e_y}+\dot{z}\vec{e_z}$. Here,$\vec{e_x},\vec{e_y},\vec{e_z... | https://physics.stackexchange.com |
213,085 | [
"https://softwareengineering.stackexchange.com/questions/213085",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/56275/"
] | I understand that ssh keys are a cryptographically secured way of authenticating yourself to a service or to another person, but beyond that I'm frankly at a bit of a loss.
What confuses me is that I see many developers mention their public key on their site, but how does one have just one key? I have a few for differ... | When you generated your key pair, you created 2 keys: a private and a public (often id_rsa and id_rsa.pub).
The main point to remember is that nobody should have access to your private key apart from you. If it does fall into the hands of someone else, then you should remove your public key from any system that uses i... | I normally keep my key in the .ssh directory where is was generated. I have backups of my home directory that included the key.
When password protected, the keys become part of a two factor authentication. The password is something you know, while the key (or the device that holds it) is something you have. Without... | https://softwareengineering.stackexchange.com |
198,618 | [
"https://mathoverflow.net/questions/198618",
"https://mathoverflow.net",
"https://mathoverflow.net/users/68519/"
] | A regular embedding of a connected reductive linear algebraic group $G$ defined over $\mathbb{F}_q$ is a morphism $\varphi : G \rightarrow G'$ of algebraic groups which is a closed immersion where $G'$ is a connected reductive linear algebraic group defined over $\mathbb{F}_q$ with connected centre and $\varphi(G)$ con... | I had cause to think about this exercise recently so I thought I’d write an answer. I think Jim’s answer is sufficient but as you seem to want more details I’ll provide them here. I am aware that you know some of the following but I’m writing a complete solution for the benefit of future readers.
I’ll use a slight gen... | Leaving aside your minor changes in the notation and definition of Cabanes-Enguehard (who also adopt some arbitrary notation), my understanding of their exercise is that it responds to the obvious non-uniqueness in the target group.
Of course, the whole problem originates in embeddings like the one of a special linear ... | https://mathoverflow.net |
192,318 | [
"https://physics.stackexchange.com/questions/192318",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/84966/"
] | I am just 16 and curious to learn about Theory of Relativity. Can any one explain it simple enough for me to understand? I read that it is bending of time-space or space-time that causes gravity. How can bending of time takes place?
| When we use the terms "bending" or "warping" with respect to spacetime and gravity, you have to keep in mind that these words are not being used in a literal way. Since the majority of concepts in General Relativity are far beyond what our experiences allow us to comprehend, we have come up with a few ways of picturing... | I'll go into a bit more detail below but, basically, Einstein discovered that space and time can't be separated. They are part of the same thing, and gravity is actually produced be bending and warping this "spacetime," like a heavy ball sitting on a sheet of elastic.
<h1>Classical <em>(or Newtonian)</em> Physics</h1... | https://physics.stackexchange.com |
112,029 | [
"https://mathoverflow.net/questions/112029",
"https://mathoverflow.net",
"https://mathoverflow.net/users/26077/"
] | Is it possible to mathematically prove that the speed of gravitational waves in general relativity equals the speed of light, without linearizing the Einstein field equations? The approach via the linearization of the EFE's, which is used in many books on relativity, does not seem to provide an exact proof that disturb... | What is the speed of a wave in a non-linear theory? Answering before considering your question is important, because that answer will tell you where to look for your answer.
A useful notion is that of <em>domain of dependence</em> (see for example a decent book on GR for a detailed discussion, e.g., Wald or Hawking &a... | The question asks whether it is possible to mathematically prove the speed of propagation of gravity waves without linearization.
It does not yet appear possible. Please let me explain.
Einstein's GR postulates the equality of a curvature tensor <span class="math-container">$R_{ij}-R g_{ij}$</span> with a mass-energy q... | https://mathoverflow.net |
300,579 | [
"https://electronics.stackexchange.com/questions/300579",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/146788/"
] | Suppose In a 5-inputs XNOR gate, the inputs are 0,1,x,0,and 1. how can I determined the output is?
| Figured it out! I used the system clock (F_osc/4 in the block diagram) instead of the LP oscillator itself since I set the system frequency to 32.768kHz:
<pre><code>T1CONbits.TMR1CS = 0; // clock source = F_osc/4
</code></pre>
I'm not sure if it's a noob mistake to use the LP oscillator for Timer1 when the system ... | 82 pF on a 33 kHz crystal is waaaay too much. <b>Read the datasheet</b> of the crystal.
| https://electronics.stackexchange.com |
680,507 | [
"https://math.stackexchange.com/questions/680507",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/127778/"
] | How many different ways are there to color the objects $a_1 , a_2 , ... , a_n$ ($n > 3$) using 3 colors if every color must be used at least once?
I thought it was $3^n - 6$ to account for the 6 cases that you have to disregard (all the same color, all two colors).
| There are two interpretations: (i) The <span class="math-container">$n$</span> objects are distinct and (ii) They are identical.
(i) Let the colours be <span class="math-container">$A,B,C$</span>. A colouring can be viewed as a function from the set <span class="math-container">$\{a_1,a_2,\dots,a_n\}$</span> to the set... | A colouring can be seen as a $n$-lettered string created from the three letters $r$, $g$, $b$ (red, green, blue) containing at least one $r$, one $g$ and and one $b$. The correspondent exponential generating function is thus
$$f(x)=\left(x^1+\frac{1}{2!} x^2 + \frac{1}{3!} x^3 + \ldots \right)^3 $$
and so the answer ... | https://math.stackexchange.com |
78,542 | [
"https://datascience.stackexchange.com/questions/78542",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/101581/"
] | What do we take into consideration while deciding which technique should be used when dealing with a particular dataset? I understand that there isn't any hard and fast rule to this. Do we use XGBoost only when there are a lot of features in the dataset and RandomForest for otherwise cases? Or are we suppose to hit and... | Decision Tree is very useful if you want to be able to explain where your result comes from you can often print the tree and see how your model came to this answer.
Random Forest can also provide such information, but you'll have to browse all trees and make some "stats" into them, which is not as easy. But R... | There are quite a few reasons to choose a specific ml algorithm for a given dataset, e.g based on the problem you are trying to solve for example if it is a classification or a regression problem.
Learning about different algorithms and common problems that they are used for and if they are suitable to work under certa... | https://datascience.stackexchange.com |
44,912 | [
"https://security.stackexchange.com/questions/44912",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/32789/"
] | Why do some websites (especially banking sites) display more characters in the password field than were actually typed? This is sometimes apparent just after clicking the login button.
| Some online banks, and some other web services, clear the password field and/or replace with a fixed number of characters so that if (for any reason) the login form submission failed or the user double clicked on the <kbd>Login</kbd> button then he won't be able to make the same submission again. The "rationale" behind... | Another reason password entry fields (not limited to banks' web sites; I've seen several native PC applications, notably Lotus Notes, do this) are sometimes filled with random numbers of characters is to prevent anyone from shoulder-surfing the <em>length</em> of one's password and using that information to narrow the ... | https://security.stackexchange.com |
120,488 | [
"https://dba.stackexchange.com/questions/120488",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/47866/"
] | I need to determine the dates which are the "3rd Friday of each month" for a date range of "1.1.1996 - 30.8.2014" in SQL Server.
I expect I should use a combination of <code>DENSE_RANK()</code> and <code>PARTITION BY()</code> to set "rank = 3". However, I am new to SQL and unable to find the correct code.
| Given:
<ul>
<li>Friday is called "Friday" </li>
<li>The 3rd Friday of the month will always
fall from 15th-21st of the month
<pre><code>select thedate
from yourtable
where datename(weekday, thedate) = 'Friday'
and datepart(day, thedate)>=15 and datepart(day, thedate)<=21;
</code></pre></li>
</ul>
You could als... | In order to get a language/culture independent answer, you need to account for different weekday names and start of the week.
In Italy, Friday is "Venerdì" and the fisrt day of the week is Monday, not Sunday as in US.
<code>1900-01-01</code> was a monday, so we can use this information to calculate the weekday in a ... | https://dba.stackexchange.com |
55,887 | [
"https://physics.stackexchange.com/questions/55887",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/21235/"
] | Let's say we have a normal circuit with a light bulb, with wires and a battery.
When one places a capacitor in this circuit, how is the light bulb able to light up, even when the capacitor prevents the flow of charge? Also, why does it dim and then go out eventually?
Then when the battery is removed from this circu... | First, note that the light bulb is essentially just a glorified resistor. As current flows through the filament, Joule heating causes the filament to get hot and emit light.
When one places a capacitor in a circuit containing a light bulb and a battery, the capacitor will initially charge up, and as this charging up ... | This question is straight out of chapter 20 of the third edition of Matter & Interactions by Chabay and Sherwood (Wiley, 2010). The deep, underlying issue here is that for the first time ever that I'm aware of, an introductory physics textbook correctly teaches that a capacitor's fringe field must not be neglected!... | https://physics.stackexchange.com |
23,407 | [
"https://cs.stackexchange.com/questions/23407",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/16431/"
] | Let A = <span class="math-container">$(Q, \Sigma, \delta, S, F)$</span> be a deterministic finite automaton associated with the language
<span class="math-container">$L \subseteq \Sigma^*$</span>
<blockquote>
<span class="math-container">$L' = \{y \in \Sigma^*:\exists x\in L. |x| = |y|\}$</span>
<span class="math-conta... | Hint: Replace every transition labelled $a \in \Sigma$ with a transition labelled $\Sigma$.
| Here is a proof using the closure properties of regular languages. Let $\pi: \Sigma^* \to a^*$ be the monoid homomorphism defined by $\pi(u) = a^{|u|}$. I let you verify that
$L' = \pi^{-1}(\pi(L))$. Since regular languages are closed under homomorphisms and inverses of homomorphisms, $L'$ is regular.
| https://cs.stackexchange.com |
217,580 | [
"https://mathoverflow.net/questions/217580",
"https://mathoverflow.net",
"https://mathoverflow.net/users/79963/"
] | Consider a compact manifold $M$ with boundary and corner. As an example, we could have the cube $\{(x_1, x_2,..x_n) \in \mathbb{R}^n : x_i \in [0,1]\}$. We could very well define the Laplacian $\Delta$ on such an $M$ with Dirichlet or Neumann boundary conditions. In such situations, are the eigenfunctions of the Laplac... | There is an extensive literature on elliptic problems on domains with corners. Grisvard's book (Elliptic Problems in Nonsmooth Domains) is a good place to start.
| Let me add some other references about elliptic regularity on singular manifolds:
1) B.-W. Schulze and his co-authors have developed a comprehensive calculus for elliptic pseudo-differential operators (in particular differential operators) on singular spaces, including elliptic regularity and Fredholm properties:
1.1... | https://mathoverflow.net |
743,414 | [
"https://physics.stackexchange.com/questions/743414",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/320629/"
] | <blockquote>
Exercise 1:
In frame <span class="math-container">$S$</span>, event <span class="math-container">$B$</span> occurs <span class="math-container">$2 \cdot 10^{-6} \mathrm{ s}$</span> after event <span class="math-container">$A$</span> and at <span class="math-container">$\Delta x = 1.5 \mathrm{ km}$</span> ... | The exercises are actually way different. Notice that in the first one, there is a frame of reference in which both events happen at the same <em>time</em>. Hence, they are spacelike related. In the second exercise, you are told there is a frame of reference in which both events happen at the same <em>location</em>, an... | Remember that you can use also another Lorentz equation. In this case, we are looking at a frame in which two events happen at the same point, not at the same time. Therefore, we must have <span class="math-container">$\Delta x'=0$</span> and thus <span class="math-container">$v=\Delta x/\Delta t$</span>.
| https://physics.stackexchange.com |
595,126 | [
"https://math.stackexchange.com/questions/595126",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/113813/"
] | Can someone help me construct this proof using strong induction?
<blockquote>
Use strong induction on $l$ to show that for all $l \geq 1$, a full
binary tree with $l$ leaves has $2l-1$ vertices total.
</blockquote>
| "Strong" induction is really not needed here. For the inductive step, just pick any pair of leaves with the same parent and consider the tree obtained by removing them both; it has $l - 1$ leaves and you can proceed by "weak" induction. I imagine the proof you're "supposed" to write involves splitting your tree at it... | Establish a basis: $l=1$ is a full binary tree isomorphic to $K_1$ which has $2(1)-1=1$ vertices.
Induction Hypothesis: Fix a $k$ and assume for all $l<k$ we have $2l-1$ vertices on a full binary tree with $l$ leaves.
Induction: Now consider the case when $l=k$. Suppose I have a graph $G$ that is a full binary tre... | https://math.stackexchange.com |
34,136 | [
"https://dba.stackexchange.com/questions/34136",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/18810/"
] | I've got Postgres 9.2 and PgBouncer (The version that came with the 'stack builder'). I have a .net application connecting via Npgsql version 2.0.12 on a win7 machine.
My application can happily connect straight to the postgres server, but always fails to connect via the pgbouncer.
My connection string to npgsql (aft... | Ah! Darn!
I found the answer.
My connection string had SslMode=Prefer
I removed this, and voila! the pgBouncer worked!!
My connection string is now:
<pre><code>Timeout=40;Server={0};Port={3};User Id=someone;Password={1};Database={2};Pooling=False;Protocol=3;
</code></pre>
Thanks everyone for your time and effort.
| The userlist.txt file contains the accounts for connecting from your client <em>to</em> PgBouncer. The <code>[databases]</code> section contains the login information for connection <em>from</em> PgBouncer to the backend PostgreSQL server. You appear to be mixing these two up.
When you connect from your client to Pg... | https://dba.stackexchange.com |
141,545 | [
"https://security.stackexchange.com/questions/141545",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/129390/"
] | Two input authentication uses both username (may be available publicly) and password (kept secret).
For the sake of comparison, assume the length of username is the same as the length of password, i.e., <code>n</code> characters. Also assume we can only use case insensitive letters from a to z. If both username and pa... | When signing up for a service, you have a good chance of getting "This name is already in use, choose another" - or something to that effect.
In the system you propose, this would tell you that the access code is in use - great, open a new browser and log in with this access code! You've just hijacked somebody else's ... | User name is an <em>identifier</em>, a label that indicates which user you are, and identifies which resources belong to you (or refer to you).
Password is an <em>authenticator</em>, a way of proving that you are permitted to assume that user identity.
User names can't be secret, as an information system needs this k... | https://security.stackexchange.com |
337,749 | [
"https://softwareengineering.stackexchange.com/questions/337749",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/198652/"
] | <blockquote>
The first 90 percent of the code accounts for the first 90 percent of
the development time. The remaining 10 percent of the code accounts
for the other 90 percent of the development time.
— Tom Cargill, Bell Labs
</blockquote>
What does that exactly mean in practice? That programers do substan... | Imagine it like this: When you start working on software you can write huge amounts of code in relatively short time. This new code can add huge amount of new functionality. The problem is that, often, that functionality is far from "done", there might be bugs, small changes (small in business small) and so on. So the ... | It is a reference to a common scenario, that sadly still occurs today:
<ol>
<li>The team is asked to estimate (ie guess) the amount of work needed to write all the code,</li>
<li>The project proceeds with numerous internal and external pressures to "stay on time and budget",</li>
<li>So for a significant percentage of... | https://softwareengineering.stackexchange.com |
106,390 | [
"https://physics.stackexchange.com/questions/106390",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/-1/"
] | A staple of science fiction and fantasy is a blade (knife, sword, ...) that cuts through literally any solid object (wood, steel, concrete, skulls, ...) without effort, often even without the need to apply any pressure.
What (in theory) would be required to construct such a blade? Would, say, a diamond wedge that is o... | Pressure is a measure of the force applied to a given area. Blades are sharp because they have a <strong>small cross sectional area</strong>, allowing you to create very high pressure whilst applying only a modest force. This force generates so much of <strong>shear stress</strong> on the object getting cut that it cru... | A blade must be near-atomically thin to generate immense local pressures to cleave "anything." The edge must also be immensely stiff to avoid contact deformation. The brittle edge could be mounted in a tough matrix to avoid brittle failure, or erode as it cut to maintain its edge (e.g., pattern-welded Samurai/Damascu... | https://physics.stackexchange.com |
285,749 | [
"https://physics.stackexchange.com/questions/285749",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/127932/"
] | If $\vec F$ is a conservative force and $W$ is the work done on an object by that force, than the following are true:
$$\Delta U=-W$$
$$\int dU=-\int\vec F\cdot d\vec r$$
$$\int dU=\int -\vec F\cdot d\vec r$$
$$dU=-\vec F\cdot d\vec r$$
But then our professor immediately skipped to $$F_x=-\frac{dU}{dx},$$ and I really ... | Starting from your forth line:
$dU = -\vec{F} \cdot d\vec{r} = -F_x dx -F_y dy -F_z dz$
On the other hand, we expand $dU$ as
$dU = \frac{\partial U}{\partial x} dx + \frac{\partial U}{\partial y} dy + \frac{\partial U}{\partial z} dz$,
which implies
$F_x = -\frac{\partial U}{\partial x}$
and so on.
| You'll want to look into a mathematical concept called a "directional derivative". For a small displacement along a curve with direction and magnitude $\operatorname{d} \mathbf{r}$, the change in a function $U$ along that curve is given by: $$\operatorname{d} U = (\nabla U) \cdot \operatorname{d} \mathbf{r}.$$ So, whe... | https://physics.stackexchange.com |
738,839 | [
"https://physics.stackexchange.com/questions/738839",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/348896/"
] | Say we had an object lying on a inclined plane, at an angle of <span class="math-container">$\theta$</span> to the horizontal, and the object has a mass of <span class="math-container">$m$</span>. If we take the obejct to have an acceleration of 0 perpendicular to the plane, (i.e it accelerates down the slope), we can... | Only gravity <span class="math-container">$w$</span> and the normal force <span class="math-container">$N$</span> are involved. Set up Newton's 1st law in the perpendicular direction (let's call it the <span class="math-container">$y$</span> direction):
<span class="math-container">$$\sum F_y=0\quad\Leftrightarrow\quad... | If a car is travelling round a smooth banked curve at an angle <span class="math-container">$\theta$</span> to the horizontal and is at a constant height on the bank then its vertical acceleration is zero and by resolving forces vertically we can conclude that
<span class="math-container">$N \cos \theta = mg$</span>
Th... | https://physics.stackexchange.com |
68,035 | [
"https://softwareengineering.stackexchange.com/questions/68035",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/22905/"
] | I have to develop an application (in C#) that has to work with files. The application consists of two versions: a web version and a windows version. Therefore, the files must be stored on a place where both versions can access the files.
In both versions, the files can be edited and new files can be created.
I've bee... | Unless you have other requirements I'd keep it simple, use a share. You might want to think about who has access to the files, perhaps you don't want other people randomly removing them if they are controlled by your code, but you can do this with ACLs quite easily.
WebDav and Sharepoint would both work but I think bo... | How about an XML file? The program can access the xml file normally. However for the web application, you'd need to write a servlet which retrieves the data on behalf of the web user. Perhaps it requires more time to do it this way, but once you have a servlet which does this, you can essentially use the same patter... | https://softwareengineering.stackexchange.com |
43,195 | [
"https://physics.stackexchange.com/questions/43195",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/15576/"
] | What is the difference between the weight of an object and the mass of an object?
| Weight is the force with which gravity pulls on a mass.
Maybe the simplest way to explain the difference is that on the Moon or on Mars, your <em>weight</em> is reduced because gravity is weaker there, but your <em>mass</em> is still the same.
| Yes of course, According to physics the Mass and Weight are different from each other. Following is their main difference,
<strong>Mass</strong>:
<ul>
<li>Mass is the amount of matter contained in a body.</li>
<li>Mass of the body is the constant quantity and does not change with the change of position or location.</... | https://physics.stackexchange.com |
96,888 | [
"https://softwareengineering.stackexchange.com/questions/96888",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/32706/"
] | As an independent vendor being contracted to write a product for a company, is it reasonable to ship only the source code and user documentation while leaving out design documents, architecture diagrams, unit tests etc (basically anything else not strictly needed to run the product or extend it)?
The goal is to make t... | Contractually they'd be smart to include some sort of clause about the scope of the documentation they want. They should compensate you for presenting them in an understandable and professional format (redo the napkin sketch).
It's not unethical to only do what is asked and paid for, but to go out of your way to with... | Did you charge them for developing the unit tests and documents? and did they pay the invoice? If so, then they are entitled to receive what they paid you for.
Otherwise next week we'll see a question here on p.se.com titled 'How do we force our contractor to give us all the technical documentation and unit tests we... | https://softwareengineering.stackexchange.com |
153,961 | [
"https://mathoverflow.net/questions/153961",
"https://mathoverflow.net",
"https://mathoverflow.net/users/45250/"
] | By the Hopf-Poincaré theorem, the sum of the indices of the zeros of a vector field on the d-dimensional torus must equal zero. Given an even number of points $x_i$ on a d-dimensional torus, and numbers $n_i=\pm 1$ such that $\sum_i n_i=0$, does there always exist a vector field on the torus which has a zero of index $... | Yes, this can be done with the Pontryagin-Thom construction. This answer is essentially the same as Pietro Majer's, by the way, but perhaps a bit more "hands on" and also more sketchy.
The first step is to note that vector fields on the $d$-torus with zeros at points $x_i$ can be constructed from maps from the $d$-tor... | Sketch of a construction. The underlying local fact is: given a field on a manifold $M$, and a point $p$ where it does not vanish, you can modify it on any nbd of $p$ and create a couple "zero - antizero" of the field. This way you can make a field with any larger number of zeros of given index, the only constraint bei... | https://mathoverflow.net |
67,251 | [
"https://mathoverflow.net/questions/67251",
"https://mathoverflow.net",
"https://mathoverflow.net/users/39663/"
] | We have convex sets $C_1=Conv(yy^{T}|y^{T}y=a,y\in R^{M})$ and $C_2=Conv(yy^{T}|y^{T}y=a,y\in R_{\geq 0}^{M})$. Clearly $C_2\subset C_1$. Does there exist a PSD matrix $A$ having $tr(A)=a,A(i,j)\geq 0$ $\forall i,j$ and $A\in C_1\setminus C_2$?
| There is such an $A$ if and only if $M\geq 5$.
To see this, first note that the condition that $A$ be a convex combination of terms $yy^T$ each with trace $a$ is irrelevant. As long as $A$ is positive semidefinite (and symmetric), it can be written as a convex combination of terms $yy^T$. If $A$ has trace $a$ then l... | Maxfield and Minc (1962) in their paper entitled <em>On the matrix equation $X'X=A$</em>, quote an example due to Hall (1958), <em>A survey of combinatorial analysis</em>, which shows that for $M\ge 5$ we can find counterexamples of the desired kind. Here is their example:
<blockquote>
The matrix $$A = \begin{bmatri... | https://mathoverflow.net |
3,425 | [
"https://mechanics.stackexchange.com/questions/3425",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/1771/"
] | Hypothetically speaking, let's say I've equipped a vehicle with an aftermarket audio system with an amplifier that has a higher draw (in amps) than my alternator produces causing my average voltage (while system is active) to drop to around 10-11 volts. How drastically can I expect this to effect the life of my alterna... | After installing high wattage stereos with stock alternators, I have burned out alternators in less than 30 minutes, and I have also had them last months. It would be prudent to plan for an early failure.
| Better consider an alternator upgrade when it blows up. You'll probably be blowing it up pretty soon. :-)
| https://mechanics.stackexchange.com |
15,208 | [
"https://softwareengineering.stackexchange.com/questions/15208",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/1554/"
] | I'm in the initial planning phase of a re-write project and I'm deciding between silverlight/silverlight oob/wpf. TL;DR at the end.
It's a LOB app that handles leads/customers/appointment calendars. Not too complicated. I'm independently researching these options elsewhere, but I thought I'd ask around. Some rough ini... | Well - interesting problem. You forgot to mention that you can have a Silverlight full trust application as of SL4, so if you're thinking of WPF you might want to consider that instead. It would need installing (ClickOnce), but you seem to be moving away from that.
I've not done anything with OOB yet, but I'm pretty s... | Your analysis is very good.
The only other thing that I would mention is that not ALL of the framework is available to you in a Silverlight application. That restriction MAY tilt your choice towards WPF, but you'll need to see how that restriction applies or not to your application needs.
| https://softwareengineering.stackexchange.com |
70,684 | [
"https://dsp.stackexchange.com/questions/70684",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/33977/"
] | Are classic signal processing/statistics based approaches to optimum detection/estimation still relevant/important compared to ML based approaches using DL?
There was a time when speech processing was done using HMM. While not completely unused most speech processing now rests firmly in the DL area. I'm wondering if ev... | This is an open discussion.
In my opinion, any field we couldn't figure near optimal solution (And in most cases we only have Linear Optimal solution) using Deep Learning will replace classic methods given enough data to work with.
The power of data driven features vs. intuitive (Though mostly right yet never cover eve... | Yes, classic signal processing/statistics is still relevant. For example in optimum detection/estimation, if a sub task demands you to find out the nature of a group of signals obtained(by nature I mean the distribution- if it's Gaussian or Uniform or so on), then the preferred way of finding that out would be using me... | https://dsp.stackexchange.com |
614,339 | [
"https://physics.stackexchange.com/questions/614339",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/283053/"
] | Suppose <span class="math-container">$|x\rangle$</span> is an eigenvector of the position operator <span class="math-container">$\hat{x}$</span> and let <span class="math-container">$|\psi\rangle$</span> be an arbitrary state on this Hilbert space. What is the correct interpretation of the complex number <span class="m... | The first of your two suggestions doesn't make sense, "the probability amplitude of finding the particle at position <span class="math-container">$x$</span> in the state <span class="math-container">$|\psi\rangle$</span>". The particle is <em>either</em> in the state <span class="math-container">$|\psi\rangle... | The quantum state of a quantum mechanical system can be described with a vector <span class="math-container">$|\psi\rangle$</span> of a Hilbertspace <span class="math-container">$\mathcal H$</span>. If you want to characterize the quantum state completely, you have to introduce a "complete set of compatible variab... | https://physics.stackexchange.com |
382,787 | [
"https://electronics.stackexchange.com/questions/382787",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/86793/"
] | We are told that heat increases a resistor's resistance (or decreases its conductance) and current decreases when resistance is increased.
So with less current, less heat would be dissipated, which drops the resistance and causes more current to flow, and then again, more current, more heat...
It seems like an endless... | I believe it's possible to build a simple physical model with the ideas you provided.
In a simple DC circuit, under a constant voltage V and ohmic resistance R it is possible to use the power equation:
$$P = V i = \frac{V^2}{R}$$
If we suppose the system is made of a wire with constant lenght L and cross section area... | This could be analyzed in the same way as a control circuit with feedback. From a practical sense, the heating will be much slower than the other effects, so that will dominate the loop equations. As such, it will exponentially approach equilibrium, unless there's other elements of the system that limit its response ... | https://electronics.stackexchange.com |
410,601 | [
"https://mathoverflow.net/questions/410601",
"https://mathoverflow.net",
"https://mathoverflow.net/users/8628/"
] | Let <span class="math-container">${\cal U}$</span> be a free ultrafilter on <span class="math-container">$\omega$</span>. Is the linearly ordered set <span class="math-container">$(\omega+1)^\omega/{\cal U}$</span> complete?
| No. Every ultraproduct by a free ultrafilter on <span class="math-container">$\omega$</span> is <span class="math-container">$\aleph_1$</span>-saturated. And infinite <span class="math-container">$\aleph_1$</span>-saturated linear orders cannot be complete!
<em>Proof:</em> Let <span class="math-container">$L$</span> be... | No. <span class="math-container">$(\omega+1)^{\omega}/\mathcal{U}$</span> is not complete whenever <span class="math-container">$\mathcal{U}$</span> is a non-principal on <span class="math-container">$\omega$</span>. Observe that if <span class="math-container">$(\omega+1)^{\omega}/\mathcal{U}$</span> is complete, then... | https://mathoverflow.net |
59,277 | [
"https://dsp.stackexchange.com/questions/59277",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/40934/"
] | Consider I am given two functions of one random variable each for example x=cos(at),y=rect(bt) where a and b are random variables.And I am given Probability density function for a and b then if I am asked if the two functions are independent or not so, I want to confirm that before proceeding I will have to convert t... | The canonical definition of independence of two random variables <span class="math-container">$X$</span> and <span class="math-container">$Y$</span> is
<blockquote>
<span class="math-container">$X$</span> and <span class="math-container">$Y$</span> are called independent random variables if
for every choice of B... | It happens that if <span class="math-container">$X$</span> and <span class="math-container">$Y$</span> are independent then so will their functions <span class="math-container">$g(X)$</span> and <span class="math-container">$h(Y)$</span> be; but not <span class="math-container">$g(X,Y)$</span> and <span class="math-con... | https://dsp.stackexchange.com |
709,950 | [
"https://physics.stackexchange.com/questions/709950",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/236734/"
] | When deriving the mathematical description of a field, we set the current density and charge to zero in Maxwell's equations.
However, this condition is not absolutely true anywhere on earth.
Yet, we are able to apply EM waves for problems in communication, medicine etc. How is that possible that instead of ignoring the... | First of all, a general solution of a system of inhomogeneous linear equations (such the Maxwell equations with sources) can be always decomposed into a particular solution of the inhomogeneous equations <em>and</em> the general solution of the homogeneous ones (i.e., the Maxwell equations without sources, of which EM ... | Free space solutions of maxwells equations show that wave like solutions can theoretically exist. Plane wave solutions to the homogenous wave equations are not created by charges and currents, and thus these solutions don't prove that EM waves are generated by charges.
This is not the only way that we can show wave lik... | https://physics.stackexchange.com |
21,544 | [
"https://mechanics.stackexchange.com/questions/21544",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/11697/"
] | My car has disc brakes in the rear, and the calipers have a small lever, which is pulled by the parking brake cable. (The parking brake uses the discs, so there is no additional drum or similar.)
When resetting the pistons, they also have to be rotated (clockwise in my case).
This system is quite common for many car ... | The operation is simpler then you might think. In broad strokes there is a screw type setup in the center of the piston. One part of the screw is loosely attached to the piston, the other part is attached to the lever. When the breaks are applied using hydraulics the piston has some degree of freedom to move beyond the... | The general answer to the title of your question is that in cars with rear disc brakes, the parking brake either actuates the disc calipers (usually with much less force) or has a small drum brake housed within the hub assembly. As an example of the latter, Subaru's have a small drum brake within the hub that works aga... | https://mechanics.stackexchange.com |
47,128 | [
"https://engineering.stackexchange.com/questions/47128",
"https://engineering.stackexchange.com",
"https://engineering.stackexchange.com/users/34925/"
] | As a layman, how can I know the shear strength of a particular size bolt?
For example, if I want to attach something to the internal wall of my van ( 1mm stainless steel ) using a rivnut and standard metric bolt and it should hold up to 50 kg, how would I figure out the right size and grade to use? Nothing specialty, ... | For a layman, you shouldn't use bolts stressed in shear. Bolts seldom hold shear forces. Bolts are used to hold two surfaces together so that the joined components take the shear stress. It is the friction of the two surfaces that support the load. This is the principle that holds wheels on automobile hubs. The fri... | For complete layman's terms, an m6 bolt can hold up an elephant. For 10 kg, the bolts won't be the weak link, it will be whatever you are bolting into. Use 3 or 4 and you will probably be fine
The fine print: I'm considering an average female Asian elephant (6000 lb or 2700 kg), grade 8 bolts (the expensive "top s... | https://engineering.stackexchange.com |
417,834 | [
"https://physics.stackexchange.com/questions/417834",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/109375/"
] | Black color substance is assumed to absorb all the light/radiation that falls on it. White color substance on the other hand, reflects all the radiation that falls on it. Since all the momentum of photons is absorbed by black substance, change in momentum is $\Delta p$ whereas for white substance, change in momentum is... | As you say the white surface reflects all photons. It redirects the photons, which fly away in a different direction transporting their energy away again from the surface. The temperature of the white material is not impacted. The black surface however absorbs the photon energy, which will tend to increase its temperat... | The photons after reflecting from the white surface have same energy as before. So, no energy is transferred to white surface by the photons.
However, the photons falling on black surface lose all its energy (as no photons are reflected, we can say they all come to a halt). This energy raises the temperature of black ... | https://physics.stackexchange.com |
224,037 | [
"https://softwareengineering.stackexchange.com/questions/224037",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/59626/"
] | I am working (alone) on an application that is a music portal where you can search for artists, songs etc ...
The C# solution consists of:
<ul>
<li>a class library that contains all the stuff to access the web service</li>
<li>the application itself</li>
</ul>
I've recently started using Git and figured out the bene... | A Visual Studio solution is a whole entity. The source control providers work on the solution level. The solution is something that you can compile in itself, for a single project it may not be the case. There can be commits when you need to touch multiple projects at once so that the solution compiles after each atomi... | I run in the same question as yours and here is my way to apply git:
<ul>
<li>for each project folder inside the solution folder, I initialize the git repository. </li>
<li>for the solution folder, I also initialize the git repository and add project folders into the .gitignore file of the solution repository.</li>
</... | https://softwareengineering.stackexchange.com |
22,246 | [
"https://dba.stackexchange.com/questions/22246",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/10753/"
] | I am using SQL Server 2008 R2, when I looked into the services for SQL Server I saw there are two different services with different instances.
<ol>
<li><code>SQLEXPRESS</code> </li>
<li><code>MSSQLSERVER</code></li>
</ol>
Can anyone help me to get clear the difference between #1 and #2?
Thanks in advance
| You can install multiple copies of SQL Server onto the same machine. To keep them apart, all "copies" (called <strong>instances</strong>) except the first (default) one have to be named.
So by default, when you install your first instance of SQL Server, you won't give it any instance name - it'll be the default instan... | MSSQLSERVER is the default instance (accessed via <code>localhost</code>) and SQLEXPRESS is a named instance (accessed by <code>localhost/SQLEXPRESS</code>)
Did you install twice?
| https://dba.stackexchange.com |
15,959 | [
"https://biology.stackexchange.com/questions/15959",
"https://biology.stackexchange.com",
"https://biology.stackexchange.com/users/6096/"
] | We know that everybody's DNA pattern is different in the world. Then how can ´we transfer blood from one person to another person and this person can survive ?
| A few components to my answer.
<ol>
<li>Red blood cells do not contain a nucleus, therefore, they do not harbour DNA.</li>
<li>The major determinant of blood compatibility is the blood antigen. There are only 4 types: O, A, B, AB. This is genetically encoded, and is expressed as a set of sugar coats on the blood cell ... | Because one's DNA doesn't have to exactly match another person at all 3 billion locations for the transfusion to be successful. As it turns out, there are only a few proteins that determine whether a person's blood is a match to someone else's. And as it turns out, in the vast majority of the human population, there ... | https://biology.stackexchange.com |
184,219 | [
"https://softwareengineering.stackexchange.com/questions/184219",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/62035/"
] | Recently I found about two new programming languages(Vala and google's GO) which don't support method or function overloading and intend on not supporting them in the future ever! The creators of these languages say that overloading is evil and should not be used!(in system programming?)
I was told that the creators o... | I can see where they're coming from, but as rules I would say these are way too specific for 100% restriction in a language design.
Gigantic 18+ layer convoluted inheritance schemes are pretty well-established as an anti-pattern for instance but inheriting 1-3 levels deep can be perfectly reasonable for solving certai... | Classes, method overloading, operator overloading... they are all tools. They are suitable in some circumstances, not in others. Don't try to apply labels like "evil" to such things; instead, strive to understand them so you can decide for yourself whether they are useful or not.
| https://softwareengineering.stackexchange.com |
3,098,737 | [
"https://math.stackexchange.com/questions/3098737",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | <blockquote>
Prove that <span class="math-container">$\lim_{(x,y)\to(0,0)}(xy+y^{3})=0$</span>.
</blockquote>
I am trying to determine how to set the <span class="math-container">$\delta$</span>. Here is my rough work, which isn't much:
<span class="math-container">$|f(x,y)-0|=|xy+y^{3}|\leq |y||x+y^{2}|$</span>
I... | <strong>Hint</strong>
For <span class="math-container">$\vert x \vert, \vert y \vert \le 1$</span>, you have
<span class="math-container">$$\vert xy+y^3 \vert \le \vert x \vert \vert y \vert + \vert y \vert^3 \le \vert x \vert \vert y \vert + \vert y \vert\le 2 \vert y \vert$$</span>
| Using polar coordinates (not really necessary here, but useful in harder cases):
<span class="math-container">$$|xy + y^3| = |r^2\cos\theta\sin\theta + r^3\sin^3\theta|\le r^2 + r^3,$$</span>
and <span class="math-container">$r = \|(x,y)\|$</span> (euclidean norm), so...
| https://math.stackexchange.com |
85,947 | [
"https://dsp.stackexchange.com/questions/85947",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/64776/"
] | I have two signal spectrums. I represent them with polynomial coefficients and then do convolution.
fig 1 & 2 are the two signal spectrums and their corresponding polynomial representations. (I have used <code>numpy.polynomial.polynomial.Polynomial.fit</code> and <code>convert().coef</code>). As far as I know, the ... | It looks like your inverse is very poorly behaved. Your original signal <span class="math-container">$x[n]$</span> is very very small at the edges. That means that your inverse <span class="math-container">$y[n] = 1/x[n]$</span> is very very large. From the looks of it it's mostly amplified noise.
Your fit in in the fl... | What I am seeing in your polynomial fits to an inverted signal is an astonishingly good fit.
A least-squares fit minimizes the sum of the squares of the differences between the sample and the curve fit. Polynomial fits tend to have that oscillatory behavior that you are seeing in your fit. Given that you appear to be... | https://dsp.stackexchange.com |
287,896 | [
"https://electronics.stackexchange.com/questions/287896",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/16832/"
] | I'm a software developer trying out some electronics projects so this might be a stupid question but here it goes:
Is it possible to put a coil around the mains wire (220V/50Hz) that would be able to power a small Arduino circuit (5V/15mA) by induction only?
I've seen some power consumption monitoring devices that se... | No, wrapping a coil around a straight wire does not make a transformer.
To derive electric power from a magnetic field, the field has to go "thru the hoop" of a coil of wire. The magnetic field around a straight wire is circular around that wire. If you wrap another wire around the first wire, that second wire is es... | Olin is correct about the geometry of the magnetic field created by a single straight wire such as you would like to use. The magnetic field around such a wire takes the shape of a cylinder which is co-axial to the wire. This cylinder is made up of an infinite number of circular magnetic flux tubes (the "magnetic field... | https://electronics.stackexchange.com |
431,327 | [
"https://electronics.stackexchange.com/questions/431327",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/216816/"
] | I have a measured a periodic signal waveform <span class="math-container">\$v(t)\$</span> that can be modeled as:
<span class="math-container">\$v(t) = f(\phi) A \sin(2\pi ft + \psi)\$</span>
where <span class="math-container">\$A\$</span> is a constant amplitude, <span class="math-container">\$f\$</span> is signal f... | You've made some wrong assumptions about what the parts of the system are. The part you're describing as a non-powered passive splitter is actually a powered active splitter. Splitting one aerial signal into six with a passive splitter is unlikely to give you sufficient signal on any of the six outputs, especially if t... | It is desirable to have the amplifier be as close to the aerial as possible and certainly prior to the signal being split, however getting mains power to said locations is often problematic.
The solution to this is amplifiers that are powered via one of the output coax lines. The amplifier is sited close to the aerial... | https://electronics.stackexchange.com |
66,188 | [
"https://physics.stackexchange.com/questions/66188",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/25077/"
] | In an unbiased PN junction, when the carriers recombine to form a depletion layer , it is said that immobile ions are formed.
We know that the conduction band electrons in N type are not associated with any particular atom. So when the conduction band electron diffuses to the P type region, which atom becomes an ion?
| None, really. Such junctions form in semiconductor crystals. Those are remarkable materials. Let's look at electrons in solids first.
Many atoms have weakly bound valence electrons in outer orbits. These orbits would have specific energies if the atom existed in isolation. But when man atoms are packed together and t... | Before the N type material is brought into contact with the P type material there will be an equilibrium between [N-type doping atoms with all electrons] and [Ionized N-type doping atoms]. So some atoms all across the bulk of the material will be ions, which ones will change over time.
When the N-type material is bro... | https://physics.stackexchange.com |
26,777 | [
"https://softwareengineering.stackexchange.com/questions/26777",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/10180/"
] | I have developed a web application for a client which is live to their customers. Recently, the application has been sold in such as way that it is replicated to other domain names so there are 4+ instances of the application which I am to develop and maintain.
At the moment, each copy of the app has it own SVN reposi... | Its going to depend on the specifics of course, but ideally you'd want a <em>single</em> source code base with extensions for the client specific code. For example, you might organize the client specific code into plugins the way a content management system would. Or use a templating system for custom GUIs. I would ... | The typical approach is to have a single core, which can then be configured as needed in a separate module. For simple cores, you can make do with if-statements based on variables in the configuration module.
For more advanced cores, you need to be able to provide customer specific code overriding standard functiona... | https://softwareengineering.stackexchange.com |
69,559 | [
"https://physics.stackexchange.com/questions/69559",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/26429/"
] | Energy eigenstates provide a convenient basis for solving quantum mechanics problems, but they are by no means the only allowable states. Yet it seems to me that particles/systems are assumed to be in energy eigenstates "in nature".
Some examples of what I mean:
<ul>
<li>Solving the Schrodinger equation for the Hydro... | If we take a system, and let it evolve for some indefinite amount of time, it will be in an incoherent mixture of energy eigenstates. Many systems we encounter in nature have been sitting for some time, and not interacting with the environment (much). These can be considered to be in energy eigenstates.
For example, s... | Is there a more sophisticated answer than "This is done because isolated low-temperature systems tend to give up energy more often than they receive it, and thus, gravitate toward their ground states. Thus, even if they start in a mixed energy state, the system will radiate away energy until it reaches its ground stat... | https://physics.stackexchange.com |
2,701,429 | [
"https://math.stackexchange.com/questions/2701429",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/220391/"
] | So I have this matrix <code>A = [1.9, 0.025; 0.1, 1.225];</code>
And I want to multiply it with the vector <code>v = [1;0];</code>
I want to plot the sum of it up to 25 iteration, so I have a <code>for</code> loop and I use the sum function like this:
<pre><code>iter = 25;
v = [1;0];
for k = 1:iter
s(k) = sum(A^... | Your computations are O.K. The statement above is wrong. If $A$ is idempotent, then $I_n-A$ is idempotent !
| The correct statement is $I-A$ is idempotent.
If $$0 = A^2 - A = A(A - I)$$
then the minimal polynomial $\mu_A(x)$ divides $x(x-1)$.
If $\mu_A(x) = x$ then $A = 0$ and if $\mu_A(x) = x-1$ then $A = I$, both cases being trivial. Thus assume $\mu_A(x) = x(x-1)$.
Recall that for any matrix $B$ and scalar $\lambda$ we ... | https://math.stackexchange.com |
51,279 | [
"https://astronomy.stackexchange.com/questions/51279",
"https://astronomy.stackexchange.com",
"https://astronomy.stackexchange.com/users/47503/"
] | Rosetta Comet Orbiter (RCO) crashed into the surface of a comet after the comet passed near Jupiter, which would be out-of-range for its antenna to communicate with Earth. So, the ESA made the difficult decision to just let go and crash the darn thing. (Talk about going out with a bang! geez!) Anyway, I saw a mission c... | Yes, it did. But not by much.
The comet has a mass of about <span class="math-container">$10^{13}$</span> kg. Rosetta had a mass (after fuel had been used up) of about 1300kg. The "impact" was at 0.9 m/s. This means that the spacecraft had a momentum of about 1200 kg m/s
After the impact, and in the frame ... | It had already moved the comet to whatever extent it was able, when the probe had gone into orbit around the comet. The future trajectory would be determined by the centre of mass of the comet and everything gravitationally bound to it (i.e. the probe). Becoming part of the surface of the comet isn't going to cause an... | https://astronomy.stackexchange.com |
622,583 | [
"https://electronics.stackexchange.com/questions/622583",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/315142/"
] | I need to find the energy consumed per product.
Let's say we have product cycle of 10 seconds.
The machine reports the average wattage of each second.
<pre><code>00:01 450 watts
00:02 400 watts
00:03 350 watts
00:04 600 watts
00:05 725 watts
00:06 400 watts
00:07 550 watts
00:08 200 watts
00:09 ... | Power is defined formally as the rate of change of energy. So the answer quoting the inverse as Energy is the integral of power is correct. This energy calculated has units of Joules when P is in Watts and t in seconds. This is equivalent to the watt-second (non preferred unit).
Assuming constant power over the integra... | Energy consumption can be calculated by integrating power over time:
<span class="math-container">$$
E = \int P \ dt
$$</span>
So, what you are doing is basically correct: The total consumption after a period of time (e.g. 10 seconds) is the sum of power-duration product of each of those 10 samples.
However, remember t... | https://electronics.stackexchange.com |
445,276 | [
"https://physics.stackexchange.com/questions/445276",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/214881/"
] | I'm confused about the action-reaction pair in a rocket. is it
1 The action of the explosion of gases in the combustion chamber on the rocket, and the reaction of the rocket on the gases, or
2 The acceleration of the exhaust by the rocket nozzle(action) and the acceleration of the rocket by the exhaust(reaction).
... | Your expression <span class="math-container">$\intop_{M}-\langle dA,dA\rangle(dvol)_{M}$</span> is the usual Lagrangian for electromagnetism written in a highbrow way. You can work this out with Hodge duals and <span class="math-container">$dA=F$</span> if you've never seen it before. That is why it is no surprise you ... | Your alternative lagrangian is not gauge invariant, since if you shift <span class="math-container">$A_\mu$</span> by the gradient of a scalar function, the expression clearly changes; <span class="math-container">$F_{\mu\nu}$</span> is antisymmetric in the indices <span class="math-container">$\mu \nu$</span> so basic... | https://physics.stackexchange.com |
220,163 | [
"https://mathoverflow.net/questions/220163",
"https://mathoverflow.net",
"https://mathoverflow.net/users/36254/"
] | Let $X$ be a complete intersection in $\mathbb{P}^n$ of multidegree $(d_1,\ldots,d_r)$. If we're working over a finite field $\mathbb{F}_q$, the Ax-Chevalley-Warning theorem says that if $X$ is in the Fano range, i.e., $$ \sum d_i \leq n,$$ then $|X(\mathbb{F}_q)| \equiv 1 \pmod q.$
In the case that $X$ is not in the ... | Extension of <strong>Daniel Loughan</strong>'s example (which is also known, but not as
well-known as it should(?) be): if a prime $p$ is of the form $dn+1$ then
the Fermat hypersurface $\sum_{i=1}^d x_i^d = 0$ in ${\bf P}^{d-1}({\bf F}_q)$
is smooth and its number of rational points is not congruent to $1 \bmod p$.
... | How about $$x_1^{q-1} + \cdots + x_{q-1}^{q-1} = 0 \subset \mathbb{P}^{q-2} \quad ?$$
This is a fairly well-known example which seems to satisfy your criteria.
| https://mathoverflow.net |
560,175 | [
"https://stats.stackexchange.com/questions/560175",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/126967/"
] | When there are missing items (answers) in a questionnaire it is sometimes usual to impute them with the mean of the valid items. For example here are 3 items/question which can have a four values from <code>1</code> to including <code>4</code>.
<ul>
<li><code>[2, 3, missing]</code> would result in <code>[2, 3, 2.5]</co... | It's commonly done to impute based on the mean of the other items, but that's more because it's simple, easy to do and has some vague logic to it ("Well, if they are all high or low, maybe the missing one would be, too, right?"). However, that does not mean it is a good (never mind the best) option.
Let's ima... | Here is a balanced algorithm: On the <span class="math-container">$i^{th}$</span> questionnaire, if there are <span class="math-container">$k_i$</span> valid responses which sum to <span class="math-container">$s_i$</span>, then:
<ul>
<li>round <span class="math-container">$s_i/k_i$</span> up if <span class="math-conta... | https://stats.stackexchange.com |
1,230,302 | [
"https://math.stackexchange.com/questions/1230302",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/204572/"
] | I am trying to show that
$$ \int^{\pi}_{0} \frac{\cos^2 x}{a^2 \cos^2 x + b^2 \sin^2 x} \ dx = \frac{\pi}{a(a+b)} $$ where $ a,b > 0$.
I have tried a few things, but none have worked.
For example, one approach was
$$ \int^{\pi}_{0} \frac{\cos^2 x}{a^2 \cos^2 x + b^2 \sin^2 x} \ dx = \int^{\pi}_{0} \frac{\cos^2 x... | If we set $x=\arctan t$ we simply have:
$$I(a,b)=\int_{0}^{\pi}\frac{\cos^2 x}{b^2\sin^2 x+a^2\cos^2 x}\,dx = 2\int_{0}^{+\infty}\frac{dt}{(1+t^2)(a^2+b^2 t^2)}$$
that is straightforward to compute through partial fraction decomposition or through the residue theorem.
| Suppose we seek to evaluate
$$\int_0^\pi \frac{\cos^2 x}{a^2\cos^2x + b^2\sin^2x} dx
= \frac{1}{2}
\int_0^{2\pi} \frac{\cos^2 x}{a^2\cos^2x + b^2\sin^2x} dx$$
with $a,b>0.$
Put $z = \exp(ix)$ so that $dz = i\exp(ix) dx$ and hence
$\frac{dz}{iz} = dx$ to obtain
$$\frac{1}{2}\int_{|z|=1}
\frac{(z+1/z)^2... | https://math.stackexchange.com |
86,049 | [
"https://mathoverflow.net/questions/86049",
"https://mathoverflow.net",
"https://mathoverflow.net/users/20692/"
] | Let $f:\mathbb R\to\mathbb R$ be a periodic function. We say $f$ is without minimum period if, $\forall t$ such that $f(x+t)=f(x)\forall x$, there is a $t'$ such that $0<t'<t$ and $f(x+t')=f(x)\forall x$.
The easiest examples of such functions are constant functions.
Dirichlet's function ($1$ if $x\in\mathbb Q$ a... | I guess what you can get as a set of periods is exactly any additive subgroup of the reals. Certainly the periods are closed under addition. On the other hand, for any subgroup $G$ or $\mathbb R$, mimic the Dirichlet function by defining $f$ to be the indicator function of $G$. Here the set of periods is exactly $G$ it... | this is the complete question, sorry.
Let $f:\mathbb R\to\mathbb R$ be a periodic function. We say $f$ is without minimum period if, $\forall t$ such that$f(x+t)=f(x)\forall x$, there is a $t'$ such that $0 < t' < t$ and $f(x+t')=f(x)\forall x$.
The easiest examples of such functions are constant functions.
Di... | https://mathoverflow.net |
21,625 | [
"https://scicomp.stackexchange.com/questions/21625",
"https://scicomp.stackexchange.com",
"https://scicomp.stackexchange.com/users/18469/"
] | I work with scientific simulation, but what I like the most in programming is creating graphical user interfaces and 2D or 3D outputs to visualize the evolution of the simulations. I know most input data in scientific simulation are in text files, not in GUIs, and that more usually statistical graphs are scientifically... | This answer is my personal opinion. I believe that high performance numerical solvers are not a good place to use MVC pattern. Additional layers and specific data flow will not help to get a better performance. MVC and MVVM (Model-View-ViewModel) are commonly used for GUI developing. If you want to develop pre/post pro... | Whilst the MVC/MMVM patterns can be useful where appropriate, I would avoid trying to shoehorn the numerics into a pattern led by your chosen GUI (or other UI) framework. Unless you multi-thread in some fashion, the GUI itself will be unresponsive as the simulation progresses until its completion.
You mention that yo... | https://scicomp.stackexchange.com |
730,334 | [
"https://physics.stackexchange.com/questions/730334",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/333207/"
] | If there is only two inertial frames in empty space with no at rest frame they can plot their trajectories from, how can the two frames determine their worldlines, coordinate trajectories, point of departure and arrival, proper time and distance, issues of time dilation, in the emptiness of space?
If that is too many q... | <span class="math-container">$M$</span>=The ramp
<span class="math-container">$m$</span>=mass moving on the ramp
<strong>When <span class="math-container">$M$</span> is free to slide on the ground with no friction:</strong>
Momentum:
When you consider <span class="math-container">$m$</span> and <span class="math-contai... | Let's start with physical principles holding in classical mechanics, and then look at your problem.
<h2>Physical principles and equations</h2>
<ul>
<li><strong>second principle of dynamics</strong> (dynamical equation for translation of the center of mass): time derivative of the momentum of a system is equal to the re... | https://physics.stackexchange.com |
377,216 | [
"https://electronics.stackexchange.com/questions/377216",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/-1/"
] | Can there be a circuit or a device(probably a semiconductor) which allow flow of current only at a minimum threshold voltage?
I've heard about comparator but I am not able to understand how they work(their internal structure and circuitry).
| Synchronous speed, n1 = (<strong>120</strong> * fn)/p where p = <strong>poles</strong>
Synchronous speed, n1 = (<strong>60</strong> * fn)/p where p = <strong>pole pairs</strong>
Most text book presentations perform calculations using poles, but some use pole pairs.
In an induction motor, the rotor poles are formed b... | An induction motor (such as a squirrel cage rotor type) can be regarded as a rotating shorted turn and as such the (north and south) poles created by induction are irrelevant for speed calculation. There is one pole pair if you want a number.
If it helps, try thinking about the rotor as being a fixed magnet with N and... | https://electronics.stackexchange.com |
48,157 | [
"https://datascience.stackexchange.com/questions/48157",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/70406/"
] | I am seeking some directions for a proper path to research the solve for this problem:
My company made all our employees take a "StrengthFinders" test, which results in every employee being assigned their top five (ordered) "strengths" from a possible list of 34 strengths. We have 500 employees. I am supposed to ide... | You have just 500 data points...
Excel of course is the worst possible tool though.
Anyway, build a dictionary. Put everybody in there 6 times: 1 with all five strengths, and 5 times with one strength omitted. Then you can easily identify the largest groups, and you can also perform various completion operations easi... | Assign each of the 34 traits a unique prime number.
Compute the product of the 5 prime numbers of each person.
Compare every person's value to find a match.
To find 4 matching traits out of 5, make the product from 4 of the 5 traits. You'll find 5 unique combinations. 1*2*3*4 , 1*2*3*5, 1*2*4*5, 2*3*4*5, and 1*3*... | https://datascience.stackexchange.com |
147,762 | [
"https://dba.stackexchange.com/questions/147762",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/104276/"
] | <pre><code>SELECT top 50 column1, column2,column3,column4
FROM (Select Distinct column1,column2, column3, column4 from Table
Where Column2>= '2016-07-01' )
Order By Column2;
</code></pre>
The result is not what I am expecting. I want to make columns <code>Column1</code>, <code>Column2</code>, <code>Column3</code>, ... | I don't think it is possible to get your expected result set using a single Select-statement. IMHO you must create an empty copy of the result table and then use following single-row Insert to prevent any duplicate values. Repeat this until you get the expected <em>n</em> rows.
<pre><code>insert into result_table
sele... | Reverse your query...
SELECT DISTINCT column1, column2,column3,column4
FROM (Select top 50 colum1,clumn2, column3, column4 from Table)X
Where Column2>= '2016-07-01'
Order By Column2;
| https://dba.stackexchange.com |
318,817 | [
"https://softwareengineering.stackexchange.com/questions/318817",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/80416/"
] | There's a bug report that the original reporter no longer cares about and I can't reproduce the issue. Is there an English terminology used to tag those issues? Should they be kept open and tagged with that term (e.g. “timed out”) or should they be closed?
Note that this question is not the same as the one about bug n... | If nobody thinks it is a problem anymore, then the ticket can be closed.
The reason for closure can be any descriptive reason, such as "not a defect", "not reproducible" or "not relevant anymore".
| It depends of the workflow you use, and essentially the relations between developers and customers.
In Extreme Programming, it belongs to the customer representative to decide what to do with the ticket. She may consider that it's high priority and you should work on it, or low priority, or should be removed. Whether ... | https://softwareengineering.stackexchange.com |
88,072 | [
"https://mathoverflow.net/questions/88072",
"https://mathoverflow.net",
"https://mathoverflow.net/users/5274/"
] | Hi,everyone.
I want to know that how to compute the henselization of some simple rings, for example: $k[x]_{(x)}$ and $R[X]_{(X)}$ where $k$ is a field and $R$ is a excellent DVR.
thank you very much!
| In both cases, the henselization $A^h$ of your ring $A$ is its algebraic closure in its completion $\hat{A}$.This follows for instance from Artin approximation (Algebraic approximation of structures over complete local rings, Publications Mathématiques de l'IHÉS, 36, 1969, p. 23-58): any system of polynomial equations ... | Perhaps a better source for this result is Nagata's 1962 book, "Local Rings". Chapter VII begins with a section on constructing the henselization of a local ring and its properties. For a normal domain R the above result (note though that the separable algebraic closure, not the algebraic closure must be used in char p... | https://mathoverflow.net |
88,155 | [
"https://stats.stackexchange.com/questions/88155",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/30023/"
] | I am currently working on a anti-cheat mechanism for a multiplayer client called sa-mp.
Problem is, recently sa-mp released a new update that fixes latency and hence allows for aimbotters.
Aimbotting can be defined as using a computer program to automatically target and shoot all enemies. Problem is, how can aimbottin... | Since you have vector data, I would look for consistent movements that are too uniform for a human. For instance I would expect an aimbot to move at (for example) 10 pixels per ms in a constant general direction until it gets its target. It is unlikely to overaim (switch back and forth from right movement to left mov... | <strong>For the players:</strong>
<ul>
<li>Check how many times the shooter hits another player, if he hits more than like 5 bullets in a row (which might seem low but is high for this game),</li>
<li>Notify the admins that the player might be cheating</li>
<li>Also check if the target is moving, if the target is stan... | https://stats.stackexchange.com |
16,783 | [
"https://mathoverflow.net/questions/16783",
"https://mathoverflow.net",
"https://mathoverflow.net/users/4325/"
] | Suppose I have a compact 3-dimensional submanifold N of S X (0,1) which has one boundary component, where S is a closed surface. Must N be a handlebody?
| No. N could be homeomorphic to the exterior of any knot in the 3-sphere (i.e. a cube with a knotted hole).
| There is a class of three-manifolds which are called Handlebodies with Wormholes. M is a handlebody with wormholes if it is homeomorphic to the result of removing the regular neighborhood of a system of properly embedded arcs from a handlebody, that includes the example that Richard gave above. There is a Topology p... | https://mathoverflow.net |
212,398 | [
"https://stats.stackexchange.com/questions/212398",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/115538/"
] | We have performed two linear regressions (OLS), one with data from 2009 and one with data from 2014. All the variables are the same, both the dependent and the six independent variables. The sample sizes are however different!
We want to be able to compare the results from these two regressions. For instance, we want ... | Rather than working through the auto-covariance function, it is simpler to perform the analysis as an algebraic exercise working with the initial recursive equations for the two levels of the model. Taking time back by one unit in the upper process gives the equation:
<span class="math-container">$$Y_{t-1} = X_{t-1} ... | Here is my answer, feel free to comment and or correct. By assuming the invertibility of the process, I can discard one of the two solutions of the equation in $\lambda$.
We remind the second order equation for $\lambda$:
$$\phi \lambda^2 + (k^2 + \phi^2 +1)\lambda + \phi. $$
The discriminant of this equation is simp... | https://stats.stackexchange.com |
1,714,263 | [
"https://math.stackexchange.com/questions/1714263",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/251829/"
] | If $cis(\alpha)=a$ and $cis(\beta)=b$, prove $$\sin(\alpha-\beta)=\frac{b^2-a^2}{2ab}i$$
I started with the right side and tried "expanding" $cis$ and arrived at the following formula:
$RHS=\frac{i[\cos(2\beta)-\cos(2\alpha)]-[\sin(2\beta)+\sin(2\alpha)]}{2\cos(\alpha+\beta)+i\sin(\alpha+\beta)}$
I don't think it he... | $$a=cis({\alpha})=e^{i\alpha}$$
$$b=cis({\beta})=e^{i\beta}$$
$$RHS=\frac{b^2-a^2}{2ab}i=\frac{bi}{2a}-\frac{ai}{2b}=\frac12i(e^{i(\beta-\alpha)}-e^{i(\alpha-\beta)})=\frac12i(e^{i(\beta-\alpha)}-e^{-i(\beta-\alpha)})=\frac12i(2i\sin(\beta-\alpha))=\sin(\alpha-\beta)=LHS$$
| Recall from trig that
$$ \cos(\alpha - \beta) = \cos\alpha\cos\beta + \sin\alpha\sin\beta.$$
We know that
$$ \text{cis}(\alpha-\beta) = \cos(\alpha - \beta) + i\sin(\alpha-\beta).$$
Also, we can use the general fact that $\text{cis } \theta = e^{i\theta}$, along with basic rules of exponents, to say
$$\text{cis}(\alp... | https://math.stackexchange.com |
385,870 | [
"https://electronics.stackexchange.com/questions/385870",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/53759/"
] | I'm trying to figure out how grid tie inverters work on a basic level. From what I understand, the inverter would have to
<ol>
<li>monitor the grid wave </li>
<li>create its own wave exactly like the grid</li>
<li>make it a little bit higher voltage</li>
<li>connect the two and then the power would flow into the grid<... | Your understanding has some of the elements, but it is more like this:
<ol>
<li>Monitor the grid wave.</li>
<li>Create an inverter wave exactly like the grid matching frequency and phase angle.</li>
<li>Connect the two and then no power would flow into the grid.</li>
<li>Monitor the current output of the inverter very... | essentialy grid tie inverters are designed to be current sources they allow the grid to regulate the frequency and set the voltage and just push energy into the grid (unless the conditions on the grid are such that they should not feed it)
| https://electronics.stackexchange.com |
275,127 | [
"https://softwareengineering.stackexchange.com/questions/275127",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/100115/"
] | Should comments explain how the application control flow works? For example, the relationships between classes, how this function is used by other functions, and what we are trying to do with these objects and processes? I suppose what I'm asking is should a person be able to understand how an application works simply... | In my opinion, comments should explain anything that cannot be made self-evident by the actual code, and any detailed external documentation you need should be generated from those comments (presumably as part of your automated build/deployment system).
Clarifying what a class or function should be used for, what prob... | <strong>No.</strong>
If people cannot read the code and understand <em>what</em> the code is doing, that is a sign that your code is poorly designed, overly complex, or poorly named (or some combination thereof). Comments don't fix those problems - they only exacerbate them.
Use comments to describe <em>why</em> code... | https://softwareengineering.stackexchange.com |
20,032 | [
"https://cstheory.stackexchange.com/questions/20032",
"https://cstheory.stackexchange.com",
"https://cstheory.stackexchange.com/users/5497/"
] | I know that this question it not entirely theoretical, but I think that's the place where is more probable that someone knows the answer.
The question is: is there any OO strong typed language where I could "delay" the definition of the method's arguments types?
I'll write an example. Imagine I want to model abelian ... | What you are looking for are called "self types", and have been studied theoretically for ~20 years or so.
For example, see <em>Safe Type Checking in a Statically-Typed Object-Oriented Programming Language</em> by Kim B. Bruce, in the 1993 POPL (Principles of Programming Languages), pp. 285-298. Off the top of my hea... | In Scala you can use parametric polymorphism for this, e.g. like so:
<pre><code>trait A [ T ] {
def f ( t : T ) : T
def g ( t : T ) : T = t }
class AImpl () extends A [ AImpl ] {
def f ( t : AImpl ) : AImpl = t }
</code></pre>
(Traits are a generalisation of Java interfaces). This can be resolved at compile... | https://cstheory.stackexchange.com |
38,151 | [
"https://mathoverflow.net/questions/38151",
"https://mathoverflow.net",
"https://mathoverflow.net/users/9107/"
] | How many different rectangles (in terms of area) can fit in a 20-unit-wide square? The rectangles can be squares, and their dimensions are integers.
| If you're looking for the number of different areas realizable by fitting rectangles in a 20x20 square <em>with (integer-length) edges parallel to the coordinate axes</em>, the answer is the number of elements in {$ \{ x \times y | x,y \in \{ 1..20 \} \} $}. In Haskell, <code>length . List.nub . sort $ [x*y| x<-[1..... | The answer is 27. The 28th triangular number is 406, so 28 is out, but you can fit rectangles in the square so that 1+...+21+24+25+27+28+30+35=400. 1-20 are 1xn rectangles, and you can arrange them so that you get a 10x19 rectangle left over. Cut off a stripe with 3x9 and 3x10, so you have 7x19 left. Chop off the o... | https://mathoverflow.net |
498,770 | [
"https://electronics.stackexchange.com/questions/498770",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/252020/"
] | I have an Atmega4809 (Arduino Nano Every) connected via UART (Rx,Tx,5V,GND) to a BLDC controller. Then I have connected a little display (Rx,Tx without GND.)
When it is connected everything works fine. When I follow the connection, disconnection steps, nothing gets damaged.
The BLDC controller and display are connec... | A diode from GND pointing to 5V is a good idea. Also, resistors in series with the RX and TC (maybe 270 ohms). When you connect the BLDC, perhaps it is actually 'bouncing connection' with many small spikes over a few milliseconds. With these high currents you will get spikes of a few hundred volts lasting only micro... | You shouldn't connect/disconnect circuitry while power is applied - if the ground is disconnected first you can put damaging currents through the signal path.
| https://electronics.stackexchange.com |
152,548 | [
"https://dba.stackexchange.com/questions/152548",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/83128/"
] | the first step is to checksum difference and record into percona.checksum table
<pre><code>pt-table-checksum h=xx,P=xx,u=xx,p=xx method=dsn=h=xx,D=xx,t=xx --databases=kobe
</code></pre>
the second step is to print the repair SQL into a file
<pre><code>pt-table-sync --print --sync-to-master h=xx,P=xx,u=xx,p=xx >... | Use union between the two queries. The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.
It will also give better performance as you ... | Use the UNION keyword again. You almost had it in your second query.
<pre><code>SELECT
UM.UMKEY,
OS.MAILTITLE,
UM.ADDRESS01,
UM.ADDRESS02,
UM.ADDRESS03
FROM (OS LEFT JOIN UM ON OS.HOMEKEY = UM.UMKEY) Left Join UM as MUM On OS.MAILKEY = MUM.UMKEY
WHERE (OS.OS_EMAIL != '') And (UM.INTERNATIONAL != 'Y') And (OS.DECEASED ... | https://dba.stackexchange.com |
2,504 | [
"https://stats.stackexchange.com/questions/2504",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/795/"
] | Is it possible to test for finiteness (or existence) of the variance of a random variable given a sample? As a null, either {the variance exists and is finite} or {the variance does not exist/is infinite} would be acceptable. Philosophically (and computationally), this seems very strange because there should be no diff... | No, this is not possible, because a finite sample of size $n$ cannot reliably distinguish between, say, a normal population and a normal population contaminated by a $1/N$ amount of a Cauchy distribution where $N$ >> $n$. (Of course the former has finite variance and the latter has infinite variance.) Thus any fully ... | You cannot be certain without knowing the distribution. But there are certain things you can do, such as looking at what might be called the "partial variance", i.e. if you have a sample of size $N$, you draw the variance estimated from the first $n$ terms, with $n$ running from 2 to $N$.
With a finite population var... | https://stats.stackexchange.com |
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