qid int64 1 4.65M | metadata listlengths 3 3 | prompt stringlengths 31 25.8k | chosen stringlengths 17 28.2k | rejected stringlengths 19 40.5k | domain stringclasses 28
values |
|---|---|---|---|---|---|
228,999 | [
"https://softwareengineering.stackexchange.com/questions/228999",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/111438/"
] | We are redesigning our corporate website with the following functions:
<ul>
<li>we want to give the marketing users flexibility and freedom to add new static pages, change the static pages or contents of static pages without the developer(s)/deployment team getting involved.</li>
<li>we want developers to work on the ... | Step 1) Research the content management systems out in the world, as Mickey Sly says.
Step 2) Select one that nearly fits. Implement it as a trial. Present it as a prototype to a pilot team. Re-implement a couple of times.
Step 3) Decide if you need to implement your own CMS. I would be willing to bet a dollar that y... | Well, it sounds like you're looking for a Content Management System or CMS. There are dozens of free ones out there on the internet that are pretty easy to setup and use. Although, it seems like you're talking about building this CMS yourself. My suggestion would be to look at some open source CMSs on Google and after ... | https://softwareengineering.stackexchange.com |
157,218 | [
"https://mathoverflow.net/questions/157218",
"https://mathoverflow.net",
"https://mathoverflow.net/users/46773/"
] | Let $\mathcal S '(\mathbb R^d)$ be the space of Schwartz tempered distributions equipped with the weak-* topology. I need to know if this space is second countable, i.e. if this topology has a countable basis. To put this in context, I need this because I am considering random variables with values in $\mathcal S'$ and... | Since $\mathscr{S}'$ is the dual of an infinite-dimensional Fréchet space, the weak-* topology in $\mathscr{S}'$ is not even first countable. What people usually do is to define probability measures in $\mathscr{S}'$ which are supported in a subspace $\mathscr{W}$ which is endowed with a stronger topology which makes $... | Pedro and Alpha are right, of course. But perhaps the following remark can be helpful:
Being the dual of a Frechet space, $\mathscr S'$ (endowed with the weak* topology) is sigma-compact: if $U_n$ form a base of $0$-neighborhoods in $\mathscr S$ it is covered by the polars $U_n^\circ$ which are weak*-compact by Alaogl... | https://mathoverflow.net |
401,905 | [
"https://mathoverflow.net/questions/401905",
"https://mathoverflow.net",
"https://mathoverflow.net/users/42412/"
] | Consider a <span class="math-container">$6\times 1$</span> continuous random vector
<span class="math-container">$$
\eta\equiv (\eta_1,\eta_2,..., \eta_6)
$$</span>
satisfying the following property:
<span class="math-container">$$
\underbrace{\begin{pmatrix}
\eta_1\\
\eta_2\\
\eta_3
\end{pmatrix}}_{\equiv x_1} \sim \u... | <span class="math-container">$\newcommand{\ep}{\epsilon}\newcommand{\R}{\mathbb R}$</span>There is no necessary and sufficient condition <strong>in terms of the support of <span class="math-container">$G$</span></strong> for the following: there exists a <span class="math-container">$4\times 1$</span> random vector <sp... | I will argue that (A) is incorrect: <span class="math-container">$G$</span> always has full support.
The proof is simple. Define <span class="math-container">$a_1=\eta_1$</span>, <span class="math-container">$a_2=\eta_2$</span>, <span class="math-container">$a_3=\eta_3$</span>, <span class="math-container">$a_4=\epsilo... | https://mathoverflow.net |
251,434 | [
"https://dba.stackexchange.com/questions/251434",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/186563/"
] | Using PostgreSQL 9.3.24
<strong>Issue</strong>
Just simple operation, but I don't know how to do that best simplest way.
How can I display results only when sum of <code>available_count</code>s for same <code>product_id</code> is <code>0</code> and <code>onway</code> is <code>0</code> or <code>NULL</code>? I.e. <cod... | <blockquote>
Is there any good practice or specific reason why SQL Server should have the same memory configuration across different replicas?
</blockquote>
This is definitely not a rule or prerequisite, normally people tend to keep configuration same so that when failover happens and secondary becomes primary shoul... | That’s due to fact it (SECONDARY) has to serve same level of work load as PRIMARY. I.e. when PRIMARY is memory intensive for 32GB, and SECONDARY limits to 24GB in the incident when it’s become PRIMARY it could perform same as primary the impact (8GB Memory) could be in various factors
<ul>
<li>Additional connections m... | https://dba.stackexchange.com |
2,205,339 | [
"https://math.stackexchange.com/questions/2205339",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/376963/"
] | Let's say I have a function $F:\mathbb R \times \mathbb R \rightarrow \mathbb R$, which meets the requirements of the implicit function theorem. Now I would like to find a general expression for:
$$\frac{d^2f}{dx^2}$$
From the theorem I know that $$\frac{df}{dx} = - \Big(\frac{\partial F}{\partial y} \Big )^{-1} \frac... | Hint: use long division:
$$\frac{t^8}{t^2+1}= t^6-t^4+t^2-1+\frac{1}{t^2+1}.$$
| $$t^8 = (t^2)^4 = ((t^2+1)-1)^4 = \sum_{k=0}^{4}\binom{4}{k}(-1)^k (t^2+1)^{k} $$
leads to:
$$ \int_{0}^{1}\frac{t^8}{1+t^2}\,dt = \frac{\pi}{4}+\sum_{k=1}^{4}\binom{4}{k}(-1)^k \int_{0}^{1}(1+t^2)^{k-1}\,dt =\color{red}{-\frac{76}{105}+\frac{\pi }{4}}.$$
| https://math.stackexchange.com |
55,598 | [
"https://stats.stackexchange.com/questions/55598",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/17832/"
] | Let's say I'm analyzing the mean number of students per class for a school district. The district has imposed a hard limit on the maximum ratio: there can never be more than 30 students in a class. The district strictly enforces this rule and it is known to hold true.
I'd like to construct a confidence interval usin... | You could use resampling and order statistics. From your dataset of 100 points, select say, 100000 samples of size 100 with replacement. Find the average class size in each case. Order the averages from smallest to largest. Remove the bottom 2500 (2.5%) and top 2500 (2.5%). Now your 95% confidence interval is the range... | If the confidence interval for the mean is running into a physical boundary you need to calculate the Bayesian credible interval for the mean where the prior assigns zero probability to the impossible values.
| https://stats.stackexchange.com |
12,982 | [
"https://security.stackexchange.com/questions/12982",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/8614/"
] | I'm creating a server where people can come and talk to each other. I'm mostly doing it for a fun project, here's my question.
I'm using RSA for encryption/decryption, so only the clients will know what's being said. I don't want the server to handle any decrypted text.
My question is, what's the most efficient way o... | Public key cryptography can, in this case, be used to facilitate setting up a secure channel in which to transmit a symmetric key. Once the secure channel has been set up, it is not necessary to continue to encrypt and decrypt using the public/private keys. Instead, generate a symmetric key and use that to encrypt th... | Run a jabber server and tell them to use pidgin with OTR
| https://security.stackexchange.com |
53,271 | [
"https://electronics.stackexchange.com/questions/53271",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/9332/"
] | I drive all Manual Transmission vehicles, and after a long battle of wills with my girlfriend, she has conceded that she would learn if there was a display that showed what gear was selected, thinking this might deter me. To her future dismay, I have an Arduino and I'm not afraid to use it.
So here's what I want to bui... | This is actually harder than it looks to get repeatable.
I would not recommend hall effect sensors for this application. Their sense distances are generally small, meaning that the mounting of the sensors and magnet need to be done with some precision. You will find it very challenging to align all of the halls to... | <strong>Edit:</strong> Well, if HikeOnPast is right, this won't work for you either (as you have no shifter linkages to deal with mechanically)... The best bet for this to work for you would be to create some sort of mask that sits on top of the shifter unit under the boot with 6 optosensors, and then attach a mask to ... | https://electronics.stackexchange.com |
135,233 | [
"https://mathoverflow.net/questions/135233",
"https://mathoverflow.net",
"https://mathoverflow.net/users/50973/"
] | Are there any good way to understand $k$-polarized Abelian surfaces? I am aware that if $A \cong \mathbb{C}^2/\Gamma$ is $k$-polarized, the lattice $\Gamma$ can be taken of the form
$$
\begin{bmatrix}
1 & 0 & \tau_1 & \tau_2\\
0 & k & \tau_3 & \tau_4
\end{bmatrix}
$$
over $\mathbb{Z}$ (think of... | It is well known that any polarized abelian variety is isogenous to a principally polarized one. More precisely, given any polaized abelian variety $(A, \, L)$ there exists a principally polarized abelian variety $(B,\, \Theta)$ and an isogeny $u \colon A \longrightarrow B$ such that $L=u^* \Theta$.
Then any polarized... | I don't know what your background is in abelian varieties, but there are several equivalent ways of talking about $k$-polarized abelian varieties (I assume that you're speaking of a polarized abelian variety of type $(1\hspace{0.1cm}1\cdots1\hspace{0.1cm}k)$?). The following are equivalent:
<ol>
<li>$(A,L)$ is $k$-pol... | https://mathoverflow.net |
191,334 | [
"https://physics.stackexchange.com/questions/191334",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/25343/"
] | For sound from 20Hz to 20kHz, wavelength is 17m to 17mm, for sound at 2kHz, wavelength is 17cm.
And I saw tiny microphone which is much smaller than that. In electromagnetic, there is a smallest size for antenna of each wavelength (half wavelength???). And there is a law (IIRC) that if sampling frequency is smaller th... | Some thoughts on the subject:
The key difference between a microphone and an antenna is that the microphone is sealed from the back - it senses a <em>pressure difference</em> between the front and the back of the membrane regardless of the extent of that pressure region.
If you have a small membrane that is <em>not</... | A microphone is a transducer that converts variations in air pressure from sound waves into electrical signals.
Air pressure varies as the wavefront passes into the diaphragm (or the ribbon, or the condenser) of the microphone. The diaphragm needn't be as long as the wavelength, as it senses the wave from a "head-on"... | https://physics.stackexchange.com |
691,822 | [
"https://math.stackexchange.com/questions/691822",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/77862/"
] | <blockquote>
Show that if $z_0$ is a solution to $(2z-1)^{2014}=(2z+1)^{2014}$, then $\Re(z_0)=0$.
</blockquote>
My attempt:
$(2z-1)^{2014}=(2z+1)^{2014}\\
\implies \left(\dfrac{2z-1}{2z+1}\right)^{2014}=1=e^{2k\pi i}, k=0\space \ldots \space 2013$
Let $\omega:=e^{2k\pi i}$
Then $\dfrac{2z-1}{2z+1}=\omega\\
\impl... | If $(2z-1)^{2014}=(2z+1)^{2014}$, then the two sides have the same absolute value, from which we can take a $2014$th root, and
$$
\begin{align}
\left|2z-1\right|&=\left|2z+1\right|\\
\implies(2z-1)(2\bar{z}-1)&=(2z+1)(2\bar{z}+1)\\
4z\bar{z}-2z-2\bar{z}+1&=4z\bar{z}+2z+2\bar{z}+1\\
-2\left(z+\bar{z}\right)... | Since $|2z-1 | = |2z+1|$, this tells us that $2z$ lies on the perpendicular bisector of the line from $-1$ to $1$, IE it lies on the line with real part 0.
| https://math.stackexchange.com |
136,880 | [
"https://mathoverflow.net/questions/136880",
"https://mathoverflow.net",
"https://mathoverflow.net/users/4231/"
] | The list of finite <em>simple</em> groups of Lie type has been understood for half a century, modulo some differences in notation (and identifications between some of the very small groups coming from different Lie types). Call this collection of isomorphism classes of finite groups $\mathcal{S}$. But it's not clear... | I feel a bit funny posting this as an answer to someone who has written a book with "finite groups of Lie type" in the title.
But I also find the matter both confusing and interesting (and full of conflicting terminology!), so here's my outside take on this, being a topologist of training:
(My own literature referenc... | I (with co-authors) recently experienced a similar problem with the term "(finite) classical group". We initially tried to write down a precise definition, but eventually gave this up as a bad job, because we couldn't come up with anything that seemed right.
It is very unclear to me what you want to include in $\mathc... | https://mathoverflow.net |
438,039 | [
"https://stats.stackexchange.com/questions/438039",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/260367/"
] | Suppose I have a vector v1 with values in the set {-1,0,1}
<pre><code>> v1
0 0 -1 1 0 0 0 0 0 0 0 0 0 0 -1 1 0 0 0 0 0 0 0 0 0 0 1 -1 0 0 0 0 0 0 0 0 0 0 -1 1 0 0 0 0
</code></pre>
How to generate a vector v2 (with values in the set {-1,0,1}) that has zero correlation (or clo... | You can generate a vector that has, on average, zero correlation simply by randomly permuting your original vector.
<pre><code>v2 = v1[sample(length(v1),length(v1)]
</code></pre>
This won't get you a zero-correlation vector every time, but if you run this many times, you'll see that the average correlation value is z... | Very simple. You can easily create three vectors by taking any number in the set and repeating that number till the length of v1. R code:
rep(1, length(v1))
or take value 0 or -1
| https://stats.stackexchange.com |
253,025 | [
"https://softwareengineering.stackexchange.com/questions/253025",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/138830/"
] | I've noticed that the string library doesn't require one to allocate space before writing the input/output. How does the string library allocate that memory dynamically, i know of the 'new' keyword in c++ and 'malloc' in c but when i try something like this, my program stops working.
<pre><code>char* str = new char[st... | <blockquote>
How does std::string in c++ allocate memory?
</blockquote>
as needed, transparently to the user.
<blockquote>
i know of the 'new' keyword in c++ and 'malloc' in c but when i try something like this, my program stops working.
</blockquote>
<pre><code>char* str = new char[strlen(str) + 1];
cin>>... | If you're not a fan of <code>std::vector</code> you may not want to hear this but, by default, it'll work pretty much the same as <code>std::vector</code>.
When a <code>string</code> reads from a stream, it <code>push_back</code>'s each character. What it does when it runs out of space is actually defined by the alloc... | https://softwareengineering.stackexchange.com |
542,277 | [
"https://math.stackexchange.com/questions/542277",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/21813/"
] | Given that $0 \leq f' \leq f$ on $\mathbb R$ and $f(a)=0\in\mathbb R$ for some $a\in\mathbb R$, how do I prove that $f$ is identically zero? Using the mean value theorem naively didn't really get me anywhere.
| Since $f'\ge 0$, then
$$
f(x)=f(a)+\int_a^x f'\ge 0 \mbox{ for any } x>a
$$
For the other side,
$$
f'-f\le0
$$
Then
$$
0\ge e^{-t}[\,f'-f\,]=(e^{-t}f)'
$$
Therefore
$$
e^{-x}f(x)=e^{-a}f(a)+\int_a^x (e^{-t}f(t))'\le 0 \mbox{ for any } x>a
$$
So, for $x>a$, $0\le f(x)\le 0$.
For $x<a$ same idea.
| Hint: Suppose $a + 1 > x > a$. Then
$$f(x) = f(x) - f(a) = f'(c)(x-a)$$
For some $c \in (a,x)$. But
$$f'(c)(x-a) \leq f(c)(x-a) = (f(c) - f(a))(x-a) = f'(d)(c-a)(x-a)$$
For some $d \in (a,c)$. Suppose $f'$ is bounded by $M$ on $[a-1,a+1]$ (note that $f'$ is bounded on this set since $f$ is continuous so bound... | https://math.stackexchange.com |
15,378 | [
"https://dsp.stackexchange.com/questions/15378",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/8442/"
] | Let's suppose I have a signal <span class="math-container">$F(t)$</span> that is periodic, with two periodicities <span class="math-container">$P1$</span> and <span class="math-container">$P2$</span>, with <span class="math-container">$P1>P2$</span>
Suppose that I know the values of the two periodicities. Using th... | If you have two periodic functions $f(t)$ and $g(t)$, where $f(t)$ has fundamental frequency $f_0$ and $g(t)$ has fundamental frequency $2f_0$, then their sum $h(t)=f(t)+g(t)$ is periodic with fundamental frequency $f_0$. Without any further knowledge about $f(t)$ or $g(t)$ there is no way to separate the two, because ... | Assume a model for waveform generation:
<span class="math-container">$$y(t) = a_0*\sin(2*\pi*f*t) + a_1*\sin(2*\pi*2*f*t)$$</span>
Case 1:
<ul>
<li><span class="math-container">$f(t)$</span> contains <span class="math-container">$\left[a_0, a_1\right] = \left[1, 0\right]$</span> and <span class="math-container">$f =... | https://dsp.stackexchange.com |
20,783 | [
"https://physics.stackexchange.com/questions/20783",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/7382/"
] | I'm stuck in a small calculation. I try to solve a small exercise.
You should calculate for a given entropy the Gibbs free energy, where the entropy is given as
$S\left(U,V\right) = \frac{4}{3}\left(\alpha V U^3\right)^{1/4} \qquad\dots\qquad U$ is the internal energy, $V$ is the volume and $\alpha > 0, \text{cons... | Much depends on how "deep" do you want the description to be. Let's assume the beam-splitter works as an external potential (in QM) and does the scattering. The 1-particle scattering operator is the unitary operator of the "Hadamard gate",
$u=\begin{pmatrix} 1& 1\\ 1& -1 \end{pmatrix}\frac{1}{\sqrt2}.$
This ... | This isn't terrible hard to do.
Say the PBS reflects vertical polarised light and transmits horizontal.
So we have a 4 mode system with the following transformation
<span class="math-container">$$\left[\begin{array}{c}
a^\dagger _{1H} \\
a^\dagger _{1V} \\
a^\dagger _{2H} \\
a^\dagger _{2V} \\
\end{array}\right] \ri... | https://physics.stackexchange.com |
120,702 | [
"https://cs.stackexchange.com/questions/120702",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/115623/"
] | Given two arrays whose elements lie between <span class="math-container">$[1,10^5]$</span> and the size of arrays is <span class="math-container">$[1,10^5]$</span>, how can we find the total number of pairs of elements from these arrays such that their product is a perfect square? The arrays may have same elements.
Fo... | There are <span class="math-container">$9592$</span> primes below <span class="math-container">$10^5$</span>. You can convert each number in each array to a sparse binary vector of length <span class="math-container">$9592$</span>, signifying the parity of the power of each prime. Using radix sort, sort each of the arr... | The numbers involved are small, which makes this quite trivial. The numbers are ≤ <span class="math-container">$10^5$</span>. We have <span class="math-container">$10^5 < 47^3$</span>, and there are 14 primes < 47.
So for each of the numbers x, y in X, Y, we calculate a 14 bit integer with bit #i set if x, y is... | https://cs.stackexchange.com |
113,989 | [
"https://physics.stackexchange.com/questions/113989",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/34304/"
] | Suppose we have a disc with a hole, when computing moment of inertia of this about the disc's centre. Why do we subtract the moment of inertia of the removed part from the moment of inertia of complete disc?
| Because the moment of inertia for a point mass is:
$$ I = mr^2 $$
When calculating the moment of inertia for continuous bodies we use calculus to build them up from infinitesimal mass elements, so effectively to calculate the moment of inertia of the disk (without hole) we're doing:
$$ I_{disk} = \sum_i^{disk} m_ir^... | Moment of inertia of a composite body is found out by adding together the moments of inertia of constituent parts(of course, taken with respect to the same axis). Formally, we say that moment of inertia is an additive property. Which simply put here, means that:
$$I_{disk(without\ hole)}=I_{cut\ out\ part}+I_{disk\ wi... | https://physics.stackexchange.com |
48,045 | [
"https://mathoverflow.net/questions/48045",
"https://mathoverflow.net",
"https://mathoverflow.net/users/6094/"
] | I am puzzled by the amazing utility and therefore ubiquity of
two-dimensional matrices in comparison to the relative
paucity of multidimensional arrays of numbers, <em>hypermatrices</em>.
Of course multidimensional arrays are useful:
every programming language supports them, and I often employ them myself.
But these us... | Note that in linear algebra matrices describe at least two different things: linear maps between vector spaces (we consider only finite-dimensional vector spaces here) and bilinear forms. When thinking of matrices as tensors, linear maps between $V$ and $W$ are elements of the space $V^* \otimes W$, whereas bilinear fo... | To such a complex problem, there cannot be a unique answer. I see many, which all justify the tremendous interest that mathematicians have devoted so far to matrices, rather than to hypermatrices.
<strong>Ubiquity</strong>. Matrices are used by every species of mathematicians, and beyond, by a large fraction of scient... | https://mathoverflow.net |
737,381 | [
"https://math.stackexchange.com/questions/737381",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/139961/"
] | Let $n \geq m$ and let $C$ be a $n \times m$ full rank matrix, that is $rank(C) =m$. Considering that $D$ is a diagonal positive semidefinite matrix, under which conditions is the $ m \times m$ matrix $X=C^\mathsf{T}DC$ invertible? Is it enough to have $rank(D) \geq m$?
| Since $X=C^TDC$ is generally positive semi-definite, $X$ is nonsingular if and only if it is positive definite, that is, $v^TXv=v^TC^TDCv>0$ for all nonzero $v$. Another way, how to say this, is that $D$ is positive definite on the column-span (range) of $C$. Also, $X$ is nonsingular if and only if the intersection ... | If $D$ is positive definit, and $C$ has full rank ($C\in \mathbb R^{n,m}$, $n\ge m$, $rank(C)=m$), then $C^TDC$ is positive definite, hence invertible: Take $x\ne 0$ then
$$
x^TC^TDCx = (Cx)^TD(Cx) >0
$$
as $D$ is positive definite and $Cx\ne 0$.
| https://math.stackexchange.com |
1,336,307 | [
"https://math.stackexchange.com/questions/1336307",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/250122/"
] | Problem:
There are 2 girls and 7 boys in a chess club. A team of four persons must be chosen for a tournament, and there must be at least 1 girl on the team. In how many ways can this be done?
Now,the solution suggested by the book is C(7,2)+2xC(7,3)=91 ,Which I understand,But I don't understand why my answer which i... | Your answer has some double counting. You may choose girl A then when choosing three more people you choose girl $B$ and two boys. You also may choose girl B then when choosing three more people you choose girl $A$ and those same two boys.
Your formula counts that possibility twice but it should count only once.
You ... | You answer is incorrect because you will have duplicates in your answer.
If you choose a girl A from the girls and then girl B, boy C, and boy D from the rest, that will be the same choosing girl B from the girls and then girl A, boy C, and boy D.
| https://math.stackexchange.com |
132,292 | [
"https://electronics.stackexchange.com/questions/132292",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/52516/"
] | What is the difference between overload and short-circuit protection for motors? I do not understand the need for SC protection when a motor has overload protection. Shouldn't the overload protection trip under SC conditions (and really fast due to the inverse time characteristics)?
| Overload and short-circuit protection protect against two different parameters being exceeded: heat (power integrated over a relatively short time) and current.
A high current can be a an almost instantaneous problem: a wire that can't shed its heat can get too hot and burn through, or set fire to surrounding materia... | I'm not sure in what contexts a motor itself would require short-circuit protection, but motor <em>driver</em> circuitry certainly can require short-circuit protection separate from overload protection. Many parts of a motor and its associated control system will heat up and cool down relatively slowly, and may be exp... | https://electronics.stackexchange.com |
508,937 | [
"https://physics.stackexchange.com/questions/508937",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/226560/"
] | All examples of lorentz invariant quantities that I have come across seem to be scalars: rest mass, proper time, spacetime interval,dot product of two 4 vectors etc. Another thing is that these are all index contractions.
So, <strong>is there any lorentz invariant quantity which isn't a Lorentz scalar?</strong>
(My ... | “Scalar” and “Lorentz invariant” are synonyms in the context of Special and General Relativity.
However, it is possible to have constant tensors whose components don’t actually change when transformed, such as the Minkowski metric tensor <span class="math-container">$\eta_{\mu\nu}$</span> in Special Relativity. We don... | Every Lorentz invariant is a <strong>Lorentz</strong> scalar. That is just true by definition.
The trick here is to specify what an object is a scalar, vector, tensor, spinor, etc <strong>with respect to</strong>.
For instance, the electric charge <em>density</em> is a scalar with respect to spatial rotations, but ... | https://physics.stackexchange.com |
327,285 | [
"https://math.stackexchange.com/questions/327285",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/33464/"
] | We know that any metric space is Hausdorff with respect to the topology induced by the metric, or that any metrizable space is Hausdorff. Then the contrapositive is:
not Hausdorff $\stackrel{?}{\Rightarrow}$ not metrizable ?
I was thinking of a very weird contrapositive, which is not Hausdorff $\Rightarrow$ not metri... | To prove that $V_1$ is irreducible, it is sufficient to prove that $\mathrm{End}_{\mathbb{C}G}(V_1)$ is one-dimensional. (<b>Subproof</b>: if $V_1$ has a non-zero proper subrepresentation $W$ then $V_1 = W \oplus C$ for some complementary representation $C$. The projection maps onto $W$ and onto $C$ corresponding to th... | There is a way of reconciliating the point of view of Beginner and that of Mark Wildon. Let $v=\sum x_i e_i$ a non-zero vector in $W$. Then $x_i \not= 0$ for some $i$ and translating $v$ by an element of $G$, one may assume that $i=1$. Let $H$ be the stabilizer of $e_1$ in $G$. By assumption it acts transitively on $\{... | https://math.stackexchange.com |
15,397 | [
"https://softwareengineering.stackexchange.com/questions/15397",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/6651/"
] | Am I wrong if I think that Python is all I need to master, in order to solve most of the common programming tasks?
<strong>EDIT</strong>
I'm not OK with learning new programming languages if they don't teach me new concepts of programming and problem solving; hence the idea behind mastering a modern, fast evolving, wi... | You are right and wrong.
<strong>Right:</strong> Knowing a single tool very well is very marketable and to be desired. And Python is good for OO, for scripts, for functional-ish programming, and it has excellent mathematical and scientific libraries.
<strong>Wrong:</strong> Python doesn't teach you everything a go... | <h1>Yes</h1>
You would be wrong to think that any single language can do everything without considering other languages.
<blockquote>
I think that in the fast evolving tech industry, specialization is key to success.
</blockquote>
I can't think of a better way to shoot yourself in the foot. Learning and mastering ... | https://softwareengineering.stackexchange.com |
24,246 | [
"https://electronics.stackexchange.com/questions/24246",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/1870/"
] | I've been teaching myself the fine art of monitor repair - replacing capacitors on a trio of lcd screens. Apparently they all had capacitors that had suffered from the plague - though a mild case of it since they bulged a tiny bit, as opposed to leaked all over the place., and replacing them has allowed two of them to ... | In some types of simulation, the more states the merrier. If you are doing only single-clock synchronous logic, I would suggest three states (high/low/undefined), but expand out the JK flip flop's logic so as to include the full computation of the next value. For example, if one has a circuit in which both J and K in... | I would go with the raise an error and refuse to simulate.
Having a floating input is never a good idea.
| https://electronics.stackexchange.com |
16,711 | [
"https://dba.stackexchange.com/questions/16711",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/3351/"
] | I'm comparing a bunch of tables from different databases on different servers to a Master record. I need to know which servers, identified by <code>locationID</code>, have the non-matching rows because they might need maintenance.
I've got a simple <code>EXCEPT</code> query where I compare a table where each row is th... | You can also do this with dynamic SQL without having to manually build out all the column names.
<pre><code>DECLARE @sql NVARCHAR(MAX), @c1 NVARCHAR(MAX), @c2 NVARCHAR(MAX);
SELECT @c1 = N'', @c2 = N'';
SELECT
@c1 = @c1 + ',' + QUOTENAME(name),
@c2 = @c2 + ' AND m.' + QUOTENAME(name) + ' = s.' + QUOTENAME(name)... | I would recommend:
<ul>
<li>Creating a <code>Hash</code> field that is a persisted computed column with a definition along the lines of <code>HASHBYTES('SHA1', Field1 + Field2 + Field3...)</code></li>
<li>Comparing just that <code>HASH</code> value from your "master" to your other records</li>
<li>Displaying all the a... | https://dba.stackexchange.com |
39,022 | [
"https://chemistry.stackexchange.com/questions/39022",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/2108/"
] | I have read that no work is done during free expansion and temperature of an ideal gas undergoing free expansion doesn't change.
Let us assume an ideal system of piston and gas which has vacuum around it. The gas will expand but we say there is no work done by the gas. Why so ? Gas applies a pressure(& hence a fo... | Little bit tricky question. As you say, the work is force acting along the path. But there is no force acting, as there is weightless piston with vacuum on the opposite side.
To visualize the process better, imagine an inflatable balloon in outer space, when you puncture it. All gas molecules continue to fly in the di... | I have seen some answers on this similar question they all point to same thing that pressure outside is zero so from <span class="math-container">$W = p.\Delta V$</span>, work done will be zero or as said in the answer from @ssavec that there is no force acting on the piston, the point is that you cannot have a balloon... | https://chemistry.stackexchange.com |
547,937 | [
"https://electronics.stackexchange.com/questions/547937",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/23950/"
] | I worked out how to drive a ledmatrix (8x8) width monochrome leds with 2 <em>SN74HC595N</em> on a STM32F103C8 over SPI. It works. (Un) fortunately I got my hand on a free a bag of <strong>SN74LS595N</strong>'s.
I tried to drive the leds with that, because I think that should be possible.
However, when I simply replace ... | STM32F103C8 is a 3.3V chip. The 74LS TTL series requires 5V. There are consequences to having a mixed voltage system that are not really worth explaining here. You should get the actual part unless you want to read and understand <strong>all</strong> the differences and make accommodations (spoiler: it will be more tro... | Some points:
<ul>
<li>LS chips are 5V only, you will need a stable 5V supply, not 3.3V.</li>
<li>LS chips are far less forgiving in power supply issues. You might need (much) better Vcc decoupling than you needed with the HC's, at the very least one 100 nF cap per chip.</li>
<li>LS chips require TTL (5V) input; the 2.0... | https://electronics.stackexchange.com |
3,435 | [
"https://security.stackexchange.com/questions/3435",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/1244/"
] | Given that I have two pieces of data:
<ul>
<li>A large piece of encrypted data.</li>
<li>A small, fixed-size piece of random data.</li>
</ul>
I want to be able to transmit both pieces of data together, but I don't want the second piece of data to be readily available to a third party, but easily retrievable at the ot... | If the intended receiver knows "how it was hidden", but not the attacker, then this means that there is a "secret convention", shared between the sender and receiver, and applied to the piece of random data which you want to keep confidential. Regardless of how you want to call it, this is encryption (both legally and ... | No, encrypted data is supposed to look completely random. so if you hide your random data in the encrypted data there's no way the attacker can say what is random or encrypted.
| https://security.stackexchange.com |
1,085,608 | [
"https://math.stackexchange.com/questions/1085608",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/203854/"
] | Hello can someone check my work and tell me what to do next.
<blockquote>
Find the values of $\sin z = 0$
</blockquote>
we know that; $$\sin z = {{{e^{zi}} - {e^{ - zi}}} \over {2i}}$$
So;
$${{{e^{zi}} - {e^{ - zi}}} \over {2i}} = 0$$
$${e^{zi}} - {e^{ - zi}} = 0$$
Then I multiply with $e^{jz}$;
$${({e^{zi}})^2} -... | Since $f \in L^2(\mathbb{R}) \iff \hat{f} \in L^{2}(\mathbb{R})$, then it suffices to work in the frequency domain. Using the unitary, ordinary frequency convention for the Fourier transform, we know that
\begin{align}
\mathcal{F}[f^{(n)}](\xi) = (2\pi i\xi)^{n}\hat{f}(\xi)
\end{align}
But since $f$ is a $\mathrm{sinc}... | Using the power series of $\sin$, one can easily see that $f$ is smooth on $\mathbb R$. So each $f^{(n)}$ exists and is continuous on $\mathbb R$ and to ensure that $f^{(n)}$ belongs to $L^2(\mathbb R)$ all you need to know is its summability at $\pm \infty$.
For $x\neq 0$, one can write $f(x)$ as $f(x)=h(x)g(x)$ with... | https://math.stackexchange.com |
627,065 | [
"https://electronics.stackexchange.com/questions/627065",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/182924/"
] | I can't understand why it is dangerous to leave the secondary side of a current transformer open.
| If you short a transformer secondary, then it is a zero ohm short circuit, an infinite load. At least for tranformers that are used to step voltage up or down that would not work because it would not be able to step voltage up or down, because a short circuit requires that output voltage is zero.
But CT are another thi... | When the secondary winding of a power or distribution transformer is short-circuited, protective devices, such as fuses / circuit breakers, would blow / trip to clear the fault.
The secondary short circuit current would reflect on the primary side as well.
Protective devices would be available in the primary as well as... | https://electronics.stackexchange.com |
60,070 | [
"https://electronics.stackexchange.com/questions/60070",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/10862/"
] | I want to make a device that I can place on a capacitive touch screen and it will start tapping the screen repeatedly. Is there any way to do this with the Arduino?
| This can be done if you get an Arduino plus a servo controller that has a rig that controls a stylus. The stylus must be able to work with capacitive touch screen (not all do).
Look for Arduino Servo Controller shield
| Yes, this is quite possible to do. Simply place a small piece of copper foil on the screen. Connecting the foil to ground will simulate a finger press. You can use an N-channel FET to switch the foil between connected and not connected. The gate of the FET goes to your microcontroller.
| https://electronics.stackexchange.com |
314,696 | [
"https://physics.stackexchange.com/questions/314696",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/127650/"
] | Is it possible? If so, can energy also easily be captured using any type or particular types of liquid? For example, NaCl in liquefied form has sodium positive and chloride negative charged free moving ions. It can conduct electricity but storing electrical energy in it raises some questions in my mind.Can it be done? ... | Absolutely. The only what you need is a reversible, first-order phase transition where the phase with the higher energy is liquid.
For example, in a <0C environment, you can store energy in liquid water. If you meld a cup of ice (store energy in it), and as it freezes down, it releases that energy. Thus it works as... | Let's say it's an extremely windy night, and you're producing much more energy than the grid uses. You can store the energy produced by the wind turbines in water as simply as lifting it high up somewhere - like pumping it from a lower dam into one that's higher, thus increasing the water's potential energy. You expend... | https://physics.stackexchange.com |
480,206 | [
"https://physics.stackexchange.com/questions/480206",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/226311/"
] | Would it be possible in the standard model to have atom like systems in which muons (or tauons) take the place of electrons? Why don't we see more of them?
For instance it could be related to some mechanism leptogenesys, but I don't know much about this subject..
How the difference between muonic and electronic atoms ... | Absolutely they can exist. In fact, physicists often creat muonic hydrogen to study things like the structure/size of the proton with more accuracy.
The reason we don't see muonic/tauonic atoms in nature is that these particles decay very quickly, whereas the electron, being the lightest of the three generations of le... | Muonic atoms can be made from one antimuon and one electron. The antimatter version of this antimuonium is where one muon is orbited by a positron. The decay product goes like this:
Now the electron orbits the W+ boson and normally the W+ would decay into a positron and neutrino. However electron and W+ react way befo... | https://physics.stackexchange.com |
310,743 | [
"https://softwareengineering.stackexchange.com/questions/310743",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/203810/"
] | I am designing a JSON based data schema to describe the visual appearance of blocks similar to HTML elements.
It should feel familiar for somebody knowledgable in HTML/CSS but is more restrictive due to constraints of the enviroment that it will be used in.
Now I am unsure how to model "width".
Acceptable are either p... | Let's just say in a year from now you decide that you need support <code>em</code> or whatever other type for width.
For compatibility I would go for the first alternative as I think this change might be difficult with one of the other two.
Another solution might be to define types:
<pre><code>{
"width": 50, //A... | the 1st so it matches actual css, and is the simplest.
| https://softwareengineering.stackexchange.com |
136,239 | [
"https://physics.stackexchange.com/questions/136239",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/33975/"
] | I heard my friend, a researcher, say that we can, in theory, explain every event happening in the universe using the Conservation of momentum and energy. He added that we may not be able to do that "now" as we do not have all the variables required to describe the Universe completely.
My question is:
1) Is it possibl... | No. Consider the radioactive decay of an atom. The energy and momentum is conserved but the direction of emission is not known. We do know that on <em>average</em>, i.e. if we take many identical atoms and observe their decay, that the direction of emission is isotropic. However, for a single event we just can't seem t... | No. There are more conservation laws: conservation of angular momentum, electric charge, color charge, weak isospin.
Without the charge conservation you cannot explain why electrons do not decay into neutrinos.
| https://physics.stackexchange.com |
375,823 | [
"https://electronics.stackexchange.com/questions/375823",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/108973/"
] | EM waves are classified into types according to the frequency of the wave: these types include, in order of increasing frequency, radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
Its said that electromagnetic waves are produced by the accelerating/oscillating ch... | <blockquote>
Can i ideally produce all these electromagnetic waves of different frequencies by changing the frequency of current/voltage to the conductor ?
</blockquote>
That is necessary, but not sufficient: your conductor must be designed to <em>radiate</em>. That's why antennas have a specific shape. Typically, ... | <blockquote>
Is there any equation that relates the two frequencies ?
</blockquote>
Yes, the two frequencies are equal.
<blockquote>
Or simply to generate the electromagnetic wave of 8×10^14 Hz (light), i will have to oscillate the charges with the frequency of 8×10^14 Hz in the conductor ?
</blockquote>
Yes, bu... | https://electronics.stackexchange.com |
2,560,234 | [
"https://math.stackexchange.com/questions/2560234",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/467203/"
] | I am trying to solve the following integral but I am not sure where to begin.
$$\int{\frac{1}{\sqrt{x+y+z} + 1}}dx$$
I tried substituting $u = \sqrt{x+y+z}$ but I keep getting stuck.
How do I proceed?
| So I figured where I went wrong and thanks to those who commented I did the following:
$$
u = \sqrt{x+y+z} \\
du = \frac{1}{2}(x+y+z)^{-\frac{1}{2}}(1)dx = \frac{1}{2}\frac{1}{\sqrt{(x+y+z)}}dx = \frac{1}{2}\frac{1}{u}dx = \frac{1}{2u}dx \\
dx = 2u\ du
$$
Therefore:
$$
\int\frac{1}{\sqrt{x+y+z}+1}dx = \int\frac{2u}{... | $$\int\frac{1}{\sqrt{x+y+z}+1}\rm dx$$
Let $u={x+y+z}$
And $\rm du=\rm dx$
Your integral becomes $$\int \frac{1}{\sqrt{u}+1}\rm du$$
Let $w=\sqrt{u}+1,\;\;\;\rm dw=\frac{1}{2\sqrt{u}}\rm du=\frac{1}{2(w-1)}\rm du$
$$\int \frac{1}{\sqrt{u}+1}\rm du=2\int\frac{w-1}{w}dw=2(w-\ln|w|)=2(\sqrt{u}+1-\ln|\sqrt{u}+1|)=2(\s... | https://math.stackexchange.com |
42,338 | [
"https://scicomp.stackexchange.com/questions/42338",
"https://scicomp.stackexchange.com",
"https://scicomp.stackexchange.com/users/36539/"
] | Consider the following equation on <span class="math-container">$(0,1)$</span>, with Dirichlet boundary conditions on both ends.
<span class="math-container">$$
\frac{d}{dx}\left(k(x)\frac{du}{dx}\right) = 0
$$</span>
Let us solve this using simple linear finite elements. The computed displacement <span class="math-con... | What you are trying to do has a name: it's called "recovery" and is a particular case of "postprocessing" the solution.
In your approach of finding an approximation to derivatives by minimization, remember that intuitively you think that <span class="math-container">$\phi\approx u_x$</span>, but in ... | Your statement is true for the onedimentional case with linear elements.
However, if you consider higher dimensions with <strong>non-constant metrics</strong> (bilinear or trilinear) then your seconde derivatives won't necessarily vanish.
Moreover, if you drop <strong>(reinterpret) the element local perpective</strong>... | https://scicomp.stackexchange.com |
58,382 | [
"https://dba.stackexchange.com/questions/58382",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/26836/"
] | <pre><code>SELECT
smart_allusers_karen.lastlogin,
smart_allusers_karen.supemail,
smart_allusers_karen.regionname
FROM
smart_allusers_karen
WHERE lastlogin < MAX(smart_allusers_karen.lastlogin) - INTERVAL 30 DAY AND
lastlogin > MAX(smart_allusers_karen.lastlogin) - INTERVAL 60 DAY
ORDER BY lastlogin
</code... | Perhaps you can split the query up to catch the DateTimes in separate variables
<pre><code>SELECT lastlogin INTO @LastLogin FROM smart_allusers_karen ORDER BY lastlogin DESC LIMIT 1;
SET @ThirtyDaysAgo = @LastLogin - INTERVAL 30 DAY;
SET @SixtyDaysAgo = @LastLogin - INTERVAL 60 DAY;
SELECT
smart_allusers_karen.la... | It is not allowed to use an aggregate function that way, i.e. in the <code>WHERE</code> clause. One way to do it is to have the <code>MAX()</code> calculated in a subquery:
<pre><code>SELECT
k.lastlogin,
k.supemail,
k.regionname
FROM
smart_allusers_karen AS k
WHERE
k.lastlogin < (SELECT MAX(las... | https://dba.stackexchange.com |
604,206 | [
"https://stats.stackexchange.com/questions/604206",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/205653/"
] | There are 3 random variables, <span class="math-container">$X_1$</span>, <span class="math-container">$X_2$</span> and <span class="math-container">$Y$</span>. We know <span class="math-container">$$corr(X_2, Y)>corr(X_1, Y)>0$$</span>, but <span class="math-container">$$corr(X_2 - X_1, Y)<0$$</span>
In other ... | It is possible. Simple algebra shows that
<span class="math-container">\begin{align}
\rho_{X_2 - X_1, Y} = \frac{\sigma_{X_2}\rho_{X_2, Y} - \sigma_{X_1}\rho_{X_1, Y}}{\sigma_{X_2 - X_1}}.
\end{align}</span>
So although <span class="math-container">$\rho_{X_2, Y} > \rho_{X_1, Y}$</span>, the sign can be flipped if ... | A small example in R:
<pre><code>X1 <- c(0, 1, 3)
X2 <- c(0, 1, 2)
Y <- c(0, 1, 2)
cor(X1, Y)
# 0.9819805 # sqrt(27/28)
cor(X2, Y)
# 1
cor(X2-X1,Y)
# -0.8660254 # -sqrt(3/4)
</code></pre>
It would not be possible with covariance, as <code>cov(X2-X1, Y) = cov(X2, Y) - cov(X1, Y)</code>
| https://stats.stackexchange.com |
30,661 | [
"https://mathoverflow.net/questions/30661",
"https://mathoverflow.net",
"https://mathoverflow.net/users/2060/"
] | What are nice examples of topological spaces $X$ and $Y$ such that $X$ and $Y$ are <strong>not</strong> homeomorphic but there do exist continuous bijections $f: X \to Y$ and $g: Y \to X$?
| Recycling an old (ca. 1998) sci.math post:
" Anyone know an example of two topological spaces $X$ and $Y$
with continuous bijections $f:X\to Y$ and $g:Y\to X$ such that
$f$ and $g$ are not homeomorphisms?
Let $X = Y = Z \times \{0,1\}$ as sets, where $Z$ is the set of integers.
We declare that the following sub... | Here's a continuum analogue of Gerhard Paseman's answer: Let $X$ and $Y$ be topological spaces whose underlying sets are $\mathbb{R}$. As topological spaces, $X$ is the disjoint union of the open interval $(0,\infty)$ with a discrete space whose points are nonpositive reals, while $Y$ is the disjoint union of $(-1,0)... | https://mathoverflow.net |
312,677 | [
"https://physics.stackexchange.com/questions/312677",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/47472/"
] | I've been exploring some philosophies regarding movement and energy. One of the fun questions I've been able to play with is how momentum fits into those philosophies. I have to ask "can you have momentum without energy?" I know the opposite is true: you can have potential energy without momentum. It's the question... | How about virtual particles in the Breit-frame (the frame in which no energy is transferred)? For example, consider a head on collision between 2 electrons with 4-momenta:
$p^1_{\mu} = (E, {\bf \vec{p}})$
and
$p^2_{\mu} = (E, {\bf -\vec{p}})$
(with, of course, $E^2 = p^2 + m^2$).
So they backscatter 180 degrees i... | In special relativity, energy and momentum of a particle form the energy-momentum 4-vector, to which certain constraints apply:
Massive particles come with a rest frame where their momentum is zero and their energy corresponds to their mass.
Massless particles have nonzero energy and momentum in all frames, equal in ... | https://physics.stackexchange.com |
494,502 | [
"https://stats.stackexchange.com/questions/494502",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/279731/"
] | I am looking for if I can do some simulation in R for verifying the output of Bayes theorem , if you have some other technique I am open to know that also. Anything that can help me in verification would be useful.
Actually my problem is something like this:-
<blockquote>
<em>I have probabilities of dropping a call bas... | Here is the simulation in R which tries to verify the <strong>Probability of dropping during 50-60sec</strong> by using <strong>P(50-60sec dropped/Not dropped 0-20sec).</strong>
<pre><code>#This will have ones with the probability of 0.77
not_20_sec<-rbinom(n=10000, size=1, prob=1-0.23)
#Below code will put 'ones'1'... | Your first problem is that your probabilities don't sum to 1.
Here's some Python that generates 100,000 samples. I'll leave it to you to write it in R.
<pre class="lang-py prettyprint-override"><code>import numpy as np
n50_60 = n_not0_20 = 0
for i in range(100_000):
u = np.random.uniform()
if u <= 0.23:
... | https://stats.stackexchange.com |
87,269 | [
"https://security.stackexchange.com/questions/87269",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/73628/"
] | I'm learning session management and I have two questions for which I could not find answers on the web.
Once the user is authenticated, the server creates the Session ID and sends it to the client (user) in the form of a cookie. This cookie is then subsequently used in requests the client sends to the server to identi... | The connection between the client and the server does not use public key encryption (that is only used for the initial key exchange). A different algorithm is used for encryption (usually a symmetric encryption), such as AES-256-CBC on a TLS 1.2 connection. So unless you intercepted it, no one but the intended browser ... | <strong>Answer 1:</strong>
if the server uses SSL/HTTPS(verified by third party-not self-signed certificate), cookies and session IDs travel as cipher-text over the network, and if an attacker (Man in the Middle) uses a packet sniffer, they can not obtain any information. They can not decrypt data because the connecti... | https://security.stackexchange.com |
492,113 | [
"https://electronics.stackexchange.com/questions/492113",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/190132/"
] | Let's say we're trying to control the speed of a motor to 20 revs/sec, and we know that our controller output for that speed is approximately 5. We currently have a Proportional controller to drive the motor to our 20 revs/sec setpoint, with some steady state error.
If we included Feedforward, but tuned it poorly, suc... | <blockquote>
If we included Feedforward, but tuned it poorly, such that it outputs 2 instead of 5, will the following error still be less than it would have been with only the Proportional controller?
</blockquote>
If your system can have such a simple feedforward, why not just multiply it be 5/2 and get it to resul... | Proportion error is reduced by the value of open-loop error gain in the controller unless the motor load demands more voltage than is possible.
Define how where error is amplified.
How does Feedforward fit into the block diagram? How does load affect speed regulation?
| https://electronics.stackexchange.com |
123,705 | [
"https://electronics.stackexchange.com/questions/123705",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/50524/"
] | The background is, I'm trying to understand how well a typical transistor obeys the Ebers-Moll equation at constant (junction) temperature.
See this snapshot taken from the Fairchild datasheet of the 2N3904 transistor.
<img src="https://i.stack.imgur.com/xU50z.png" alt="2N3904 Vbe against Ic">
My question is what is ... | It's the junction temperature \$T_J\$, and yes, they would typically take such a measurement in microseconds before the junction temperature changes.
The upward slope of at high base and emitter currents is due to resistive (non-ideal) behavior that is temperature-sensitive, or due to the reduction in beta at high cu... | A cold soak to ensure even temperature distribution and then testing at temperature. Remember that the heat equation does not allow for instantaneous changes so the values are tracked as the die heats up. With enough data you can back calculate that out for verification, but more importantly there is very little self... | https://electronics.stackexchange.com |
189,360 | [
"https://physics.stackexchange.com/questions/189360",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/83601/"
] | It's a theoretical problem. I mean I drawn a circle and I consider this circle exist.
I would like to understand how works this device composed of a circle and two charges:
<img src="https://i.stack.imgur.com/boSw4.png" alt="enter image description here">
Two charges fixed on a circle, one positive, one negative tu... | This problem is an example of the wave-particle duality (which is a simplistic model of a complex quantum interaction). Everything can be considered as a wave or a particle, although the higher the energy (that is the larger the particle's mass) the smaller the wave length. So, a low energy photon (like a radio photo... | There is simply no such thing as an electron that "goes from place to place". That electrons are little hard balls is simply an implicit assumption that we carry over from the macroscopic into the microscopic world. It's also a false assumption.
The best way to think of an electron is in form of quantization and conse... | https://physics.stackexchange.com |
357,134 | [
"https://physics.stackexchange.com/questions/357134",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/97850/"
] | Let's say there's a field that acts perpendicularly to the direction of velocity of a particle (& it does not depend on the speed; it's a constant force) and hence does no work. Would this field be conservative?
The work done by this field along a closed loop would be zero but I don't see how potential would be de... | No, it would not because conservative forces, <em>by definition</em> depend only on the position. Your force should depend also on the velocity (its direction) because it is perpendicular to it.
Forces whose work is always zero are not necessarily positional so that they are not necessarily conservative. There are tw... | A force $\vec F$ is to be considered conservative, if it does no work along a closed trajectory,
$$\oint_C \vec F(\vec r)\cdot \mathrm d \vec r=0$$
where $C$ is a closed curve. You can intuitively think of a particle moving with $\vec v$ along a circle in $\mathbb R^2$, such that $\vec F \bot \vec v$. So $\vec F$ poin... | https://physics.stackexchange.com |
28,116 | [
"https://dba.stackexchange.com/questions/28116",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/990/"
] | I've Table1 and Table2, <strong>Table1's PK is the FK of Table2</strong>. Now i need to <strong>perform a select from Table1 if and only if all the records in Table2 corresponding to a Table1 record has values</strong> (if any field value is null then skip).
ie, for an id in Table1 there have multiple sub ids in Table... | This is called an "anti-join" (or anti-semijoin). One way to write it in SQL is to use the <code>NOT EXISTS</code> construction:
<pre><code>SELECT t1.*
FROM Table1 AS t1
WHERE NOT EXISTS
( SELECT *
FROM Table2 AS t2
WHERE t2.FKcolumn = t1.PKcolumn
AND t2.columnX IS NULL
)
</code>... | <pre><code>select t1.* from table1 t1
where exists(select 1 from table2 t2 where t2.FKcolumn = t1.PKcolumn
having count(*)=count(t2.columnX) and count(*)>0)
</code></pre>
| https://dba.stackexchange.com |
448,515 | [
"https://electronics.stackexchange.com/questions/448515",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/211775/"
] | A 460-V 60-Hz four-pole Y-connected induction motor is rated at 25 hp. The equivalent circuit parameters are R<sub>1</sub> = 0.15 Ω, R<sub>2</sub> = 0.154 Ω, X<sub>M</sub> = 20 Ω, X<sub>1</sub> = 0.852 Ω, and X<sub>2</sub> = 1.066 Ω.
The rotor power factor is found using θ = tan-1(X<sub>2</sub>/R<sub>2</sub>/s) = tan-... | Motor inductance basics current ALWAYS lags voltage. PF is only negative when operated as a generator.
| As to why, the rotor current is induced BY the stator current. So when acting as a motor, it is ALWAYS going to be taking place AFTER the stator induces it. Power Factor is just the measurement of that lag.
| https://electronics.stackexchange.com |
188,840 | [
"https://security.stackexchange.com/questions/188840",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/177752/"
] | This applies to any country, but here's some context:
<ul>
<li>Website: BBC iPlayer (a major UK streaming website, registration mandatory)</li>
<li>Server: Linode, London data centre</li>
<li>VPN: SoftEther on HTTPS port 443</li>
<li>Country of stay: South Africa</li>
</ul>
This is a standard situation for residents ... | You have to protect for the greater threat. You have 2 scenarios:
<ol>
<li>Malicious user/troll trying to deny service to a fellow user. This is a relatively common concern but is rare to see in practice</li>
<li>Attackers brute forcing accounts: Although your site may not have any data useful for an attacker the cred... | As asked, I'll make up my own answer.
<blockquote>
If a malicious user B want to block the user A he could try to log with A username's 4 times every 5 minutes. Then A couldn't log to the app.
</blockquote>
That's the issue when locking accounts: you don't know if user is actually legitimate until they passed the l... | https://security.stackexchange.com |
117,751 | [
"https://softwareengineering.stackexchange.com/questions/117751",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/20831/"
] | First the history:
I found out DateTime data types cannot be null (Yes they can be nullable using the ? nullable declaration).
I was told this is because they are value types and placed on the stack (not on the heap).
I can understand why the stack is quicker and why value types need to (mostly) be placed on the s... | The heap and stack are not relevant. It is about Binary representation of values.
In a reference type, a zero binary representation means null, and other binary representations point to memory that contains the binary representation of the value.
In a value type, all binary representations, including zero, directly r... | <blockquote>
I found out DateTime data types cannot be null.
</blockquote>
Your questions show that you have a pretty vague idea about how the whole system works. Let's start by picking apart what you mean by "DateTime data types cannot be null". I think what you mean here is that <em>the null literal cannot be imp... | https://softwareengineering.stackexchange.com |
1,598,436 | [
"https://math.stackexchange.com/questions/1598436",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/191101/"
] | Is $$\sin^2(z) + \cos^2(z)=1$$ still true for all $z \in \Bbb{C}$? I've tried rewriting it using complex definitions of $\sin$ and $\cos$, and I don't see why it wouldn't, but the text I'm reading asks this question as if it shouldn't hold?
| You can derive this identity from nothing more than the fundamental differential identities that define $\sin$ and $\cos$. For suppose you have functions $s(x)$ and $c(x)$ (that play the obvious roles) with $s'(x) = c(x)$ and $c'(x) = -s(x)$, where $s(0) = c'(0) = 0$ and $c(0)=s'(0)=1$. (If you are only given the more ... | You can also prove this with inversions of Euler's identity:$$
\begin{align}
\operatorname{e}^{iz} &= \cos z + i \sin z \Rightarrow \\
\cos z & = \frac{\operatorname{e}^{iz} + \operatorname{e}^{-iz}}{2},\ \mathrm{and} \\
\sin z & = \frac{\operatorname{e}^{iz} - \operatorname{e}^{-iz}}{2i}.
\end{align}$$
Th... | https://math.stackexchange.com |
62,293 | [
"https://dba.stackexchange.com/questions/62293",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/146055/"
] | What is the difference between covering and overlapping constraints use in DBMS?
| In reference with below Employee Entity Relational Model
<img src="https://i.stack.imgur.com/ggS3P.png" alt="enter image description here" />
<ul>
<li>Overlap Constraints : Can "Karthik" (employee) be an Hourly_Emps as well as a Contract_Emps entity? (Allowed/Disallowed) - In this scenario the hourly employee... | <strong>Overlap Constraints:</strong> It determines whether XYZ person be both an hourly_emp and a contract_emp? Answer is NO. Can he be both a contract_emp and a senior emp emtity? Answer is YES.
So we can write something like this.
<strong>'Contract_emp OVERLAPS Senior_emp'</strong>
In the absence of such a statemen... | https://dba.stackexchange.com |
351,194 | [
"https://mathoverflow.net/questions/351194",
"https://mathoverflow.net",
"https://mathoverflow.net/users/150655/"
] | Does anyone know examples of a numerically non trivial nef divisor with Iitaka dimension 0? (Unfortunately, this question might be trivial as right now I can't think of any effective divisors with Iitaka dim 0 other than exceptional divisors)
| Here's an example that works over an uncountable field.
Let <span class="math-container">$E$</span> be an elliptic curve in <span class="math-container">$\mathbf P^2$</span> and let <span class="math-container">$p_1, \ldots, p_9$</span> be 9 points on <span class="math-container">$E$</span>. Blowing up these points, ... | This is not exactly what was asked, but I think it is close to the spirit of the question:
Mumford's example of a non-ample divisor that is positive on every curve gives an example of a numerically non-trivial nef divisor with Iitaka dimension <span class="math-container">$-1$</span>.
Here is a sketch, you can find ... | https://mathoverflow.net |
19,523 | [
"https://chemistry.stackexchange.com/questions/19523",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/9791/"
] | <blockquote>
A buffer with a pH of $\mathrm{4.79}$ contains $\mathrm{ 0.39\ M}$ of sodium benzoate and $\mathrm{0.10\ M}$ of benzoic acid. What is the concentration of $\mathrm{[\ce{H} ]}$ in the solution after the addition of $\mathrm{0.058\ mol}$ of $\ce{HCl}$ to a final volume of $\mathrm{1.6\ L}$?
</blockquote>
... | In that equation, the acid and base are a conjugate acid/base pair. We assume HCl quantitatively dissociates and protonates benzoates. The concentration of the acid increases, and the base decreases.
Empirically, the pH should go down in this case. Without seeing your specific calculation, we can't comment where y... | The concentration of [H] ion will be $2.45x10^{-5}$ if the pH was 4.61.
| https://chemistry.stackexchange.com |
230,815 | [
"https://physics.stackexchange.com/questions/230815",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/101812/"
] | The formula $v^2 = u^2+2GS$(where $v $ is final velocity, $u $ is initial velocity, $G $ is acceleration due to gravity and $S $ is the linear displacement) assumes that gravity remains constant throughout.
What is the actual formula which takes into consideration the change in acceleration with height. Im in 9th grade... | You don't need acceleration.
Just calculate change in potential energy with altitude and make sure your initial Kinetic energy is sufficient that your final velocity is greater than zero.
You cannot use your formula without integration.
As for actual formula it would be
$$g= GM/r^2$$
where M is mass of earth and r is ... | The formula for acceleration at a given height above the Earth would be:
\begin{equation}
a = -gR^2/r^2
\end{equation}
where R is the radius of the earth and r is the radius you are at. This is because gravitation varies with the square of the distance between two objects. I'm afraid you no need calculus to be able to ... | https://physics.stackexchange.com |
315,937 | [
"https://softwareengineering.stackexchange.com/questions/315937",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/223143/"
] | When I have a lot of data that needs to be validated, should I create a new class for the sole purpose of validation or should I stick with in-method validation?
My particular example contemplates a tournament and a event/category class: <code>Tournament</code> and <code>Event</code>, which models a sport tournament a... | <strong>It is OK to delegate any logic by means of composition</strong> if that logic is going to change dynamically during the execution of a program. Complex validations like the ones you explain are as good a candidate as any to be delegated to another class via composition.
Remember though that validations can occ... | <blockquote>
I forget about absolutely any pre-check when using my model classes' setters and similar methods to add data
</blockquote>
That's the problem. Ideally you should prevent your objects to have invalid state: Don't allow instantiation with invalid state and if you must have setters and other state-changing... | https://softwareengineering.stackexchange.com |
81,571 | [
"https://dba.stackexchange.com/questions/81571",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/51177/"
] | I am developing a simple blog application in which users can write posts and others can like, comment, share etc. i am facing difficulty in designing database.
What I have tried is:
<pre><code>user (user_id(pk), user_name)
post (post_id(pk), post_content, likes, user_id(fk))
comment(comment_id(pk), comment, post_id(... | Try something like this
<pre><code>user (user_id(pk), user_name)
post (post_id(pk),user_id(fk), post_content)
comment(comment_id(pk),post_id(fk),user_id(fk), comment)
likes(post_id(fk), user_id(fk) status)
</code></pre>
In <code>status</code> column in <code>likes</code> table you can save it as <code>like</code> ... | All you need to do is add <code>user_id</code> to your <code>comment</code> table. That way each comment can be linked to a post and a user.
| https://dba.stackexchange.com |
529,961 | [
"https://stats.stackexchange.com/questions/529961",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/53690/"
] | In Bayesian statistics, parameters are said to be random variables while data are said to be nonrandom. Yet if we look at the Bayesian updating formula
<span class="math-container">$$
p(\theta|y)=\frac{p(\theta)p(y|\theta)}{p(y)},
$$</span>
we find probability (density or mass) conditioned on the data as well as the co... | The Bayesian approach to (parametric) statistical inference starts from a statistical model, ie a family of parametrised distributions,
<span class="math-container">$$X\sim F_\theta,\qquad\theta\in\Theta$$</span>
and it introduces a supplementary probability distribution on the parameter
<span class="math-container">$$... | Maybe the confusion comes from the short hand <span class="math-container">$p(\theta|y)$</span> which actually means <span class="math-container">$p(\theta|Y=y)$</span>, the random variable <span class="math-container">$Y$</span> interpreted as generating the data takes the fixed value <span class="math-container">$y$<... | https://stats.stackexchange.com |
335,380 | [
"https://electronics.stackexchange.com/questions/335380",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/166298/"
] | I am trying to create a "dimmer" with non-dimmable bulbs.
The idea: use 6, 10 or even 20 bulbs (some cheap non dimmable LED or CFL), use a rotary switch that in position 0 lights up none of the bulbs, in position 1, lights 1 up ... to position n where it lights up the first n bulbs without complicated electronics (I a... | It looks to me like the monitors are 100W each, and the subwoofer is 160W i.e. 360W total. 300W step-downs cost around £40 each on amazon, so if you bought 2 that would be £80 and cover everything you need.
Those manuals list different versions for different countries/voltages, so it's likely that the only difference... | Yes, you can, i.e. it is possible. The specifics of which involve much knowledge of the details of the internal workings than you have. Why not just use a 240v to 120v adapter?
| https://electronics.stackexchange.com |
209,421 | [
"https://softwareengineering.stackexchange.com/questions/209421",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/46506/"
] | Developing a web application which should allow User to schedule appointment based on their TimeZone. And I am storing the User scheduled datetime as server datetime into database field. While showing schedule information retrieved the value from Database and converted into user timzone. processing in Code base I am c... | <strong>Welcome to one of the hardest problems in non-computational programming - properly representing dates and times to end users.</strong>
Realistically, timestamps should be stored in a fixed single representation regardless of how they will be interpreted, because no matter how hard you try, you will always have... | The easiest way to deal with date/time information depends on the requirements of your application.
If the date and time are entered by the user and the server does no processing with that date/time information except to store it in a database and there is no expectation that the entered time gets adapted to the locat... | https://softwareengineering.stackexchange.com |
106,466 | [
"https://physics.stackexchange.com/questions/106466",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/38153/"
] | I am creating a program that will calculate danger at water depths for sensitive equipment and maybe humans.
How do I calculate the gauge pressure at a depth $D$. Lets say $D = 5000$ meters.
My guess is to calculate the atmospheric pressure so I can find the $Pa$.
Any help is appreciated.
| Hydrostatic pressure at any height below the water surface is calculated by
$P=hdg$ where $h$ is the height below the open water surface, $d$ is the density of water and $g$ is the acceleration due to gravity. So if you want to calculate gauge pressure at height $h$ then use formula
$P=hdg+P^\circ$ where $P^\circ$ is... | Contrary to the accepted answer:
The <strong><em>gauge</em></strong> pressure, $P_{gauge}$ at a depth $H$ in a fluid of density $D$, where the acceleration of gravity is $g$, is given by:$$P_{gauge}=DgH$$
since the gauge pressure is the difference between the absolute pressure and the current atmospheric pressure.
Th... | https://physics.stackexchange.com |
84,825 | [
"https://chemistry.stackexchange.com/questions/84825",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/45286/"
] | My chemistry textbook says that $K_{p}$ indicates the equilibrium position. It then says that $kp$ will not change if pressure is increased, but it also says that the position of equilibrium does change if pressure is increased.
Let me restate that: $K_{p}$ <strong>tells you</strong> the equilibrium position. Increasi... | You're using the word "pressure" too casually. There are partial pressures of species and total pressure (which is the sum of all partial pressures of all species, regardless of if they're participating in the equilibrium or not).
Partial pressures of reactants and products factor into the equilibrium constant $K_{\ma... | Kp tells you the value of the partial pressure term at equilibrium. The partial pressure term itself may vary but at equilibrium, it is always the value of Kp.
The partial pressure term mentioned is the expression for Kp used to calculate Kp but the concentrations are not at equilibrium but at the point in time when y... | https://chemistry.stackexchange.com |
82,432 | [
"https://softwareengineering.stackexchange.com/questions/82432",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/15072/"
] | I am completely clueless about the inner workings of an operating system, but I can more or less guess the approximate behaviour of many functions. One thing that I am not able to figure out, though, is multitasking.
In theory, the operating system manages time, according the CPU for small intervals to the various pro... | The OS programs a timer to kick in every few microseconds (or milliseconds, depending on system speed). This timer raises the hardware interrupt, which causes the CPU to stop whatever it is currently doing, dump all its contents onto the stack and process the interrupt routine indicated by the address provided by the i... | It varies from system to system.
In nonpreemptive multitasking systems (such as the original Oberon, or the original Apple Macintosh), the operating system periodically "polls" all tasks, giving them an opportunity to do work. The tasks are expected to play nicely together. If they just have a little bit of work to ... | https://softwareengineering.stackexchange.com |
13,759 | [
"https://datascience.stackexchange.com/questions/13759",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/10902/"
] | I've installed the R package <code>tm</code>. Now I am trying to load it, but it's giving me this error and warning everytime, I try.
The package is in this location-
<pre><code>‘C:/Users/Google/Documents/R/win-library/3.3’
</code></pre>
Error Message--
<pre><code>Error in loadNamespace(i, c(lib.loc, .libPaths())... | <blockquote>
It worked after I <strong><em>reinstalled the R</em></strong>
</blockquote>
.
So I'll suggest to try doing this if anyone facing the same issue. Two of my mates also solved this issue by following the same approach.
Cheers!
| This is a dependency error. You're just missing one of the packages that <code>tm</code> assumes you have.
One solution is to simply directly install that package:
<pre><code>install.packages("slam")
</code></pre>
another is to use a package manager that takes care of this for you:
<pre><code>install.packages("pacm... | https://datascience.stackexchange.com |
108,861 | [
"https://mathoverflow.net/questions/108861",
"https://mathoverflow.net",
"https://mathoverflow.net/users/8699/"
] | Are there any examples of $H < G$ such that for any pmp ergodic action of the group $G$ on a standard proba space $(X,\mu)$ there exists a set $A$ of $\mu(A)>C$ such that the action of the subgroup $H$ on $A$ is ergodic? It seems that this can happen only in finite index case...
<strong>Conclusion:</strong>
The... | We seem to be talking past each other in the limited space provided by the comments, so maybe I can express myself better in the room provided by the answer box. You indicated that you were focused on countable discrete groups. For countable discrete $G$ and $H < G$ the following are equivalent.
<ol>
<li>There ex... | You can take $G$ to be any simple Lie group with finite center, and $H$ to be any non-compact Lie subgroup. Then, if the action of $G$ is ergodic, so is the action of $H$. This statement is called the "Moore ergodicity theorem".
In fact, both the actions of $G$ and $H$ will be automatically mixing. This follows from ... | https://mathoverflow.net |
280,271 | [
"https://dba.stackexchange.com/questions/280271",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/60253/"
] | I setup a new postgres database (v12) by export/importing from an old one (v9.5).
When I query the new one, it's drastically slower despite being on a more powerful machine. Shared buffers are set to 90 gb, hard drive is 500k iops m.2, 128 threads, etc.
If I run COUNT(*) on both tables, version 12 is over twice as fast... | The main issue was the database needed to have VACUUM run on it. I'm surprised that a full database export/import wouldn't just automatically do it considering the performance gain was 100x faster.
<ol>
<li>Run VACUUM on the database (100x performance gain)</li>
<li>Disable JIT in the config when running many fast SQL ... | the execution plan on Postgres 12 shows it went parallel and apparently It made it worse , where in Postgres 9 it went single threaded.
seems like postgres overestimated in every single step and eventually thought that is too much work and consequently it went parallel and made a huge scene for nothing.
probably If you... | https://dba.stackexchange.com |
134,620 | [
"https://softwareengineering.stackexchange.com/questions/134620",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/40120/"
] | Unfortunately, there are not currently any books on the subject of 'Learn Android Development with Scala', which would be the ideal solution. I'm guessing that I'll need to pick up at least 3 books here, one for each topic. However, I have no idea what order to read them in, whether to read them simultaneously, what pa... | <strong>Experience gained making those apps are more important.</strong>
It would be a waste of space to simply list the applications that you have worked on in the past and simply leave it at that. It doesn't matter if these are personal applications or commercial applications.
What is important is the quality of t... | <ol>
<li>Come up with a company name (and make a web page for it)</li>
<li>Call yourself the owner</li>
<li>Add the products to your CV under that company</li>
</ol>
Because, after all, if you are writing software for the android market (especially if you are selling them), then you <strong>are</strong> your own compa... | https://softwareengineering.stackexchange.com |
61,716 | [
"https://electronics.stackexchange.com/questions/61716",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/1705/"
] | I am looking to combine a cheap wifi baseband/mac chipset with a STM32 F1 or F2 MCU to run the IP stack.
I have to use the cheap USB wifi dongles or use SDIO/SPI interface and port the drivers.
I am facing a problem in finding the correct chipset which has datasheets / opensource drivers to continue with this work. ... | The Raspberry Pi is generally powered from a 5V wall-wart type of DC power supply. The secondary is galvanically isolated from the mains voltage for reasons of personal safety (a fault will not expose the end user to the mains voltage).
The DC return of your bridge rectifier circuit is most certainly <em>not</em> isol... | I'm not entirely happy with the answers given about dividing voltages etc. as they are not (IMHO) a good approach, from both safety and engineering angles.
I would suggest a basic opto-isolator circuit or even a small relay with a 120v coil as safer, simpler and more effective ways to detect the presence of mains volt... | https://electronics.stackexchange.com |
650,921 | [
"https://physics.stackexchange.com/questions/650921",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/306789/"
] | I have tried multiple ways but failed several times so can anyone plz guide me about this.
| If you are talking about two projectiles colliding in mid-air, then the equations for projectile motion can give you conditions just before the collision. Then you need to know things about the collision: Does it conserve energy? Can it be described in 2D or do you need to go to 3? Do you have any information about th... | Think of projectile motion as 2 individual motions, one along x-axis and one along the y-axis. Now if you are saying that the balls are rebounding, make an apparent graph of the motion in x-axis and y-axis and then solve it using basic laws of motion like you do in 1D and write the x-axis part in î and y-axis part in j... | https://physics.stackexchange.com |
207,786 | [
"https://stats.stackexchange.com/questions/207786",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/112494/"
] | I have two neural networks. If I take only weights (the activation functions for both are the same), is there a way to tell the percent similarity of these two networks?
| You can try to estimate the similarity between two units, using their weights and the similarity matrix between units from which they receive inputs. This would lead to a process similar to back-propagation, but going from bottom to top. At first you estimate the similarity between each pair of 1st layer units, then ... | You could consider the whole number of neurons as your population, and by checking the weights of the neurons, through this formulation say how similar these two are:
similarity = (number of neurons with same weight values) / (total number of neurons) * 100
As you mentioned, we are considering that the number of neur... | https://stats.stackexchange.com |
288,688 | [
"https://physics.stackexchange.com/questions/288688",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/123178/"
] | I've learned in BCS theory about its ground state by applying Bogoliubov annihilation operator on it to be zero; however, in the textbook the total momentum of electrons is set to be zero. It's okay to me for this state to be a ground state for the effective Hamiltonian; however, I cannot understand why this state exhi... | Actually, it is <em>not</em> always assumed that there are no free charges. It depends on the particular problem that one wants to study. When one wants to study free space propagation of light, for instance, then yes, we would assume that there are no free charges. The presence of free charges would mean that the ligh... | My usual disclaimer: please view this as an <strong>attempt</strong>, at an answer, as I self study and this is the best way to research and learn. If it is incorrect, I will delete it.
Free charges are those excess charges that can shift into electrostatic equilibrium, so you end up with a time independent electric f... | https://physics.stackexchange.com |
225,865 | [
"https://softwareengineering.stackexchange.com/questions/225865",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/116526/"
] | I am starting to work on a new commercial software project that may use elements of "open source" code.
I have been instructed to ensure that I only use code which is licensed for commercial use and although we <em>may</em> decide to make it open source in the future it should be <em>required</em> to be open source.
... | There are no open-source licenses that do not allow the redistribution of source; that's a contradiction in terms. What there are, though, are non-viral open-source licenses that do not <strong>require</strong> the redistribution of source.
It sounds like what you're looking for is code with a license less "strong" t... | No, not that I ever heard of.
Your firm is looking for "dual license" code, code that is available EITHER under an open source license OR under a commercial license. When such code is acquired under a commercial license, the licensee generally is not obligated to release the source of his modifications to anyone.
Wi... | https://softwareengineering.stackexchange.com |
493,998 | [
"https://electronics.stackexchange.com/questions/493998",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/249314/"
] | When the speed of the charge is close to the speed of light, is Gauss's law still valid?
As far as I know, the invention of Gauss's law preceded the theory of special
relativity, so if the answer is yes, then why is Gauss's law still valid when the speed of the charge is close to the speed of light?
| I finally have a moment to write out something.
<h2>Intro</h2>
First off, it's better if I say a little something about Gauss's Law -- it's not just one law. There are two important versions, one for electricity and one for magnetism, and together they form up half of Maxwell's equations (which have a differential form... | Here is my intuition behind Gauss's Law in integral form.
First a couple words about relativistic correctness (simplified view). Coulomb law is not relativistically correct since when either of the charges move, the law does not work any more. Gauss's Law is relativistically correct, since when charge moves the law sti... | https://electronics.stackexchange.com |
722,421 | [
"https://physics.stackexchange.com/questions/722421",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/342755/"
] | I was told that the entropy change is <span class="math-container">$0$</span> during adiabatic expansion. However, according to <span class="math-container">$\mathrm dS=\frac{C}{T}\,\mathrm dT$</span>, <span class="math-container">$\delta S$</span> is not zero because temperature is not constant during adiabatic proces... | You are thinking about it in the wrong way. What Maxwell's equations tell you is that when you have a time-varying magnetic field, there must also be an electric field present that satisfies the Maxwell-Faraday equation. The two fields co-exist (and indeed are different aspects of THE electromagnetic field). Similarly,... | Maxwell was being unnecessarily pedantic. The only way an electromagnetic field <em>can</em> vary is with time. <em>Any</em> field can only vary over time. So he was really only describing a simple variation of the field.
A variation of the <em>electromagnetic</em> field necessarily implies a variation in both its elec... | https://physics.stackexchange.com |
273,938 | [
"https://physics.stackexchange.com/questions/273938",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/40433/"
] | The coupling constant in the QED lagrangian is clearly the electric charge $e$. However, one often hears the statement that the expansion parameter in QED is the fine structure constant $\alpha = e^2/4\pi$, not $e$.
Unfortunately, I've never seen a formal proof that the sum of all contributions to S-matrix which are ... | Your latter option is what is meant - the perturbation series is an expansion in loop orders, and the power of $\alpha$ is what counts the loop order.
| Every perturbative correction of a diagram comes with an internal photon which contributes two other vertices to the diagram. Hence the corrections should be of order $\alpha$, $\alpha^2$ $\dots$
| https://physics.stackexchange.com |
326,410 | [
"https://softwareengineering.stackexchange.com/questions/326410",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/163709/"
] | I recently ran across some code during a review that baffled me a bit. The dev was using structs to organize code within a larger file. I personally don't like the practice but wanted to hear others thoughts.
Assumptions:
This was an ASP.MVC app with several app specific areas.
The Settings.CS file has a lot of dif... | Examples with terms like "Foo", "Bar", "Other" and "More" are seldom helpful when it comes to design issues. So lets assume the "real" code looks more like this:
<pre><code> public struct NetworkSettings
{
public string ConnectionString { get; set; }
public ProtocolType Protocol { get; set; }
public... | I've seen this used as well. A possible reason is that with say 10+ settings, it becomes hard to pick the right one even with Intellisense. But if they're broken up by category/ subcategories, when you type "." to get Intellisense predictions you only get 3-4, then you can drill down by area.
It's just a way of makin... | https://softwareengineering.stackexchange.com |
415,682 | [
"https://softwareengineering.stackexchange.com/questions/415682",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/360411/"
] | I'm working on a system with a main entity, let's say "Members", these members have relationships to other entities and it's important to keep the history to be able to query and know the relationships at a given time in history.
We mainly have two kind of relationships:
<ul>
<li>Company (most exist and can o... | I don't know what Jim Perry meant, my mindreading capabilities are very restricted. Actually, I disagree to his literal statement: even in a hypothetic "more realistic context" an expression like
<pre><code> if (<very complex boolean expression>)
return true;
else
return false;
</cod... | He is referring to code where the reader looses context such as the if statement covering twenty lines, or in a language where logic operators operate by success (have an object)/failure (have an error) necessitating a conversion to a boolean type.
| https://softwareengineering.stackexchange.com |
276,381 | [
"https://dba.stackexchange.com/questions/276381",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/39142/"
] | We have a logshipping setup, where the <code>LS_Restore</code> job on the secondary database is switched off every night for the datawarehouse to grab its data. The restore cycle is set to 15 minutes and if <code>LS_Restore</code> would kick in, the datawarehouse connection would get severed - the interface therefore s... | you can use the below query
<pre><code> DECLARE @total_buffer INT;
SELECT @total_buffer = cntr_value
FROM sys.dm_os_performance_counters
WHERE RTRIM([object_name]) LIKE '%Buffer Manager'
AND counter_name = 'Database Pages';
;WITH src AS
(
SELECT
database_id, db_buffer_pages = COUNT_BIG(*)
FROM sys.dm_os_buffer_desc... | Just be aware that the suggested queries focuses on the Buffer Pool (cached data).
A database can have other types of memory usage as well, like exec plans (when database bound), working memory for queries, locks, etc. I don't know of a way to summarize those since many are, sort of indirect consequences of doing thing... | https://dba.stackexchange.com |
237,816 | [
"https://electronics.stackexchange.com/questions/237816",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/104309/"
] | I am working with Altium.
What verification do you particularly do when going from schematic to layout in terms of electronic and in terms of capture ?
| With Altium, it's a nicely integrated environment with very tight coupling between schematic and layout aspects. Altium has a concept of <strong>component links</strong> which you can use to verify that every component on the PCB is linked to something on the PCB. Additionally, as Bence mentioned in a comment above, th... | If the pins of the parts got properties like input, output, power, ground etc. there are some checks possible for open inputs, two outputs in the same net, unconnected power or ground pins.
You may highlite a net to see what is connected to it and what not.
| https://electronics.stackexchange.com |
156,045 | [
"https://electronics.stackexchange.com/questions/156045",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/41777/"
] | Is it possible for the base current to exceed the collector current for a BJT in saturation mode? (If the base current was increased without the collector current increasing at all)
| First let's see how to implement a function using single decoder. It is a straight-forward task having the truth table. Decoder will output <code>1</code> on the lines selected by the input, so if we just "or" the outputs corresponding to the truth table lines with X=1, we will get the exact required function:
<img src... | Inputs of first decoder be A0 and A1. Outputs \$N_0, N_1, N_2\$ and \$N_3\$ (\$N_0\$ will be high for A='00')<br>
Inputs of second decoder be B0 and B1. Outputs \$M_0, M_1, M_2\$ and \$M_3\$
Let the final output be Y. Then,
If A = '00' then, \$Y = `0`\$<br>
if A = '01' then, \$Y = M_0 \$<br>
if A = '10' then, \$Y = ... | https://electronics.stackexchange.com |
32,259 | [
"https://chemistry.stackexchange.com/questions/32259",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/12743/"
] | I'm getting a little confused about the definition of chelation and its implications (I should probably point out I'm only an A2 Chemistry student). The IUPAC defines chelation as the following:
<blockquote>
The formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) li... | I think you should not focus on the ligand to define chelation. You should instead focus on the complex that is formed. In fact, the very same multidentate ligand may participate in binding in more than one way:
<ul>
<li>by forming more than one bond with the same central atom, therefore forming a chelated complex;</l... | This is what my textbook says:
<blockquote>
All polydentate ligands are the example of chelating ligands.
However, it must be noted that <span class="math-container">$\ce{NH2NH2}$</span> and <span class="math-container">$\ce{N(CH2CH2)N}$</span> cannot act as chelating ligands due to the formation of a three membered ri... | https://chemistry.stackexchange.com |
107,841 | [
"https://mathoverflow.net/questions/107841",
"https://mathoverflow.net",
"https://mathoverflow.net/users/2191/"
] | Let $X$ be a smooth projective variety (over an algebraically closed field; it could be the field of complex numbers); $Z$ is its hyperplane section. When there exists an etale $U/X$ such that:
<ol>
<li>$Z$ can be lifted to $U$.</li>
<li>There exists a morphism from $U$ to a curve $C$ such that $Z$ is the preimage of ... | If $X$ is a curve, then just take linear projection from a disjoint codimension 2 linear subspace in the spanning hyperplane of $Z$. If the dimension of $X$ is $2$ or more, then there never exists such an étale neighborhood and morphism. For $Z$ of dimension $n-1 \geq 1$, for the normal sheaf $\mathcal{N}_{Z/X... | Suppose that $H$ is an hyperplane such that $Z = H \cap X$. Let $L$ be a subspace of codimension 1 of $H$; the pencil of hyperplanes passing through $L$ defines a morphism $X \smallsetminus L \to \mathbb P^1$ such that the inverse image of a point is $Z \smallsetminus L$. If you want $U$ to surject onto $X$ you can cov... | https://mathoverflow.net |
25,337 | [
"https://mathoverflow.net/questions/25337",
"https://mathoverflow.net",
"https://mathoverflow.net/users/5952/"
] | If you want to compute crystalline cohomology of a smooth proper variety $X$ over a perfect field $k$ of characteristic $p$, the first thing you might want to try is to lift $X$ to the Witt ring $W_k$ of $k$. If that succeeds, compute de Rham cohomology of the lift over $W_k$ instead, which in general will be much easi... | This paper of Serre gives an example (I've justed pasted I. Barsotti's math-sci review).
(The paper can be found in Serre's "Collected Works vol. II 1960-1971)
<blockquote>
Serre, Jean-Pierre Exemples de
variétés projectives en
caractéristique $p$ non relevables en
caractéristique zéro. (French) Proc.
Nat. A... | A general method for for constructing schemes with "arbitrarily bad" deformation spaces (including the non-existence of liftings from char. p to char. 0) is in the following paper:
R. Vakil "Murphy's Law in algebraic geometry: Badly-behaved deformation spaces", Invent. Math. 164 (2006), 569--590.
| https://mathoverflow.net |
445,726 | [
"https://stats.stackexchange.com/questions/445726",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/271444/"
] | I'm leveraging the Python packages lime and shap to explain single (test-set) predictions that a basic, trained model is making on new, tabular data. WLOG, the explanations generated by both methods do not agree with user intuition.
For example, when leveraging the methods in a healthcare setting, they might list the ... | Just to state this up-front: most machine learning models just try to predict. They do not find/show causal effects, understand what is going on, model disease mechanisms or medical relationships. I.e. model explanations may not point to what happens in terms of causality/disease mechansim, but only highlight what appe... | It could also be that the model is training on noise in the data. Many ml models are over-parameterized and can thus can predict even randomly assigned targets.
Depending on what kind of model you are using you can try to reduce the size of the model. Or use other methods usually used to prevent overfitting and see if ... | https://stats.stackexchange.com |
577,850 | [
"https://physics.stackexchange.com/questions/577850",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/271193/"
] | This is a problem I saw in a stat mech textbook, and I think it is a fairly common problem.
Given the entropy function: <span class="math-container">$$S = - \sum_{i=1}^N p_i \log p_i$$</span>
Maximize <span class="math-container">$S$</span> subject to constraints:
<span class="math-container">$$ \sum_{i=1}^N p_i = 1 \\... | One may apply a following trick, let <span class="math-container">$f(\mu) = \sum e^{-\mu e_i}$</span>, then:
<span class="math-container">$$
\frac{\partial f}{ \partial\mu} = -\sum e_i e^{-\mu e_i}
$$</span>
the constraints lead to following differential equation:
<span class="math-container">$$
\frac{\partial f}{ \p... | You cannot solve for <span class="math-container">$\mu$</span> unless you know the <span class="math-container">$e_i$</span>'s. But that shouldn't bother you, because in the context of the canonical ensemble, <span class="math-container">$\mu$</span> is defined to be the (inverse) temperature, and all quantities are wr... | https://physics.stackexchange.com |
390,615 | [
"https://physics.stackexchange.com/questions/390615",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/154582/"
] | I am learning about Electron Capture.
$$p+e\to n+\nu_e$$
My question is whether the $W^+$ boson or the $W^-$ boson is involved in this transfer. I can consider this two ways:
1) Since the electron is losing a negative charge in this situation, this would indicate that this should be mediated by the $W^-$ boson.
2)... | The duration of the wave is fixed as it travels along the string because the speed of each part of the wave profile is the same at every point along the string. This happens because the string is assumed to be a linear medium. The high-frequency parts of the wave (the rising and falling edges at half amplitude, where t... | When the wave hits the boundary, it starts to slow down because the density of the second portion of rope is higher than the first. Slowing down the wave has the effect of squishing the peak. You can intuit this by considering the motion of the two tails of the wave packet.
Consider the forward tail as it hits the bo... | https://physics.stackexchange.com |
162,938 | [
"https://mathoverflow.net/questions/162938",
"https://mathoverflow.net",
"https://mathoverflow.net/users/54780/"
] | If $\gamma$ and $\eta$ are two smooth curves in a smooth manifold $M$, is it possible to find a diffeomorphism of $M$ such that $f \circ \gamma = \eta$? What if one removes the assumption of smoothness, working with topological manifolds (or even plain topological spaces) and homeomorphisms?
(Later edit: Argghh! I apo... | If your definition of "curve" is embeddings of say $[0,1]$ in a manifold, then the result follows in dimensions $n\geq 2$ by the isotopy extension theorem, together with the observation that the unit tangent bundle of a connected manifold is connected. In dimension $n=1$ the diffeomorphism would not always be isotopic ... | In the case that $M=\Sigma$ is a closed, oriented surface, this question has a rather complete answer; Farb and Margalit call it the "change of coordinates principle" in "The Primer on Mapping Class Groups." It says that given $\alpha, \beta$ two simple, homotopically non-trivial, non-separating (i.e. $\Sigma\backslas... | https://mathoverflow.net |
264,930 | [
"https://electronics.stackexchange.com/questions/264930",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/78981/"
] | I have a Microchip PIC board (18F46K22) which has a VIN pin to connect to a power source (7vdc + supply).
What happen if I feed 3.3V to the VIN which goes through the 5V linear regulator (ST 78M05)?
The 18F46K22 is able to run on 5v or 3.3v supply. I would like to force the board to run on 3.3V so it will solve my lo... | What will happen is that with a light load the 7805 will still drop around 1.5-1.6V and you'll see around 1.6-1.7V at the output, not enough for the board to operate correctly.
You'll need to bypass the regulator. If you jumper directly from input to output on the 7805 (the two outside pins) it should work fine and t... | Look for "headroom" or "dropout voltage" in the 7805 (or any other linear regulator) datasheet - that will state the minimum voltage differential across the regulator for correct operation.
You would be very lucky to get 1 volt from a 7805 with a 3.3 volt input.
| https://electronics.stackexchange.com |
611,066 | [
"https://physics.stackexchange.com/questions/611066",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/256066/"
] | In order for a process which involves heat flow from surroundings to the system to be reversible, is it necessary that the temperature difference between the system and surroundings be very small at every time during the process?
| <blockquote>
is it necessary that the temperature difference between the system and
surroundings be very small at every time during the process?
</blockquote>
Yes, because that is the only way the system can be in equilibrium with its surroundings at every stage of the process. It it is disequilibrium that causes a pro... | Spontanenous heat flow is always irreversible and involves difference of temperature. However, the lower the difference of temperature, the closer the process is to being reversible. In theory, if the difference becomes zero and some heat still transfers, the process is reversible. But it is very hard/impossible to ach... | https://physics.stackexchange.com |
506,055 | [
"https://physics.stackexchange.com/questions/506055",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/243791/"
] | suppose we have an irreversible process, caused by a pressure difference <span class="math-container">$p_1 \neq p_2$</span> between two separated gases. The whole system is insulated. The two gases are separated during all the process, but not insulated between them. Applying the first law to the first gas:
<span clas... | In an irreversible process, the ideal gas law does not describe the behavior of an "ideal gas." This is because the ideal gas law applies only at thermodynamic equilibrium. In an irreversible process, there are also viscous stresses present within the gas that contribute to the force on the piston (separating the two... | I don't see how you got the last equation. Under your assumptions I get
<span class="math-container">$$Q_{1}+Q_{2}=\int P_{2}dV_{1}+\int P_{1}dV_{2}$$</span>
Am I missing something?
| https://physics.stackexchange.com |
121,473 | [
"https://physics.stackexchange.com/questions/121473",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/25083/"
] | The gravitational binding energy of a sphere is: $U=\frac{3GM^2}{5r}$, the mass defect is $\Delta E=\Delta m c^2$. Putting: $M=\frac{4}{3}\rho\pi r^3$, we get:
$$U=\frac{16}{15}G\rho^2\pi^2 r^5$$. Now if we put: $U=\Delta E$, we get for $r$:
$$r=\frac{1}{2}\frac{\sqrt5c}{\sqrt{G\rho\pi}}$$ that means:
$$r\approx\frac{2... | No, it is not correct because of relativity.
Because you use $E=mc^2$, clearly, you are using relativistic effects so you need to take the full theory of relativity into account. While you might think that you have only used the special theory of relativity, you are also considering gravitational effects – the gravita... | Next to the obsious problem with the relativity (as
Luboš Motl said), I think there is another problem as well.
The gravitational binding energy is calculated to the infinite distance as reference.
Thus your calculation doesn't mean, that the mass "disappeared" - your calculation means only, that disintegrating this... | https://physics.stackexchange.com |
121,340 | [
"https://mathoverflow.net/questions/121340",
"https://mathoverflow.net",
"https://mathoverflow.net/users/19313/"
] | In order to realise the K-groups of a ring as the homotopy groups of some space associated to that ring, Quillen proposed the following (roughly-sketched) construction:
Recall that $K_1(R) = GL(R)/E(R)$, so we're, at least, looking for a space $X$ with $\pi_1(X) = K_1(R)$. The classifying space of $K_1(R)$ is obviousl... | Here are some thoughts, gathered from reading many texts about algebraic K-theory. Let me start with some historical remarks, then try to give a more revisionist motivation of the plus construction.
First of all, it's true as you say that the already-divined definitions of the lower K-groups made it seem like the hig... | I don't really know if this helps, but you can in effect give the plus-construction definition of $K$-groups without explicitly mentioning homotopy groups, and without ever doing the plus construction:
$H_1BGL(R)$ is $K_1(R)$. Map $BGL(R)$ to an Eilenberg-MacLane space $BK_1(R)$ and consider the homotopy fiber.
This ... | https://mathoverflow.net |
63,931 | [
"https://stats.stackexchange.com/questions/63931",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/26629/"
] | Since it's a two-sided test, it seems plausible that <strong>I can only reject that the difference is 0</strong>. However, if the entire interval is negative or positive can I say that one is larger or smaller than the other (with confidence 1-alpha) or do I need a different test? If I need a different test to establis... | You can tell which mean is bigger by looking at the means.
But you can only reject the null hypothesis that you tested, which is that the two means are equal.
| The t-test compares means. So just compute the means of your two samples and wherever it is larger, that's the sample with the significantly larger mean.
Most statistical packages should actually give you not only a p-value, but also the difference of means (as well as the pooled standard deviation/error etc.).
The t... | https://stats.stackexchange.com |
8,715 | [
"https://security.stackexchange.com/questions/8715",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/3775/"
] | Would you prefer a private cloud for your data or a public one?
Who takes care of financial liability in case of any breach?
Do companies still keep the most critical within their data centers, but would not mind hosting their other non-critical data on the cloud?
Is the strategy to wait and watch? Or would you t... | Your company takes on any and all risks associated with any breaches. You'll find that by signing up for public cloud environments, you'll have to sign a document accepting that provider's Terms of Service. Among those TOS will be something along the lines of a full waiver of any liability on the part of the provider... | In the absence of standardized and interchangeable cloud vendors, it's at least as
important to consider the durability of any particular cloud vendor. A business
based on a particular vendor could be in deep trouble when that vendor decides to
close up shop.
| https://security.stackexchange.com |
592,423 | [
"https://physics.stackexchange.com/questions/592423",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/279251/"
] | I would be glad if someone can explain to me the concepts and the applications of Reynolds number in real-life.
My handout has this explanation and I cannot understand it clearly...
<blockquote>
Reynolds number denoted by <span class="math-container">$N_R$</span> is an experimentally deduced relationship which is used ... | I think your handout is written in quite a confusing way.
The main point of the Reynolds number is <em>similarity</em>. You can show that flows around a geometrically identical shape, but with different size, different flow speed, different viscosity will behave the same way if the Reynolds number is the same.
I like t... | Dimensionless numbers in fluid dynamics are always a ratio of two quantities. The expression that you share is only a results of that expression. The Reynolds number is defined as the ratio between inertial forces and viscous forces.
Both forces can be approximated from the Navier-Stokes equations.
The inertial term is... | https://physics.stackexchange.com |
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