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 |
|---|---|---|---|---|---|
402,076 | [
"https://mathoverflow.net/questions/402076",
"https://mathoverflow.net",
"https://mathoverflow.net/users/10366/"
] | I would like to know (as part of an attempt to streamline some calculations in the cohomology of a Morava stabiliser group) whether <span class="math-container">$1170\sqrt{-3}\sqrt{5}\sqrt{-7}-19110$</span> is a square in <span class="math-container">$\mathbb{Q}(\sqrt{-3},\sqrt{5},\sqrt{-7})$</span>. What is an effici... | The field <span class="math-container">$L=\mathbb Q(\sqrt{-3},\sqrt{5},\sqrt{-7})$</span> has a lot of intermediate fields which we can exploit. Pick one of its index two subextensions, say <span class="math-container">$K=\mathbb Q(\sqrt{-3},\sqrt{5})$</span>. The element <span class="math-container">$\alpha=1170\sqrt{... | A numerical approach: Let <span class="math-container">$x\sim\sqrt{1170\sqrt{105}-19110}$</span> be a numerical approximation of high precision (say 500 digits). Let <span class="math-container">$(b_1, \ldots, b_8)$</span> be a high precision numerical approximation of a <span class="math-container">$\mathbb Q$</span>-... | https://mathoverflow.net |
1,334,271 | [
"https://math.stackexchange.com/questions/1334271",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | Hello everyone we have exams tomorrow and i am practising vectors and i wanted some help here. Finding the area of the parallelogram spanned by vectors <-1,0,2> and <-2,-2,2>
I have not tried anything since I have no idea. I consider this as revision I have looked at several examples but most are complex and so ... | This is what I could have done.Just a hint towards exams.
<span class="math-container">$$Area = {\vert\vec a \times \vec b\vert}$$</span>
<span class="math-container">$${\vert\vec a \times \vec b\vert}$$</span>= <span class="math-container">$$ \begin{vmatrix}
i & j & k \\
-1 & 0 & ... | The area of the parallelogram spanned by two vectors is the magnitude of the cross-product. The cross product is $<4,-2,2>$, having magnitude $\sqrt{24}$.
| https://math.stackexchange.com |
206,678 | [
"https://dba.stackexchange.com/questions/206678",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/31995/"
] | I am trying to get disk space report from bunch of servers and insert them into sql table..
below is sample of what i am trying to do,Server names will be populated from a table and passed as a comma seperated list.Even passing one server also has the same result
<pre><code>set @ps = 'powershell.exe -noexit -c "
$c... | There are several problems here:
The main issue is that the CMD shell executes each line as it comes in via a return; it does not wait for an "end of command" indicator such as <code>;</code>, nor does it attempt to figure it out via parsing (like SQL does upon a batch being submitted) since there is no way to know if... | The issue may be the line feeds in the command you're trying to execute. Consider the difference between the following cases.
<pre><code>-- case 1
execute xp_cmdshell N'powershell.exe -noexit -c "echo hello"';
-- case 2
execute xp_cmdshell N'powershell.exe -noexit -c "
echo hello
"';
</code></pre>
To execute multi-... | https://dba.stackexchange.com |
178,366 | [
"https://mathoverflow.net/questions/178366",
"https://mathoverflow.net",
"https://mathoverflow.net/users/5101/"
] | Recall that a flag variety over a field $k$ is a smooth projective variety over $k$, which is a homogeneous space for some linear algebraic group.
My question concerns specialisations of flag varieties over discrete valuation rings. Namely, let $R$ be a discrete valuation ring with field of fractions $K$ and residue f... | There is a counter-example (with $k=\mathbb{C}$) due to Pasquier and Perrin, Math. Zeitschrift 265 (2010), 589-600. The generic fiber is an orthogonal grassmannian $\mathrm{Gr}_q(2,7)$, while the special fiber is a smooth
projective non-homogeneous ("horospherical") variety with an action of $G_2$.
| In the positive direction, you should consult the work of Hwang-Mok. Regarding counterexamples, there are even easier examples than the one due to Pasquier and Perrin. Let $V$ be an even-dimensional vector space, and let $\omega$ be a symplectic pairing on $V$. Let $\mathbb{P}V$ be the associated projective space wi... | https://mathoverflow.net |
112,361 | [
"https://cs.stackexchange.com/questions/112361",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/108129/"
] | To choose 3 items out of 5 items <code>[1, 2, 3, 4, 5]</code>, Donald Knuth's lexicographic algorithm (The Art of Computer Programming, Vol 4A, 2011, p. 358, Algorithm L (lexicographic combinations)) gives:
<pre><code>[[1, 2, 3], [1, 2, 4], [1, 3, 4], [2, 3, 4], [1, 2, 5], [1, 3, 5], [2, 3, 5], [1, 4, 5], [2, 4, 5], [... | Let us prove a more general result:
<blockquote>
For each <span class="math-container">$m \geq 2$</span> there is a language <span class="math-container">$L$</span> such that <span class="math-container">$L,L^2,\ldots,L^{m-1}$</span> are not regular but <span class="math-container">$L^m$</span> is regular.
</blockqu... | Unless you must deal with unary regular languages, there is no need for complex math ...
Just pick an irregular language that is able to "capture and mask" the concatenation of itself; e.g. over <span class="math-container">$\Sigma = \{a,b \}$</span>
<span class="math-container">$L = \{ (a^ib^j) (a^n b^n) \mid i, j \... | https://cs.stackexchange.com |
475,351 | [
"https://electronics.stackexchange.com/questions/475351",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/194358/"
] | The flip-flop of FPGA (at least those from Xilinx or the ECP5 family from Lattice) support both synchronous and asynchronous reset (extract from the ECP5 datasheet : "There is control logic to perform set/reset functions (<strong>programmable as synchronous / asynchronous</strong>)".
The only way I can think of is t... | The simple answer is that Xilinx, to use your example, <strong>did not</strong> implement their flip-flops in Verilog. Their flip-flops are full-custom VLSI designs where the logic cells are highly optimized.
However, we can get a notion of how Xilinx would accomplish this. If there is only one reset input signal for ... | I don't know why anybody would want this as the asynchronous reset would make the synchronous reset superfluous.
It makes more sense to do this, but with two different reset signals, an asynchronous and a synchronous reset :
<pre><code> always @(posedge clk, posedge async_reset)
if (async_reset)
q <= 1... | https://electronics.stackexchange.com |
17,728 | [
"https://physics.stackexchange.com/questions/17728",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/1635/"
] | In quantum field theory when dealing with divergent integrals, particularly in calculating corrections to scattering amplitudes, what is often done to render the integrals convergent is to add a regulator, which is some parameter $\Lambda$ which becomes the upper limit of the integrals instead of $\infty$.
The physic... | It took the insights of Wilson and Kadanoff to answer this question. Universality. It doesn't matter all that much what the precise details in the ultraviolet are. Under the renormalization group, only a small number of parameters are either relevant or marginal. All the rest are irrelevant. As long as you take care to... | I'm not sure about it, but my understanding of this is that the $\int_\Lambda^\infty$ term is essentially constant between different processes, because whatever physics happens at high energies should not be affected by the low-energy processes we are able to control. That way, we can meaningfully calculate <em>differe... | https://physics.stackexchange.com |
216,406 | [
"https://softwareengineering.stackexchange.com/questions/216406",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/48999/"
] | I get that /dev/random is a good source of entropy, and is what is usually used-- It's just as I'm reading up on GC, at least in Java, it seems accepted that the garbage collection daemon executes non-deterministically. If this it true, why don't we use the timing of the garbage collection as a source of entropy inste... | "Unspecified" and "random" are two entirely different concepts.
The exact workings of a garbage collector are not specified and are up to the garbage collector (usually implemented by a VM of sorts, but not necessarily).
Therefore, you have no specified (i.e. deterministic) time at which garbage will be collected.
<... | Firstly, we have to be careful not to fall into the trap of reasoning by manipulation of mere words. For instance, we could ask, since a NFA is a "non-deterministic finite automaton", why don't we use it to obtain random numbers? In that case, it would be because that's not what "non-deterministic" means in an NFA; in ... | https://softwareengineering.stackexchange.com |
325,961 | [
"https://electronics.stackexchange.com/questions/325961",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/161082/"
] | So first at all this question isn't really about electrical engineering, but about electricity in general. However, I think this is the best place to ask, correct me if I’m wrong. Also I’d like to mention that my knowledge about electricity is very limited, especially my knowledge about alternating current.
While work... | Yes, probably you were wearing well isolating shoes, so you got only small schock.
Yes, touching both wires would give you more current trough. It depends on how you touch these wires. The most dangerous is to touch with arm and leg, or just live wire and no shoes, since the current would pass through the heart. Let's... | Your skin has resistance. Dry skin has a relatively high resistance resulting in a lower current.
If you would have had wet hands, or one of the strands punctured the top skin layer, you would have had a painful arm.
You were probably also wearing nonconductive shoes. Further limiting the current path.
| https://electronics.stackexchange.com |
123,615 | [
"https://cs.stackexchange.com/questions/123615",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/118883/"
] | Currently I want to learn the complexity of space, I read a few of the books on it. On this I encountered this example problem. I would just like to know how to show that the following problem <span class="math-container">$\in L $</span> (deterministic logarithmic space).
Input: series of open or closed parentheses
... | Initialize a counter at zero. Whenever you see "(", increase it by one, and whenever you see ")", decrease it by one. Accept if the counter was always nonnegative and ended up at zero. The counter only takes logarithmic space to store.
| This amounts to reading the input string and keeping track of the difference <span class="math-container">$\delta$</span> between the number of open and closed parenthesis.
If <span class="math-container">$\delta$</span> ever becomes negative, then the input is not properly balanced (at some point there is one more cl... | https://cs.stackexchange.com |
202,167 | [
"https://softwareengineering.stackexchange.com/questions/202167",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/-1/"
] | There seems to be an aversion to writing even the most basic documentation. Our project READMEs are relatively bare. There aren't even updated lists of dependencies in the docs.
Is there something I'm unaware of in the industry that makes programmers dislike writing documentation? I can type out paragraphs of docs if ... | I don't think it's helpful to speculate on the motivations of people who aren't adopting something you think is good practice or who are continuing to do something you see as bad practice. In this business, the people who fall into one or both of those categories will <em>far</em> outnumber the ones who you'll see eye-... | There are two main factors in my experience:
<strong>Deadlines</strong>
Most companies are so date driven that QA, tech debt, and actual design are cut just so the project manager doesn't look bad or to hit some absurd over-promised client deadline. In this environment where even functional quality is cut, then a lo... | https://softwareengineering.stackexchange.com |
5,379 | [
"https://cstheory.stackexchange.com/questions/5379",
"https://cstheory.stackexchange.com",
"https://cstheory.stackexchange.com/users/-1/"
] | This question is something I've wondered about for a while.
When people describe the P vs. NP problem, they often compare the class NP to creativity. They note that composing a Mozart-quality symphony (analogous to an NP task) seems much harder than verifying that an already-composed symphony is Mozart-quality (which... | I don't claim this is a complete answer, but here are some thoughts that are hopefully along the lines of what you're looking for.
NP roughly corresponds to "puzzles" (viz. the NP-completeness of Sudoku, Minesweeper, Free Cell, etc., when these puzzles are suitably generalized to allow $n \to \infty$). PSPACE correspo... | I think one is led to the wrong model by trying to extrapolate from the kind of things the human brain appears to compute, and I think it would be better to take the opposite view and instead extrapolate from the computational model it is.
So, to me the complexity class that most reasonably captures the human mind is ... | https://cstheory.stackexchange.com |
412,864 | [
"https://electronics.stackexchange.com/questions/412864",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/134429/"
] | I know about active and reactive power as a big picture. Active power is what really does the job and the reactive power circulates back and forth in and out of the system. I also know the math behind them to some extend and active reactive components ect.
Where Im stuck as is that why power factor is set near unity l... | <blockquote>
Where Im stuck as is that why power factor is set near unity like 0.8
or 0.9 but never unity.
</blockquote>
The Power Factor (to be precise, displacement power factor) isn't set to 0.8, 0.9.
A generator will be capable of producing VA. If a resistor was place across the terminals then the DPF & ... | Yes, it's a bit foolish. Ultimately, motors only care about current and the magnetic field that it produces, not the voltage that produces that current. The phase relationship between them (the power factor) doesn't matter at all.
Power factor <em>limits</em> are never exactly unity, because in practical terms, it's n... | https://electronics.stackexchange.com |
60,456 | [
"https://stats.stackexchange.com/questions/60456",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/26315/"
] | I'm new to Weka (and machine learning) so this question would be a bit silly.
So I have 2 models built using J48 and RandomForest (both run with 10-fold cross-validation mode) on a 40,000-tuple training set. I also have 6 different smaller test sets, each has around 8000 tuples.
Now I want to compare how these 2 clas... | Consider the following setup. We have a $p$-dimensional parameter vector $\theta$ that specifies the model completely and a maximum-likelihood estimator $\hat{\theta}$. The Fisher information in $\theta$ is denoted $I(\theta)$.
What is usually referred to as the <em>Wald statistic</em> is
$$(\hat{\theta} - \theta)^T ... | @NRH gave a good theoretical answer, here is one that intends to be simpler, more intuitive.
There is the formal Wald test (described in the answer by NRH), but we also refer to tests that look at the difference between an estimated parameter and its hypothesized value relative to the variation estimated at the estima... | https://stats.stackexchange.com |
20,010 | [
"https://astronomy.stackexchange.com/questions/20010",
"https://astronomy.stackexchange.com",
"https://astronomy.stackexchange.com/users/6965/"
] | I'm playing around with Sérsic profiles, and would like to retrieve the effective (half-light) radius that I put in by integration of the profile. I'm not managing to, so perhaps someone can help me out.
I create the profile using astropy:
<pre><code>from astropy.modeling.functional_models import Sersic1D
from scipy.... | The conceptual problem is that the half-light radius is the radius of a <em>circle</em> enclosing half the total light, assuming that the object is circular with a radial intensity profile equal to the Sersic function.
So you need to integrate over circular rings, each of which has an area of $2 \pi r \, dr$ and an i... | There are a number of conceptual and numeric problems with your code. They contribute, but I haven't found the conceptual problem yet. I will edit this answer if / when I find it.
To start, you set the half-light radius for an exponential profile at 1, create a profile out to 1000, and with a resolution of 1. You need... | https://astronomy.stackexchange.com |
401,166 | [
"https://physics.stackexchange.com/questions/401166",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/193318/"
] | I know how it is derived and here is my concern:
$dU=-dA=-d(pV)$
<code>p</code> is not a constant here, it depends on <code>V</code>. So my logic says, that
$d(p(V)*V) = p(V)dV$
is incorrect! It should be
$d(pV)=p(V)dV+Vdp$
What's wrong with my logic, thanks
| There are a few distinct questions here, so I'll answer quote by quote. $\newcommand{\ket}[1]{|#1 \rangle}$
<blockquote>
[...] the state is simple not normalised here (we would require a factor of $\frac{1}{\sqrt2}$ at the front)
</blockquote>
People often omit explicit normalization to save space; you should just ... | The relative phase matters in all kinds of circumstances. Imagine a spin-1/2 system with $\vert +\rangle$ and $\vert -\rangle$ the spin-up and spin-down states in the $\hat z$ direction.
Consider the state
$$
\vert \psi\rangle =\frac{1}{\sqrt{2}}\vert +\rangle + \frac{i}{\sqrt{2}}\vert -\rangle. \tag{1}
$$<br>
The av... | https://physics.stackexchange.com |
489,916 | [
"https://physics.stackexchange.com/questions/489916",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/236184/"
] | I am not a physicist nor do I have a good knowledge of the topic. Pardon me if I use terms erroneously.
I observed that when I put some water in a bottle, without regards to how the bottle is placed, the surface of water is always perpendicular to the direction of gravitational force. What is the explanation behind t... | I think you mean horizontal to the direction of gravity. The answer is simple. Any part of the water surface which rises above the general level is dragged down by gravity until pressure from the rest of the liquid prevents it from sinking any lower.
| To add to other answers - you could also say that the water will, in most cases, not be <em>perfectly</em> level. In a smooth sided water bottle, for example, if you look very closely, you will see a raised curved edge where the water meets the bottle. This is called a 'water meniscus', and is to do with the surface te... | https://physics.stackexchange.com |
3,412,255 | [
"https://math.stackexchange.com/questions/3412255",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/577174/"
] | <span class="math-container">$$f:\mathbb R \to\mathbb R, f(x) = \arctan x $$</span>
<span class="math-container">$$\frac{1}{1+(k+1)^2} \lt f(k+1) - f(k) \lt \frac{1}{1+k^2}, k\in \mathbb N^*$$</span>
I have to prove this for an exercise and I am would greatly appreciate some help. So far I have tried the following: N... | <strong>Hint:</strong> <span class="math-container">$\displaystyle f(k+1)-f(k)=\int_k^{k+1}\frac1{1+x^2}\,\mathrm dx$</span>.
| You may just use the mean value theorem:
<ul>
<li><span class="math-container">$f(k+1) -f(k) = f'(\kappa)(k+1 - k) =f'(\kappa)=\frac{1}{1+\kappa^2}$</span> with <span class="math-container">$k <\kappa < k+1$</span></li>
</ul>
Hence,
<span class="math-container">$$\frac{1}{1+(k+1)^2} < f'(\kappa) = f(k+1) -f(... | https://math.stackexchange.com |
439,320 | [
"https://mathoverflow.net/questions/439320",
"https://mathoverflow.net",
"https://mathoverflow.net/users/56524/"
] | I am trying to observe the behavior of <span class="math-container">$x_n \in (0,1)$</span> defined such that the function
<span class="math-container">\begin{equation}
f_n(x):=e^{-1/x}\Bigl(1+\frac{1}{n^2 x^n} \Bigr)
\end{equation}</span>
attains its maximum inside the interval <span class="math-container">$(0,1)$</spa... | The maximum <span class="math-container">$x_n$</span> of
<span class="math-container">$$f_n(x):=e^{-1/x}\Bigl(1+\frac{1}{n^2 x^n} \Bigr)$$</span>
is the smallest solution in <span class="math-container">$(0,1)$</span> of the equation
<span class="math-container">$$x=n x^n+\frac{1}{n}.$$</span>
For <span class="math-con... | Suppose that you consider
<span class="math-container">$$f(x)=n^2 x^n-n x+1+O(x^{n+1})$$</span>
Use power series reversion to obtain
<span class="math-container">$$x_n=-\frac {f(x)-1}n+(-1)^n \frac {(f(x)-1)^n}{n^{n-1} }+O\left((f(x)-1)^{n+1}\right)$$</span> Since we want <span class="math-container">$f(x)=0$</span>, t... | https://mathoverflow.net |
52,628 | [
"https://softwareengineering.stackexchange.com/questions/52628",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/13137/"
] | I'm currently design an application that involves a lot of calculation. Now I have generally two possibilities which I have both tested:
1) During startup of the application I calculated only the most important values and these values that consume a lot of time. So the user has to wait approximately 15 seconds during ... | Startup as fast as possible, use a thread to do the most important calculations. So the user gets a feedback immediately, and can start working after 15 secs. In the background, let another thread calculate everything, so after two minutes, those 2-3 sec response times also go away.
| Use a separate thread from the GUI to do whatever calculations you need. Have your form subscribe to an event and when you are done calculating fire that event so your form acts accordingly.
Don't lock the GUI, it's very, very annoying for end users and can be taken as a sign of a poor product.
In your case, display... | https://softwareengineering.stackexchange.com |
379,008 | [
"https://physics.stackexchange.com/questions/379008",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/180904/"
] | How to determine sign of $g$? It is said that we take it negative if any body is in upward motion. So why we don't take it positive when we observe downward motion.It is very confusing.
| The speed is irrelevant. Astronauts do space walks around spaceships and space stations regularly. In the reference frame of the ship, the astronauts have very small velocities. The speed of the ship itself is only relative to some other reference frame.
| As others have pointed out, speed itself is irrelevant. Astronauts would not notice their speed in any way; there will be no drag on the rope.
However, there is another way in which speed is very relevant. As the astronauts are moving at $0.5c$ it means that all other matter they encounter will strike them with that s... | https://physics.stackexchange.com |
207,985 | [
"https://electronics.stackexchange.com/questions/207985",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/92345/"
] | I few days ago when I got my FPGA I created a module to drive my 7-Segment display. I used only continuous assignments to drive the leds.
<pre><code>module set_number(input [3:0] x, output [6:0] seg);
assign seg = x == 0 ? 7'b1000000 :
x == 1 ? 7'b1111001 :
x == 2 ? 7'b0100100 :
... | As transistor mentions in his answer, the logic represented by the two pieces of code you posted are not the same; it has differences in the bit ordering and the latter sample will display hex characters A-F.
But, you are asking about the difference between using a wire variable and an assign statement vs. a reg varia... | If your concern is regarding the binary values then it looks OK. The example code has two differences.
<ul>
<li>The bit order is reversed. This can be rearranged to suit the PCB layout.</li>
<li>The book version will display full hex character set whereas yours only handles decimal digits.</li>
<li>The book seems to b... | https://electronics.stackexchange.com |
2,257,985 | [
"https://math.stackexchange.com/questions/2257985",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/411300/"
] | <blockquote>
Show that
$$L_n''(0)=\frac{1}{2}n(n-1)$$
</blockquote>
My first thought was to use the recurrence relation $$L_n^k(x)=L_{n-1}^k(x)+L_n^{k-1}(x)$$ and the orthogonality property
$$\int_0^\infty e^{-x} x^kL_n^k(x)L_m^k(x)dx=\frac{(n+k)1}{n!},$$
for $k=2$, but doing so didn't seem to lead anywhere.
| $B$ is not closed!
Let<br>
$$f_{n} = \left\{\begin{matrix}
A(x-\frac{1}{2})^{{1+\frac{1}{n}}} & x \in [\frac{1}{2},1]\\
- A\left |x-\frac{1}{2}\right |^{{1+\frac{1}{n}}} & x \in [0,\frac{1}{2}]
\end{matrix}\right.$$
then $\left \| f_{n} \right \|_{\infty} \leqslant A$ and $f_n$ is differentiable at... | Every continuous function on $[0,1]$ is a limit of a (bounded) sequence of polynomials (Weierstrass' theorem), so this set is not closed for any $A>0$. However, for $A= 0$ it is closed being a singleton.
Of course, you may pick your favourite example of a sequence of differentiable functions converging uniformly to... | https://math.stackexchange.com |
177,432 | [
"https://physics.stackexchange.com/questions/177432",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/64376/"
] | I was thinking about light and the different frequencies. The higher the frequency the more energetic the photon.
The higher frequency photon being more energetic seems counterintuitive when considering observed time dilation. What I mean is, if something has more energy it should "slow time" more than something with... | It's probably best to not think of single photons as sources of gravitational energy. For one thing, most bulk electromagnetic fields are not eigenstates of the photon number operator.
For another thing, the thing that couples to the gravitational field is the energy density of the field. This density is proportio... | I am not sure what you mean by
<blockquote>
if something has more energy it should "slow time" more than something with low energy
</blockquote>
But even if I just neglect that for a moment. I'll use a description of waves for visual aid and simplicity.
Imagine in "not slowed down time" a wave has a period (and f... | https://physics.stackexchange.com |
99,450 | [
"https://softwareengineering.stackexchange.com/questions/99450",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/33751/"
] | New to the site so sorry if this is the wrong section.
I'm starting app development and wondering what is the best practice when initially releasing my app. Do developers tend to keep some of the features for future updates to keep users active, or do they try to release the most complete app possible?
Basically, is ... | Most people producing their first app (at least those who turn out to have a successful product) release what they call an MVP first.
MVP is Minimum Viable Product - the app at this point contains the bare minimum amount of features necessary to be a useful product.
Then, based on user/customer feedback, you can work... | If you don't hold features back for later releases your application will never be released. There are almost always new features to add to software, but at some point you have to sit down and say "The application will release with X features and Y will come later"
| https://softwareengineering.stackexchange.com |
66,198 | [
"https://electronics.stackexchange.com/questions/66198",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/22817/"
] | I need to detect the presence of large metallic objects. The detector will be on the ground. The <code>detectable object</code> will be around 60-70cm away. The detector will need to be protected to support heavy weight. The detector will be on/near/under asphalt and cars might run over the detector. I am not sure abou... | No, you are not right. AC5 is a pin of the chip. You have to place a 27 Ohms resistor near to that pin and connect the <em>actual external circuitry</em> to that resistor (on the diagram, this is called the AC5-Load).
AC5 and AC5-Load are different things: AC5 is the pin, AC5-Load is the circuitry that should be conn... | The diagram indicates that the resistor is to be placed in series to the load, i.e. the signal from the AC5 pin will pass through the resistor to the AC5 load.
| https://electronics.stackexchange.com |
444,741 | [
"https://electronics.stackexchange.com/questions/444741",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/214898/"
] | I often read that positive feedback is used to get oscillators (since the output of the circuit is summed up to its input signal and so leads to instability), while negative feedback to get amplifiers (since its output is subtracted to its input and so leads to stability).
But it seems to me that this intuitive reaso... | You are not reading the words carefully. Tomato sauce is used to make a pizza, but not everything made with tomato sauce is a pizza.
Yes, positive feedback is used to cause oscillation. But that statement <strong>does not</strong> say that a circuit with positive feedback will <strong>always</strong> oscillate.
Simil... | In general, circuits need the operating point controlled (the quiescent voltage and current of the amplifying devices), and that requires negative feedback of some sort.
With quiescent values being at DC (zero Hertz) thus the DC behavior being Negative, because DC_positive feedback would be an unstable operating point... | https://electronics.stackexchange.com |
1,247,874 | [
"https://math.stackexchange.com/questions/1247874",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/233761/"
] | <img src="https://i.stack.imgur.com/mf5Vq.png" alt="Here is a picture of the masses. ">
<em>"Three masses are connected by wires and hung vertically. Draw a Force Body Diagram for each mass and determine the tensions in the three wires."</em>
I know there are forces only in the y-direction, so I started by trying t... | Hint: $257$ is a prime number and Fermat's little theorem tells us that,
$$2^{256}\equiv 1\pmod{257}$$
Hence, we have,
$$2^{804}\equiv 2^{804~\bmod~256}\equiv 2^{36}\equiv (256)^4\times 16\equiv (-1)^4\times 16\equiv 16\pmod{257}$$
| $2^{804}=2^4(2^8)^{100}=16\cdot(257-1)^{100}=16+(\text{multiples of 257})$.
The remainder is then evidently 16.
| https://math.stackexchange.com |
530,044 | [
"https://electronics.stackexchange.com/questions/530044",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/54870/"
] | Standard metal electrical distribution panels are rectangular or square shaped boxes.
Is there any standard - IEC, NEC, etc.- contraindicating the manufacturing of a custom shaped box for an electrical panel, i.e. with round, elliptical or any other shapes?
| <blockquote>
<em>But in all these things what I explained above has nothing to do with
input impedence because Even if we increase input impedence it causes
decrease in voltage gain (for example CE with emitter resistance
amplifier) and hence overall voltage will remain same .So that's why I
don't understand how increa... | Input impedance of amplifier forms the lower arm of a potential divider with the source impedance of whatever is driving the amplifier.
The lower the amplifier's input impedance then the more the signal is attenuated before it even arrives at the amplifier.
| https://electronics.stackexchange.com |
26,810 | [
"https://mathoverflow.net/questions/26810",
"https://mathoverflow.net",
"https://mathoverflow.net/users/6129/"
] | Are there examples of subshifts (that is, closed shift-invariant subsets of the full shift {$1...n$}${}^{\mathbb{Z}}$) on which the shift is topologically mixing, which admit a shift-invariant probability measure of full support, but no invariant ergodic measure of full support ?
I guess the answer is no, but I can't ... | A construction of a topologically <strike>transitive</strike> mixing subshift with a fully supported invariant measure, but no fully supported ergodic measure, is given by Benjamin Weiss in the article "Topological transitivity and ergodic measures", <em>Mathematical Systems Theory</em> 1971. It gives a direct combinat... | Let me go a little further: I believe that there also exists a topologically mixing subshift on two symbols with no fully-supported invariant measure. I don't know of a reference for this result, but I think that a direct construction should be "not too hard", in the sense that a completely detailed proof would take up... | https://mathoverflow.net |
3,905 | [
"https://security.stackexchange.com/questions/3905",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/2497/"
] | I'm in the process of making changes to my site so that we can be a SAML 2.0 Service Provider. We will be doing IdP initiated SAML with Out-of-Band account federation.
My question is this: Given a SAML response that is posted to the target page on my site, how do I verify that the response was generated by a trusted... | Some applications are incorectly coded to look for the SID of the well known Administrator account. This was an issue in the early days of NT4 but don't think it's an issue now...
I don't think any applications match on the name "Administrator" since that is Engish specific and is a very poor programming practice. "... | The only difference is that the built-in administrator account SID is well known. This makes for bad programming logic by 3rd parties, but otherwise that is the only difference.
| https://security.stackexchange.com |
56,106 | [
"https://stats.stackexchange.com/questions/56106",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/17590/"
] | I need to test my data to see if it follows a normal distribution with specific mean and std like N~(mu, std) I know that this can be done by Kolmogorov-Smirnov test which has a function in both MATLAB and R but the default for these function is standard normal and I do not know how should I specify my mu and std in t... | <code>ks.test</code> in <code>R</code> allows one to adjust the mean and sd of the distribution to be tested against. e.g.
<pre><code>x <- rnorm(1000, 4, 10)
ks.test(x, "pnorm", mean = 4, sd = 10)
</code></pre>
| In R, you can just use the function <code>ks.test</code> with the following arguments:
<pre><code>ks.test(your_data, "pnorm", mean=test_mu, sd=test_sd)
</code></pre>
Where <code>your_data</code> is your data vector, <code>test_mu</code> is the specific mean of the theoretical normal distribution and <code>test_sd</co... | https://stats.stackexchange.com |
12,613 | [
"https://physics.stackexchange.com/questions/12613",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/4631/"
] | In his popular book on relativity, in chapter IX, "The relativity of simultaneity", Einstein describes an experiment in which a flash happens simultaneously on A and B, as defined by the fact that an observer at the middle point M can see the light coming from A and B at the same moment:
<pre><code> --tra... | Your result is correct and, of course, there is no problem with relativity. When you're on the train and you see how events unfold for an observer on the platform at point M, then you conclude that he will see the light from A first. When you're on the platform and you see how events unfold for an observer on the train... | It looks to me that you've left out other important parts of the passage that are needed to understand what's going on here.
The motion of A and B from the point of view of an observer on the train is irrelevant to determining which flash reaches the midpoint of the train first. What matters is that both flashes trave... | https://physics.stackexchange.com |
256,410 | [
"https://math.stackexchange.com/questions/256410",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/52664/"
] | I'm given a solution $y = at^n$ (fixed)
And then asked if there exists a second order linear homogeneous differential equation for it.
My current line of thinking is that I can just substitute the solution into the form of a DE and if the resulting equation "looks right", then it exists. However, it bothers me that I... | A second-order linear differential equation is of the form:
$P(t)y''(t) + Q(t)y'(t) + R(t)y(t) = G(t)$, for continuous functions $P,Q,R,G$ and $y$ twice differentiable on some open interval. The homogeneous case occurs when $G(t) = 0$ for all $t$, reducing to:
$P(t) y''(t) + Q(t)y'(t) + R(t)y(t) = 0$.
Given that $y... | Here's what I'd recommend. Multiply both sides of the equation by $t^{-n}$ (or $t^{-n+1}$ if you prefer), then differentiate both sides twice. Of course, that won't be the complete solution of said differential equation, but it will be a solution.
| https://math.stackexchange.com |
575,187 | [
"https://physics.stackexchange.com/questions/575187",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/232786/"
] | If teleportation were to be somehow invented in the classic movie scenario (basically, enter a "magic portal" that immediately moves you to a different point in space). Ignoring some obvious problems (like the difference in local speeds between the entrance and exit "portals", one could wonder what ... | Yes, it is always an operator. The inner product is defined for all vectors in a space, so <span class="math-container">$|a\rangle\langle a|\psi\rangle$</span> is always well-defined. It is also always Hermitian, since its eigenvalues are only <span class="math-container">$\langle a|a\rangle$</span> and <span class="ma... | Yes. For any vector <span class="math-container">$\psi$</span> in the Hilbert space, the projector <span class="math-container">$P_\psi$</span>, which eats a vector <span class="math-container">$\phi$</span> and spits out <span class="math-container">$\langle\psi,\phi\rangle \psi$</span>, is bounded (with operator nor... | https://physics.stackexchange.com |
272,807 | [
"https://softwareengineering.stackexchange.com/questions/272807",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/167608/"
] | Noobie to deploying any kind of web app. I have a Java (Play Framework) and MySQL server app running locally. I know that I <em>can</em> deploy it to EC2 (have once as a test), and that I can download some kind of module to get it to run on Google App Engine. But my question is -- do I need to?
I suppose I don't under... | Do you expect a situation where you would need to scale your application, and do it fast?
If yes, that's the point of cloud computing in general and Amazon's Elastic Compute Cloud in particular. You start small and you pay a small fee. But if your service is successful and you need to increase its capacity per ten, or... | The main alternative to AWS EC2 would be a dedicated server. Having dedicated hardware can be beneficial if the following are true:
<ul>
<li>You have a good sense of your capacity requirements.</li>
<li>Performance is important for your application.</li>
<li>You expect that the resource needs of your application will ... | https://softwareengineering.stackexchange.com |
616,921 | [
"https://electronics.stackexchange.com/questions/616921",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/256347/"
] | I've used silicone RTV that's intended for use on PCBs and have also used 100% silicone purchased from the hardware store to use on PCBs. I was not able to tell a difference between the two. Everything looked and smelled exactly the same. I know there could still be differences, but I'm wondering if they're just sellin... | Cheap silicone is acidic (like vinegar) while GE's Room Temperature Vulcanizing (RTV) is not, and is suitable for electronic anti-corrosive insulation on metals. The main features are thermal insulation and high voltage insulation in addition to the expected dust & water HV protection.
However, the application is o... | For electronics work you want "platinum cure", "addition-cure" or electronic grade silicone. "RTV" means "room temperature vulcanizing" and is a generic term that includes various types of curing. So is "silicone", it can refer to materials from oils and greases to rela... | https://electronics.stackexchange.com |
384,417 | [
"https://softwareengineering.stackexchange.com/questions/384417",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/-1/"
] | I have a module with the class <code>Foo</code>. <code>Foo</code> has three methods:
<ul>
<li>The constructor(<code>__init__()</code>)</li>
<li>An error handling method (<code>_error_handler()</code>)</li>
<li>The method that actually does something. (<code>run()</code>)</li>
</ul>
Then, I have a bunch of helper meth... | This depends on the relation between the task of those methods and the purpose of the object.
If your object is a report generator and uses a lot of utility functions to justify, inflect and tabulate strings, then those methods have little to do with the purpose of the object. Generating records involves justifying li... | Side from polymorphism, the difference between putting them at the module level versus the class level is really subtle. So unless you need to override these in a subclass, the answer is: it really doesn't matter much. Do what makes it the easiest to read and understand. You don't have a parameter in the module leve... | https://softwareengineering.stackexchange.com |
258,938 | [
"https://stats.stackexchange.com/questions/258938",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/147207/"
] | I have a regression dataset where the features are on the order of ~ 400 variables and the dataset itself is around 300 samples. I tried to use Random Forest Regression (RFR) on the dataset and used either out-of-bag (oob) score or k-fold cv score to judge its performance. The kind of behavior I see right now that I'm ... | Using Random Forest in a dataset as the one you described has two major problems:
<ol>
<li>Random Forest does not perform well when features are monotonic transformations of other features (this makes the trees of the forest less independent from each other).
</li>
<li>The same happens when you have more features than ... | I think you just answered yourself. In general RF are not good in high dimensional settings or when you have more features than samples, therefore reducing your features from 400 to 8 will help, especially if you have lot's of noisy collinear features. You have also less chance to overfit in this case, but beware of do... | https://stats.stackexchange.com |
123,433 | [
"https://electronics.stackexchange.com/questions/123433",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/37418/"
] | Here in the United States the electricity grid is AC. I have heard that AC allows transmission of power at greater distances with less loss. However, with the advent of solar panels, it would seem that one could generate DC power directly and power the home this way. There are no great distances involved.
Why is ... | It's not impossible, it's just more complicated and expensive. Everything in your house is designed to run from AC. Many smaller products do take DC in but they come with an AC adapter because that's the only available source of continuous, inexpensive power nearly everywhere. The voltage required can be different f... | You can feed your house DC, however the issue remains that while most devices rectify AC to DC, they are designed for an AC input. This is why you need an inverter, even if it's at some loss, you feed your electronics what they were designed for. Even then, grid tie-in solar systems that you speak of only help boost th... | https://electronics.stackexchange.com |
11,078 | [
"https://dsp.stackexchange.com/questions/11078",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/5554/"
] | I have the following inverse system
$$G(s)= s^2 + 2s + 3$$
How do I implement it in Simulink? Note that the transfer function is only accepted if and only if the order of the numerator is less than the order of denominator.
| I believe that you will want to process each channel separately over all of the images. Otherwise a mean and variance will have very little meaning. And if you convert to grayscale before training, then the network can only retrieve grayscale images, which doesn't seem to match with what the paper describes.
<blockq... | <blockquote>
For example if I have r=255, g=255, b=255, can I take pixel value as (in binary), (r<<16)+(g<<8)+b ?
</blockquote>
While this number would identify the colour, it would not help with a mean or standard deviation. e.g. pure blue would be 255 but pure red would be 16711680
The paper is unclea... | https://dsp.stackexchange.com |
4,180,885 | [
"https://math.stackexchange.com/questions/4180885",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/492702/"
] | Please mark this as duplicate if this has been asked before.
I started a deep dive in distribution theory on <span class="math-container">$\mathbb{R}^n$</span> I want to understand the singular support better. I know that there are regular distributions <span class="math-container">$T_u$</span> for <span class="math-co... | This is a worthwhile issue to consider, after all! In addition to @md2perpe's example of principal value of <span class="math-container">$1/x$</span>:
It may be illuminating to look at the analogous question on the circle, where we can show that every distribution has a Fourier expansion with coefficients of polynomial... | No, I cannot see how for example <span class="math-container">$\operatorname{pv}\frac{1}{x}$</span> could be written in that way.
Something that is true, though, is that on every compact set, <span class="math-container">$T=T_f^{(k)}$</span> for some <span class="math-container">$f \in L^1_\text{loc}$</span> and some <... | https://math.stackexchange.com |
660,348 | [
"https://math.stackexchange.com/questions/660348",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/79762/"
] | I'm wondering what open mappings are actually good for (except for inverse becomes continuous)???
My irritation came since, people stress that an open mapping not necessarily preserves closed sets (well, sure, I mean closed maps are some totally different subject since they don't describe neigbborhoods).
I cannot ima... | <em>Open and closed maps become useful when combined with continuity!</em>
<strong>Open/Closed Maps:</strong>
For a continuous and open/closed map we have:
If it is injective then it is an embedding.<br>
If it is surjective then it is a quotient map.<br>
If it is bijective then it is a homeomorphism.
<em>Note: Neit... | How about this: as soon as you know a mapping is open, you know that maxima can't be achieved on the interior of sets (if our codomain is $\mathbb{R}$ or $\mathbb{C}$ for example): consider $f: X \rightarrow Y$ an open map. Then $f(\Omega)$ is open in $Y$, so $f(\Omega)$ cannot contain an element with maximal norm.
| https://math.stackexchange.com |
2,215,768 | [
"https://math.stackexchange.com/questions/2215768",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | The question:
<blockquote>
Consider the tetrahedron with vertices $(0, 0, 0)$, $(a, 0, 0)$, $(0,
b, 0)$, and ($0, 0 ,c)$ and let $S$ be the side of the tetrahedron
with vertices $(a, 0, 0)$, $(0, b, 0)$, and $(0, 0, c)$. By finding 2
vectors in $S$, find a unit normal to $S$.
</blockquote>
I know how to find a ... | <em>Hadamard's formula</em> directly gives $\;\dfrac1R=\limsup\, \lvert a_n\rvert^{\tfrac1n}$, and there are two sorts of $\lvert a_n\rvert^{\tfrac1n}$:
$$\begin{cases}\lvert a_{2p}\rvert^{\smash{\tfrac1{2p}}}=2^{1/2},\\
\lvert a_{2p+1}\rvert^{\tfrac1{2p+1}}=3^{\tfrac{n+1}{2n+1}}\to 3^{1/2}.
\end{cases} $$
Hence $\;\d... | The sequences $(\left|2^nz^{2n}\right|)$ and $(\left|3^{n+1}z^{2n+1}\right|)$ must be bounded, which gives $R=\frac{1}{\sqrt3}$.
| https://math.stackexchange.com |
299,751 | [
"https://physics.stackexchange.com/questions/299751",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/139643/"
] | I am quite new to physics and am still wrapping my head around some of the core concepts. I have a homework question that states:
<blockquote>
A 115-kg astronaut is floating at rest relative to her spacecraft in
deep space. She throws an 18-kg tool kit at 4.6 m/s away from the
spacecraft. Show that the astronaut... | To be honest, you have taken the wrong approach to this problem. The fact that you were able to calculate a time of one second is purely a coincidence; actually, that time has nothing to do with the problem.
The equation you have in mind, which I would suggest writing
$$v_f = a\Delta t + v_0$$
involves the initial vel... | You have an interesting mixture of errors which happened by luck to have produced the correct answer.
<blockquote>
$v_F$ from $v_F = \frac{\text{Force}}{\text{Mass}}\times\text{Time} +
> v_0\text{ (initial velocity)}$
</blockquote>
Is in fact a perfectly valid equation which can be seen if it is rearranged as
... | https://physics.stackexchange.com |
195,592 | [
"https://mathoverflow.net/questions/195592",
"https://mathoverflow.net",
"https://mathoverflow.net/users/66688/"
] | I would like to find a pedagogical reference where the classification, up to isomorphism, of principal $SU(2)$ bundles over a four-dimensional compact, oriented manifold is explained. In particular I am interested in the torus $T^4$ case. Is there a similar classification when the base manifold is non-compact, in parti... | Let $X$ be an $(n-1)$-connected CW complex with $\pi_n(X) = G$. By attaching cells of dimensions at least $n+2$, we obtain a CW complex $Y$ the same $(n+1)$-skeleton as $X$, but with $\pi_i(Y) = 0$ for $i > n$. For $i \leq n$, we have, by cellular approximation,
$$\pi_i(Y) = \pi_i(Y^{(n+1)}) = \pi_i(X^{(n+1)}) = \... | Isomorphism classes of principal $SU(2)$-bundles $P\to X$ over a closed 4-manifold $X$ are classified by $H^4(X;\mathbb{Z})$. One assigns to $P$ the second Chern class (a.k.a. Euler class) of the associated $\mathbb{C}^2$-bundle. (In the oriented case, the number $c_2(P)[X]$ is also called the instanton number.) Over o... | https://mathoverflow.net |
111,001 | [
"https://stats.stackexchange.com/questions/111001",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/53620/"
] | I am trying to construct a formula, which will take student's previous exam results (for ex: SAT) taken at particular dates and predict his future test result.
One X is previous test result 1; another X is date of previous test 1 (can be converted to number of days between this test and the last test for simplicity);... | As requested, I illustrate using a simple regression using the <code>mtcars</code> data:
<pre><code>fit <- lm(mpg~hp, data=mtcars)
summary(fit)
Call:
lm(formula = mpg ~ hp, data = mtcars)
Residuals:
Min 1Q Median 3Q Max
-5.7121 -2.1122 -0.8854 1.5819 8.2360
Coefficients:
Estima... | The original poster asked for an "explain like I'm 5" answer. Let's say your school teacher invites you and your schoolmates to help guess the teacher's table width. Each of the 20 students in class can choose a device (ruler, scale, tape, or yardstick) and is allowed to measure the table 10 times. You all are asked... | https://stats.stackexchange.com |
222,361 | [
"https://security.stackexchange.com/questions/222361",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/139736/"
] | Situation: I want to set up some home servers that can store my files. Currently in the planning stage of the setup.
Problem: Security and stuff comes to mind when doing such things, especially exposed servers.
Question: Does it make a difference if I set up a VPN server at home and only expose the VPN server vs expo... | If you <em>don't</em> use a VPN, then ask yourself the following questions:
<ul>
<li>How would the connection from your client to your file server be encrypted? </li>
<li>How would the client and the server mutually authenticate each other?</li>
<li>How would the keys used to build this encrypted connection be managed... | Yes, it makes a difference. It makes the setup much more secure, especially if you use 2FA for your VPN accesss. So go ahead and do it!
| https://security.stackexchange.com |
11,690 | [
"https://mathoverflow.net/questions/11690",
"https://mathoverflow.net",
"https://mathoverflow.net/users/271/"
] | Let $R$ be a complete discrete valuation ring and let $K$ be its field of fractions. Suppose $X$ is a smooth rigid-anaytic space over $K$. Often it is convenient to have a model of $X$ whose reduction has singularities which are as mild as possible--a semistable model. This amounts to having an admissible covering o... | Here's a first pass at your question; hopefully it will suggest something more definitive.
Let's imagine we were in the simplest case, where $X$ is a disk, with its smooth model
being the formal affine line over $R$, and that $Z$ was the sub-disk of elements of
absolute value less than or equal the absolute value of ... | In dimension 1, the answer should be yes because the semi-stable reduction is well understood. The only problem would be the difference between the rigid analytic reduction and the algebraic reduction as a formal covering defines a reduced analytic reduction but non necessarily reduced algebraic reduction (= special fi... | https://mathoverflow.net |
47,090 | [
"https://astronomy.stackexchange.com/questions/47090",
"https://astronomy.stackexchange.com",
"https://astronomy.stackexchange.com/users/44120/"
] | Many people have told me that the “universe doesn’t care what you think” in my regards to it being infinite in size, and I know something that seems logical doesn’t mean anything when measured by physics yet I don’t see how the universe could be infinite and yet constantly expanding. I believe it is constantly expandin... | I'm taking a risk here; a risk of acquiring many down votes and ruining my very modest site score. But I shall proceed in the name of free-thinking.
Disclaimer: The following is viewed as naughty, troublesome thinking by the Astrophysics establishment:
Logically, how can space NOT be infinite, @Max? Do you somehow thin... | Something infinite can expand.
Consider an infinite length of elastic. There are (infinitely) beads attached to it at 1m gaps. You might label one of the beads "0", then the next one is "1", and "2" and so on. Beads on the other side are labelled "-1", "-2"... The ela... | https://astronomy.stackexchange.com |
2,331,301 | [
"https://math.stackexchange.com/questions/2331301",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/257314/"
] | Since $(AB^T) ^T = BA^T, (AA^T)^T=A^T A$?
where A, B are matrices.
I am wondering if the second equation holds.
| No, it doesn't hold.
In more detail, $$(AB^T)^T = (B^T)^T A^T = B A^T$$
Now, if you substitute $A$ for $B$ in the equation, you get $A A^T$ as the final result.
| no it doesn't
let $A = [0\;1]$
then $(AA^T)^T$ is $1$ but $A^TA$ is a 2 by 2 matrix
| https://math.stackexchange.com |
1,657,531 | [
"https://math.stackexchange.com/questions/1657531",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/191792/"
] | I was wondering what are the odds of drawing the 1 card you need when you have 4 cards left and two draws.
I instinctively think 50/50 as you have two draws, but then I think it's a 1/4 + 1/3 == > 50/50
What is correct and why?
| <strong>Outline</strong>
You are right. It must end in $0$ for it to be divisible by $5$. For it to be divisible by $3$, it has to have three $8$'s. Which means the minimum <strong>k = 3</strong> at which point the smallest number is $\color{blue}{300338880}$ which leaves a remainder $\color{blue}{6}$ when divided by ... | We need the smallest available number. We can't use an initial 0 so we start with a 3. Then come k-1 0's, saving the last zero for the units digit. Then the remaining k-1 3's, then the 8's and finally the last 0.
Split the number into 2-digit groups starting from the units digit and add these groups together. This... | https://math.stackexchange.com |
43,267 | [
"https://softwareengineering.stackexchange.com/questions/43267",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/1293/"
] | Personally I'd prefer to work on one project at a time, maintaining absolute focus on that. I find I'm at my most productive in this situation, however it's not always the way things happen in the real world.
Do you prefer working on one large project, and being totally focused on that, or be working on say 3-4 small ... | Well I used to work on one single project at work and on my personal project in the evening. It provided sufficient diversity for me to keep motivated and have fun.
It is in fact good to switch contexts now and then, it sort of refreshes your mind. If you've focused your brain on one particular thing for a long time, ... | I prefer working on several small projects OR one giant project.
When working on a several small projects, I can avoid getting stuck by switching to another project. When I'm typing away madly I don't stop for anything, but when I get bogged down it's good to have something else to switch to. With small projects the c... | https://softwareengineering.stackexchange.com |
2,732,783 | [
"https://math.stackexchange.com/questions/2732783",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/25633/"
] | Let $\delta$ $>$ $0$, $k$ $\in$ $\mathbb{N}$ and $f$ is analytic on $B(0,2\delta)$
with $|f^{(k)}(0)|$ =$k!\delta^{-k}${sup$|f(z)|$:$|z|=\delta$}. Then show that $f(z)$ = $az^k$ for some $a$ $\in$ $\mathbb{C}$. My idea was to use the Cauchy Integral Formula and get a bound on the coefficients after performing the Ta... | Making a rigorous proof for this turned out to be <em>way</em> more involved than what I'd expected.
<hr>
From Cauchy's integral formula we have that for all $n \in \mathbb N$
$$f^{(n)}(0) = \frac{n!}{2\pi i} \,\oint_{C(0;\delta)}\,\frac{f(z)}{z^{n+1}}\,dz.$$
Hence:
\begin{align}
\left|f^{(n)}(0)\right|
&=
\fr... | Lemma: Suppose $g:[a,b]\to \mathbb C$ is continuous. Let $M= \sup_{[a,b]}|g|.$ If
$$|\int_a^b g(t)\,dt\,| = M (b-a),$$
then $g$ is constant on $[a,b].$
The proof of the lemma is a nice exercise. Ask questions if you like.
To our problem: Let $M=\sup_{t\in [0,2\pi]} |f(\delta e^{it})|.$ By Cauchy's integral formula... | https://math.stackexchange.com |
141,330 | [
"https://softwareengineering.stackexchange.com/questions/141330",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/50293/"
] | I have a MySQL database and an automation script which modifies the data inside once a day. And these columns may have changed by an user manually. What is the best approach to make the system only update the automated data, not the manually edited ones? I mean yes, flagging the cell which is manually edited is one way... | I would put two DateTime columns in those tables, one called <code>lastUserEdit</code> and the other one called <code>lastAutomatedEdit</code>. That way, you always know whether the last edit on each row was done by a user or automatically by simply comparing these two dates.
| Flagging as you have mentioned is what I'd do.
Just by updating the value in an <code>ENUM column ('0','1')</code> to let you know who last updated the record.
| https://softwareengineering.stackexchange.com |
317,267 | [
"https://stats.stackexchange.com/questions/317267",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/186842/"
] | I have a count outcome and a treatment vs. comparison group independent variable. I would like to model how many times patients return following their first visit (I might actually make this outcome binary - TBD). However, the patients all have different study enrollment lengths. Rather than lose data by subsetting bas... | Two answers, based on whether you are analyzing the effect of the treatment, or simply predicting number of times returning.
<strong>If analyzing treatment effect:</strong>
You effectively have a regression
$$
y = \alpha + \beta T + \epsilon
$$
The coefficient $\beta$ measures the average effect of the treatment on t... | It rather standard to use the natural logarithm of the follow-up as an offset in Poisson or negative-binomial regression, if you want to model events per time unit. This is a pretty common and widely accepted approach. There are lots of precedents e.g. in clinical trials of asthma/COPD exacerbations, multiple sclerosis... | https://stats.stackexchange.com |
1,603,094 | [
"https://math.stackexchange.com/questions/1603094",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/251995/"
] | Consider the ODE
$$
\frac{dv}{dt}=v^2+1.
$$
I do not know how to solve this ODE. I would be very thankful to get some help.
| $$
\frac{dv}{dt} = v^2 + 1 \implies \frac{dv}{v^2 + 1} = dt
$$
Now integrate both sides.
| We separate variables to obtain $\int \frac{1}{1+v^2} dv=\int dt$. We know that $\frac{d}{dv}[\arctan(v)]=\frac{1}{1+v^2}$, so our integral just becomes
$\arctan(v)+C=t+D$. Let $D-C=C_1$ and take $\tan$ of both sides to obtain $v=\tan (t+C_1)$
| https://math.stackexchange.com |
356,642 | [
"https://mathoverflow.net/questions/356642",
"https://mathoverflow.net",
"https://mathoverflow.net/users/11145/"
] | Suppose <span class="math-container">$e : \mathbb R \to F$</span> is an elementary embedding in the language of ordered fields. Can there exist an elementary embedding <span class="math-container">$e' : \mathbb R \to F$</span> such that <span class="math-container">$e \not= e'$</span>? Note that it would have to be t... | Yes, this is possible. Let <span class="math-container">$F$</span> be a nonarchimedean strongly <span class="math-container">$\omega$</span>-homogeneous real-closed field such that <span class="math-container">$\mathbb R\subseteq F$</span> (which exists by model-theoretic general reasons). Fix an infinitesimal element ... | In fact <span class="math-container">$\mathbb{R}$</span> is elementarily embedded in several ways in any non-archimedean real-closed field which contains it. The proof is more involved than I thought before writing it, but if you don't know the arguments, I think this will make the situation of your question clearer.
... | https://mathoverflow.net |
76,628 | [
"https://cs.stackexchange.com/questions/76628",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/70335/"
] | <blockquote>
We're given the algorithm which uses stack in order to store data (stack is implemented via an array). When the stack is full the algorithm does the following:
<ol>
<li>Creates a new array double the size of the original one.</li>
<li>Copies all elements of the old array to the new array, preserving the or... | I don't think that it is possible to push an element to a <strong>full</strong> array-based stack in worst case <span class="math-container">$\mathcal{O}(1)$</span> time. However, you can rest assured that each push runs in constant <strong>amortized</strong> time whenever you multiply the length of the full array by a... | If you use <em>one</em> array for storage, and you add more and more items to the stack, then at some point you need to increase the size of the array. Most likely your "array" implementation doesn't allow increasing the array size in constant time.
So the only possible solution: Reserve all available space for the a... | https://cs.stackexchange.com |
31,249 | [
"https://electronics.stackexchange.com/questions/31249",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/6981/"
] | I am looking for serial bus, that could have addressable nodes, with maybe max. 30 nodes. I don't need multi-master, and the speed is not an issue.
The distance between the nodes could be something like 20 meters. The easier it is to implement with Microchip PIC microcontrollers, the better. I2c seems otherwise nice, b... | CAN is definitely the way to go. There are plenty of PICs that have a CAN peripheral built in. The electrical interface and up to the packet layer, including checksum generation and checking, is all built into the hardware. The hardware also handles collision detection and transmission retry. At 20 meters you shoul... | RS-485 is pretty good. It requires separate driver chips, but using such chips will help shelter the PIC from any electrical nastiness on the bus.
If there's going to be one master, I would suggest that slave devices be coded such that they essentially never talk except in immediate response to a transmission from th... | https://electronics.stackexchange.com |
135,435 | [
"https://dba.stackexchange.com/questions/135435",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/69658/"
] | Cannot find old connection string connecting to old database and not new one that has been defined. We have changed web servers on Azure and cannot find the reference to the old database because our IIS sites are still hitting the old database as well as the newly defined one.
I am hoping this is not edge case and som... | To add on to what Max is saying, you could use the following to get you the sql info and use the port information and sql text to figure out which service is doing what and then use those additional clues to help you isolate where it's coming from.
<pre><code>SELECT
des.host_name
, des.program_name
, des.l... | This doesn't directly answer your question, however it may help you identify the specific parts that are still accessing the old database server.
Run the following query on the old server:
<pre><code>SELECT des.host_name
, des.program_name
, des.login_name
, dec.client_net_address
FROM sys.dm_exec_session... | https://dba.stackexchange.com |
323,288 | [
"https://electronics.stackexchange.com/questions/323288",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/154246/"
] | I was wondering today what could possibly happen if a lightning strikes my van where I have a 12V battery negative connected to the chassis. Could that destroy the battery or start a fire?
In order to prevent it should I disconnect the battery negative during the storm?
Thanks
| The lightning can cause a fire regardless of the battery being connected. It can go through the electronics to the battery positive as well. A lightning strike on a solid car can blow electronics, fuse metal, blow out windows and start a fire in the seats or other fabric material or said electronics.
Fire risk aside, ... | Your car body (presuming that it's not a convertible) forms an effective faraday cage - hopefully you would be fine.
As alluded in other answer the tires and ecu / electronics may take a hit. The battery being connected or not is unlikely to make any difference, since the lightning will follow fastest route to ground,... | https://electronics.stackexchange.com |
220,549 | [
"https://softwareengineering.stackexchange.com/questions/220549",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/13027/"
] | <strong>TLDR:</strong> Should TIMEOUT be a public property on my static class, or a parameter to every function?
<hr>
<strong>Background:</strong>
I am releasing a c# client-api library that facilitates communicating with our REST api.
The client-api consists of an object model and one large static class with a bunc... | <blockquote>
Is it reasonable to assume users won't be micromanaging timeouts for individual requests like that?
</blockquote>
Well it really depends on the services being provided. Some operations might be very slow and he client might be able to anticipate that (for example, requesting all transactions on some acc... | I would hate to have only a global static setting because then you can have but a single timeout setting in an app and there might be times where you want to make that highly variable throughout the app. This is one reason I tend to prefer to avoid static API client implementations in favor if instance-based implementa... | https://softwareengineering.stackexchange.com |
153,676 | [
"https://mathoverflow.net/questions/153676",
"https://mathoverflow.net",
"https://mathoverflow.net/users/38200/"
] | Consider an elementary chain of models of some first-order theory $T$:
$$ (M_\alpha)_{\alpha < \kappa}, M_\alpha \prec M_\beta \; {\rm for} \; \alpha < \beta .$$
Let also $(N_\alpha)$ be another elementary chain of models of the same theory.
Assume, that for every $\alpha < \kappa$ ($\kappa$ a cardinal) we h... | No. Consider the following two linear orderings: $\mathcal{M}=\mathbb{Z}\cdot\mathbb{N}$, and $\mathcal{N}=\mathbb{Z}\cdot\mathbb{N}^*$. (Recall that $\mathcal{L}\cdot\mathcal{L}'$ is the linear order gotten by replacing each point in $\mathcal{L}'$ with a copy of $\mathcal{L}$, and that $\mathcal{L}^*$ is just $\mathc... | Here is a simple counterexample. Let $T$ be the theory of sets with no additional structure. Let $\mathcal{M}_{\alpha}$ be the set $\omega+\alpha$ with no additional structure, and let $\mathcal{N}_{\alpha}=\mathbb{N}$ for all $\alpha<\omega_{1}$. Then if $\alpha\leq\beta$, then
$\mathcal{M}_{\alpha}$ is an elementa... | https://mathoverflow.net |
231,664 | [
"https://stats.stackexchange.com/questions/231664",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/82889/"
] | Logistic regression can be penalized with L2 or L1 to avoid overfilling and/or select variables. The idea is to maximized the likelihood. Accordingly, the total quality formula is following:
<pre><code>total quality = measure of fit (likelihood of the data) - measure of the magnitude of the coefficients (L2 penalty).
... | For any regression model, we try to minimize a certain loss function say $L(\beta|x)$ (e.g. sum squared residuals for OLS). Note that we can see the likelihood function as a 'negative loss function', since we try to maximize this.
In case of a penalized regression (LASSO, Ridge, etc.) we try to minimize $$L(\beta|x) +... | Because otherwise it would favor bigger and bigger coefficients and the estimates would blow up. You rather ask, why do we add or substract anything ;)
| https://stats.stackexchange.com |
57,012 | [
"https://stats.stackexchange.com/questions/57012",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/2040/"
] | If I try and fit the linear regression
<pre><code>lm(y~V1+V3,data=x)
</code></pre>
with data:
<pre><code>structure(list(V1 = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 3L, 3L, 3L, 3L,
3L, 3L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L
), .Labe... | This is what your covariates matrix looks like:
$\begin{pmatrix}
1 & 0 & 0 & 0 & \alpha \\
1 & 1 & 0 & 0 & \beta \\
1 & 0 & 1 & 0 & \gamma \\
1 & 0 & 0 & 1 & \delta \\
\end{pmatrix}$
First column is for the intercept, the last is for <code>V3</code> a... | It's easy to see that V3 is a linear combination of the levels of V1:
<pre><code>summary(lm(V3~V1,data=x))
</code></pre>
Like so:
<pre><code>Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 3.330e+00 1.016e-15 3.278e+15 <2e-16 ***
V1B 6.600e-01 1.437e-15 4.593e+14 ... | https://stats.stackexchange.com |
6,773 | [
"https://cs.stackexchange.com/questions/6773",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/4298/"
] | I am implementing the cycle-canceling algorithm to find an optimal solution for the min-cost flow problem. By finding and removing negative cost cycles in the residual network, the total cost is lowered in each round. To find a negative cycle I am using the bellman-ford algorithm.
My Problem is:
Bellman-ford only find... | To expand upon my comment, remember, this algorithm for finding Min-Cost-Flow relies on the fact that $f$ is maximal. By first running Ford-Fulkerson to find $f$ and the resulting residual network $G_f$, the cost $f$ is then reduced by finding negative cycles in $G_f$. That is, by finding negative cycles in $G_f$ we do... | My suggestion:
You have to start the algorithm from T,
in order to find a negative cycle in your residual network. The result should be the same, but then you can reach the circle
| https://cs.stackexchange.com |
1,857,808 | [
"https://math.stackexchange.com/questions/1857808",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/160853/"
] | In queuing theory, (with a single queue and a single server) writing $A$ for the service rate (of customers) and $B$ for the arrival rate (of customers) we know that the average time a customer waits in the system is given by,
$$W=\frac{1}{A-B}$$
What is the intuitive interpretation of this equation?
What I underst... | I am canoeing up a river at a speed (relative to the water) of $A$ miles per hour, and the current in the other direction is $B$ miles per hour. If $A\gt B$, my speed relative to the shore line is $A-B$, so it will take me $\frac{1}{A-B}$ hours to travel $1$ mile.
Or if you prefer I am running up a down escalator.
| The intuition behind the delay formula for a M/M/1 queue is based on the stability assumption requiring that during my waiting delay same numbers of customers got service and arrived to the queue. Customers arrive with a rate of <span class="math-container">$B$</span> while being serviced at a rate of <span class="math... | https://math.stackexchange.com |
412,639 | [
"https://physics.stackexchange.com/questions/412639",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/198722/"
] | Suppose we have two balls, $A$ and $B$, of radius $1$ with equal mass. Ball $B$ is initially at (center is at) two on the $x$-axis, i.e. $(2,0,0)$, and has velocity $0$. Ball $A$ is initially at (center is at) some point on the $x$-axis to the left of ball $B$ (not touching ofc) and is moving right with speed $s > 0... | An issue that is glossed over in most introductory material—how you define a collision, and in particular <em>when are the initial and final states</em>—plays a roll here.
We could define a collision by considering the interaction(s) between the affected bodies. We look for a situation when the interactions (i.e. forc... | It is easier to visualize this from an inertial frame of reference that is moving to the right with half the initial speed of the left ball. From this frame of reference, both balls will initially be moving toward each other with equal speed.
Upon contact, each ball will experience a deformation, and the spatial ex... | https://physics.stackexchange.com |
99,370 | [
"https://softwareengineering.stackexchange.com/questions/99370",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/33198/"
] | In a company I used to work for, executives insisted that the code coverage with unit tests must be 99% or more. This resulted in writing more tests than code. It took us literally 3 days to write tests for a single class that took a day to implement.
As a result, however, I learnt a lot about TDD, testing tools, prac... | The amount of unit testing that is needed depends on several factors:
<ul>
<li>Product Size (The larger the project, the greater the need to include at least some unit testing)</li>
<li>Required Quality Level (If you are quickly putting software together that needs to be out as quick as possible and some minor bugs a... | Unit tests pay off at maintenance time. If you plan to have a long living application you will spend more time maintaining than you think you will now (if you have not yet tried this, you will be surprised how long a successful project may live)
What you want, is that if you accidentally <em>change</em> your functio... | https://softwareengineering.stackexchange.com |
185,077 | [
"https://electronics.stackexchange.com/questions/185077",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/82957/"
] | I have been trying to supply 5V into parallel-connected components which require a certain amount of current in total. If I wanted to increase the current from my supply, can I increase its current output without increasing the voltage?
| No you cannot, according to Ohms law, which we think is pretty well solid (It's a law, not a theory!).
As long as your 5V source can output the required current while maintaining it's regulated voltage, you are okay.
if the 5V source is weak, and cannot output enough current for the load, then the output voltage wil... | What is your voltage source? What is your load?
If you have a voltage source, you cannot change the current that your load consumes. If you increase the voltage, you will increase the current because your load is constant (V=I*R).
But you will need to watch out for the max voltage on your load and make sure you do ... | https://electronics.stackexchange.com |
188,589 | [
"https://physics.stackexchange.com/questions/188589",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/73769/"
] | EDIT: The title should rather be how/why transformers lose energy
My idea of a transformer is that it is composed of two separate wire windings around some metal core. The purpose is to increase/decrease AC voltage. The transfer of energy from the primary to the secondary winding is due to magnetic coupling or mutual... | A significant source of power loss in a transformer is the induced eddy currents in the core. Just as the varying magnetic field induces current in the secondary coil, it can also induce currents in the core itself. These currents do nothing but dissipate energy, and so are to be avoided.
To reduce eddy currents, you ... | I suspect most of the loss is simply resistive heating in the coils and possibly some heating due to hysteresis in the iron core rather than coupling of the magnetic field to any external power leakage
| https://physics.stackexchange.com |
112,447 | [
"https://mathoverflow.net/questions/112447",
"https://mathoverflow.net",
"https://mathoverflow.net/users/21816/"
] | A finite, two-player, nondegenerate, symmetric game is defined by a nondegenerate $n \times n$ <em>payoff matrix</em> $A$. If player 1 plays strategy $i$ and player 2 plays strategy $j$, then player 1's payoff is $A_{ij}$ and player 2's payoff is $A_{ji}$. It is well known that the problem of computing a symmetric Na... | In a two-by-two symmetric game, there are two possible symmetric equilibria in pure strategies. Suppose we know that these are both in fact equilibria. All this tells us is that
$A_{11}>A_{21}$ and $A_{22}>A_{12}$.
Then there is exactly one additional equilibrium, determined by the equation $${p\over 1-p}=... | Elaborating on Steven's answer, consider the Chicken game, that is the two-by-two symmetric game where each player can either cooperate (C) or defect (D). The payoff for mutual cooperation is $R$, the one for mutual defection is $P$, and, if one player cooperates and the other defects, then the cooperator gets $S$ and ... | https://mathoverflow.net |
55,930 | [
"https://mathoverflow.net/questions/55930",
"https://mathoverflow.net",
"https://mathoverflow.net/users/11786/"
] | is it possible to explicitly parametrise all the t-structures
on the derived category of finitely generated abelian groups?
| I guess the answer is the following. Take arbitrary subset $S$ of the set of all prime numbers. Let $A_S = <Z,\{Z/pZ\}_{p \not\in S},\{Z/pZ[-1]\}_{p \in S}>$. Then $A_S$ is a heart of the t-structure which is obtained by a simple tilting from the standard one. The claim is that any bounded t-structure is a shift ... | The question and Sasha's answer can be generalized to a Noetherian ring $R$. The parametrization is in terms of functions from the integers to a specialization closed subset of $Spec R$. I do not have access to precise references at the moment, but see the paper
'Invariants of t-structures and classification of nullity ... | https://mathoverflow.net |
140,979 | [
"https://security.stackexchange.com/questions/140979",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/119175/"
] | Let's say have a linux server to which only I have shell access and all other users shells are disabled. Is there any reason to be preoccupied?
| Dirty cow does not need shell access. It only needs the ability to execute own software. This can for example also be done by using a security problem in a web application to upload and execute remote code.
| A more complete answer in my opinion:
You could execute dirtycow exploit if you have:
shell access;
RCE in web server or anything else;
a social engineered customer support;
etc
You should fully understand or at least partially understand how the exploit works to understand it's possibilities of being executed.
Rea... | https://security.stackexchange.com |
136,438 | [
"https://softwareengineering.stackexchange.com/questions/136438",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/5094/"
] | I saw an interview with Joel Spolsky, where he says that Fog Creek Software intentionally keep their teams small (I believe as in four-five guys). The reason for this is to avoid a lot of the communication needed between the team members if the teams are larger. If one compares this to open source projects, where there... | If you speak with the guys working on large projects such as the linux kernel (epic amount of work done by many devs) you will find that they are working in loosely knit small 'teams'. A known set of contributors works on a particular feature, usually under 5.
Out of that team one is the feature owner, and is part of ... | A few general thoughts on practices I've seen work (regardless of team size & type of software):
<ul>
<li>continuous integration attitude </li>
<li>clean interfaces across tiers (e.g., SOA)</li>
<li>strong leadership</li>
<li>test-driven culture</li>
<li>automation</li>
<li>mature adults (not (necessarily) "old" ... | https://softwareengineering.stackexchange.com |
364,792 | [
"https://electronics.stackexchange.com/questions/364792",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/162514/"
] | I am currently working on my first li-poly power supply, and I am testing Texas instruments BQ21040 PMIC.
The device has three charging phases that I need to monitor to do some troubleshooting.
I was using my Fluke branded digital multimeter to monitor charge current between the battery and output pin of the PMIC, bu... | In your situation you <em>may</em> find it worthwhile to get a “bench” DMM. It will be line-powered instead of battery powered, so does not need to auto shutoff, and some models have other features which may be useful to your work such as built-in logging and graphing.
| <h2>Disable Auto Power Off (Fluke 115 & 117)</h2>
All the ways...
<ol>
<li>The Meter automatically enters "Sleep mode" and blanks the display if there is no function change, range change, or button press for 20 minutes. </li>
<li>Pressing any button or turning the rotary switch awakens the Meter. </li>
<li><stron... | https://electronics.stackexchange.com |
171,971 | [
"https://stats.stackexchange.com/questions/171971",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/89202/"
] | How can I calculate Standard Deviation, step-by-step, in R? Thanks!
| <pre><code>> a <- c(179,160,136,227)
> sd(a)
[1] 38.57892
> sqrt(sum((a-mean(a))^2/(length(a)-1)))
[1] 38.57892
</code></pre>
```
| So, you want to calculate the standard deviation step-by-step. So, firstly, you should calculate the sum of the differences of all data points with the mean.
Have a variable called <code>count</code> and set it to the value 0.
For that, you loop through the data set with a variable, say <code>i</code> and subtract <c... | https://stats.stackexchange.com |
3,336,008 | [
"https://math.stackexchange.com/questions/3336008",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/698871/"
] | Can anyone help me please. I want to know how to convert a decimal number that has fraction to binary number.
For example 1,7 or 24.6 etc...
I know how to convert a whole number to binary but I'm struggling with this.
Thanks in advanced for any help.
| Let's take <span class="math-container">$24.6$</span> as an example. Presumably, you know how to convert <span class="math-container">$24_{10}$</span> into <span class="math-container">$11000_2$</span>, so I won't bother to explain that in any detail.
That means we're left with <span class="math-container">$0.6_{10}$<... | Having converted the whole part, you can think of the process as finding which of <span class="math-container">$\frac 12, \frac 14, \frac 18, \frac 1{16},\ldots$</span> you need to add together to get the fractional part. If the denominator is not a power of <span class="math-container">$2$</span> the expansion will r... | https://math.stackexchange.com |
69,990 | [
"https://physics.stackexchange.com/questions/69990",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/26618/"
] | My question is simple. Given a group $G$ broken to a subgroup $H$, gauging a possibly different subgroup Hg breaks explicitly the global symmetry $G$, generating what is known as pseudo-Goldstone bosons. Why is this?
The usual answer I get is that the gauging determines a specific direction in field space, but I reall... | How can gauging only a subgroup not break the symmetry? Isn't any group element that doesn't preserve my gauge subrgoup going to change the action.
Let's I have an $SU(2)$ symmetry, and some scalar field $\rho$ in the fundamental. If I gauge the $U(1)$ subgroup corresponding to rotations by $\sigma_z$. I have a Lagra... | Main Reference Zee (Quantum Mechanics in a Nutshell).
1) Global symmetry
A global symmetry means that the Lagrangian is invariant by a transformation whose parameters are constant.
For a continuous global symmetry, if the symmetry of the Lagrangian is the group $G$, and if the symmetry of the vacuum is the group $H$... | https://physics.stackexchange.com |
81,710 | [
"https://chemistry.stackexchange.com/questions/81710",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/46596/"
] | Currently I am attempting two questions on concentration but I am not sure if I am correct. Can someone help me with these questions? These are the questions:
<blockquote>
<strong>1</strong>) Calculate the volume of pure water added to dilute $\pu{0.2 M}$ $\ce{HCl}$ solution in $\pu{250 cm3}$ to $\pu{0.12 M}$ $\ce{H... | To clarify, I've reworded your questions. Please comment if I changed the meaning:
<ol>
<li>Calculate the volume of pure water that needs to be added to dilute a 0.2M HCl solution with a volume of 250cm<sup>3</sup> to a 0.12M HCl solution</li>
</ol>
The first step would be to work out how many moles of HCl is in the... | There is no way to take a 0.2M solution of anything and get a 0.12M solution. Although it is <em>trivial</em> to take a 0.20M or even a 0.200M solution and dilute it to 0.12M. Have you ever heard of significant digits?<br>
You state that a 0.12M solution contains 0.03 moles...so if I get a million tons of 0.12M solutio... | https://chemistry.stackexchange.com |
2,558,315 | [
"https://math.stackexchange.com/questions/2558315",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/408328/"
] | We have $f:\mathbb{R} \to \mathbb{R}$ and we know that $f$ is both additive: $f(x+y)=f(x)+f(y)$ and multiplicative: $f(xy)=f(x)f(y)$ and I found out that this means that $f(x)=0$ for any $x$ or $f(x)=x$ for any $x$ but I don't know how to prove it. Can you help me? I know that if $f$ is continuous or monotone we can sh... | There is a standard trick to this. The sketch of the argument is
<ul>
<li>$f$ maps squares to squares</li>
<li>$f$ maps nonnegative numbers to nonnegative numbers (since they are precisely the squares)</li>
<li>$f$ is monotone increasing, since $x \leq y$ iff $y-x$ is nonnegative</li>
</ul>
| The zero function is obviously a match so assume that f is nonzero.
Since f(1) = f(1)^2 we find f(1) = 0 or f(1) = 1. If f(1) = 0 then it follows by the multiplicative property that f = 0. This leads to contradiction hence we may assume that f(1) = 1, by additivity f(n) = n for all n in Z.
Observe: 1 = f(1) = f(r/r) ... | https://math.stackexchange.com |
603,021 | [
"https://math.stackexchange.com/questions/603021",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/107679/"
] | The survival times $X$ and $Y$ for two components of a system are independent exponentially-distributed random variables. X has density $f_X(x) = \frac{2}{3}e^{-2x/3}$ and Y has density $f_Y(y)=\frac{1}{2}e^{-y/2}.$ The system works as long as either component continues to function. Find the probability density functio... | <em>Hint:</em>
Your general approach is correct, just note that:
$Z=max\{X,Y\}$
$P(Z \le z)=P(X \le z, Y \le z)$
$F_Z(z)=F_X(z)F_Y(z) \\
f_Z(z)=\frac{d}{dz}F(z)=\frac{d}{dz}\left(F_X(z)F_Y(z)\right)=f_X(z)F_Y(z)+f_Y(z)F_X(z)$
May be computing $F_Z(z)$ directly and then differentiating be less liable to mistakes.
| If the components are independent then you multiply their CDFs and then differentiate
| https://math.stackexchange.com |
59,823 | [
"https://electronics.stackexchange.com/questions/59823",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/19740/"
] | What is the smallest cheap AC (110v) to DC convertor available for Arduino. I want this to be embeddable in a box along with Arduino without taking more space.
Thanks in advance
| Any of a number of semiconductor materials can be and are used, indeed the first transistor was actually a Germanium (Ge) transistor. the real reason why Si is so <strong>dominant</strong> comes down to 4 principal reasons ( but #1 is the primary reason):
1) It forms an oxide that is of very high high quality, seals ... | To sketch why a semiconductor is good for creating circuits, start with your understanding that it is in between a conductor and an insulator, and add the fact that impurities (dopants) and other processing (oxide layers) can modify its behaviour to make parts of it conduct better, and other parts conduct worse. Add in... | https://electronics.stackexchange.com |
239,653 | [
"https://electronics.stackexchange.com/questions/239653",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/102080/"
] | I am working on a system similar to a 3D printer, which works with stepper motors, heated bed and an hot air fan. This system will work inside a chamber, with a temperature around 4 degrees and around 50% of relative humidity - These variables are concerning to me. I am wondering what measures should I take to avoid th... | First of all, an I-V curve does not make any sense for a capacitor. This is because a capacitor follows the following equation:
$$i = C \frac{dV}{dt}$$
Note that the current depends on the rate of change of voltage. So you can have the same current at two different voltages, if the rate of change is the same.
The re... | Your assumption is wrong:
<blockquote>
It has non linear I-V characteristics
</blockquote>
An ideal capacitor, just like an ideal resistor, has linear I/V characteristics.
Since you're obviously learning linear circuit analysis (judging by your knowledge of the superposition principle), I'm absolutely certain you... | https://electronics.stackexchange.com |
270,434 | [
"https://softwareengineering.stackexchange.com/questions/270434",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/102575/"
] | I'm working on a project, I have a question regarding the architecture:
<ul>
<li>Say I have a many python scripts on my server and there's main.py
which contains all the classes. And there's a script called
<code>copymain.py</code></li>
<li>A user named alex signs up and the url of his site is stored in
mariadb</li>
<... | It's not uncommon to do what you described. In fact, when you create a new database for your user, you basically creating a new file for that user. So, it just adding a file to the set of per user files.
The choice of when to do the work begs explanation though. Doing the sign up process in batch periodically can crea... | Generally, this is not a good set up, because it's not DRY ("Don't Repeat Yourself"). If it were me, I'd work very hard to figure out how to have one well tested script which can generate the feeds needed by the individual users. Perhaps the script would get per-user information from the database.
| https://softwareengineering.stackexchange.com |
696,632 | [
"https://physics.stackexchange.com/questions/696632",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/299453/"
] | My textbook mentions the following lines about the validity of dimensional analysis.
<blockquote>
..... if an equation fails this consistency test, it is proved wrong but if it passes it is not proved right. Thus a dimensionally correct equation need not be actually an exact (correct) equation, but a dimensionally wron... | A simple example: <span class="math-container">$x=at^2$</span> is dimensionally homogeneous, but the true equation is <span class="math-container">$x=\frac{1}{2}at^{2}\;\;$</span> (in the case <span class="math-container">$\;v_{0}=0,x_{0}=0$</span>).
| An especially awkward situation occurs when something is dimensionless. In special relativity, <span class="math-container">$\gamma$</span> is a function of <span class="math-container">$\beta:=v/c$</span>, but dimensional analysis doesn't constrain <span class="math-container">$\gamma$</span> <em>at all</em>, except t... | https://physics.stackexchange.com |
13,922 | [
"https://physics.stackexchange.com/questions/13922",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/3241/"
] | I saw on Discovery channel that the source of Earth's magnetic field is the molten iron and metals in the earth's core. The spinning of these liquid metals produces the same effect as electric current in a coil which produces a magnetic field. The scientist in the program proved the concept by conducting an experiment ... | Circulating neutral particles will not by themselves create a magnetic field. However, if the neutral particles are moving through an existing magnetic field, and the neutral medium is conducting, then the magnetic field will induce a current via the Lorentz force. That induced current will in turn create it's own ma... | The earth’s magnetic field is created by the enormous amount of water on this planet. Water being slightly more di polar than unipolar is responsible for the important Van Allen belt that allows complex life to exist on this planet bombarded by deadly proton streams from our otherwise, life giving sun.
The source is NO... | https://physics.stackexchange.com |
429,087 | [
"https://physics.stackexchange.com/questions/429087",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/203389/"
] | Consider you are travelling on a board in the space moving with 5m/s velocity. There is no force acting on it. Now consider you have 2 stones of 1kg in your hand. So the stones also had 5m/s velocity.
Now you throw both stones in opposite directions with same force providing them with same kinetic energy (i.e one in f... | This question is based on a false premise. Specifically “But for the stone you threw on back side had initial 5m/s velocity (i.e 12.5 joules of kinetic energy) but you also did work to reverse its direction that is 100 joules of energy.”
In fact, there was 0 work done on the stone thrown backwards. Remember,
$$W=\in... | This is not a bad question, it is actually a good question: but it is unfortunately tied to its particulars in a way that makes things difficult. You are right to sense a strange contradiction that is somehow "under the water."
Let's take a second to reveal it. The work-energy theorem says that the change in the kinet... | https://physics.stackexchange.com |
463,947 | [
"https://physics.stackexchange.com/questions/463947",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/223523/"
] | I have read from various sources that, the photon strikes the solar cell and creates and electron hole pair which then migrates to their respective ends of the cells, this creates a potential difference from which we harness solar energy.
Is there any limit to the maximum number of electron hole pairs that can be creat... | No, there is no limit, because electrons which leave the solar cell are replaced: an oversimplified way of thinking about it is that electrons migrate to one terminal and leave the device, while holes migrate to the other where they are filled by electrons pouring in from the terminal.
Solar cells do deteriorate over ... | The photons raise the energy of the electrons which then pass their energy to a circuit, for example your appliance or the network. The electrons just circulate.
| https://physics.stackexchange.com |
7,212 | [
"https://earthscience.stackexchange.com/questions/7212",
"https://earthscience.stackexchange.com",
"https://earthscience.stackexchange.com/users/5132/"
] | I am in O'Fallon, Missouri and today it is -15 °C (5 °F) outside. I was taught water freezes at 0 °C (32 °F). I could understand if it was exactly 0 °C (32 °F) that the water might not be turning to ice, but how can it be 15 °C (27 °F) below water's freezing point and still be liquid water? The water is still moving du... | Water melts at 0 °C (32 °F) but freezing is a more complicated affair. It is safe to say water gains the ability to freeze at 0 °C, but it can get much cooler before it actually does so resulting in supercooled water. Water in this state can rapidly solidify when suitable ice nuclei are introduced. For example, in ... | Water is a rather strange substance. With most substances, the solid phase is denser than is the liquid phase. This is not the case with water. Ice is less dense than liquid water. A side effect of this effect is that liquid water very close the the freezing point is less dense than is slightly warmer water. That very ... | https://earthscience.stackexchange.com |
246,319 | [
"https://security.stackexchange.com/questions/246319",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/236749/"
] | When creating a self-signed certificate you are asked to enter some information (First Name, Last Name, Organization Unit, Organization, City, State,...). Is it possible to update any of those fields later? (E.g. my company changed its legal name and now I want to update the "Organization" name to reflect the... | No, if you changed those informations on the certificate, the fingerprint changed, and the signature is invalid.
You will need to issue a new certificate.
| I like to compare certificates to driver's licenses. Can you take a sharpie and change the First Name, Last Name, Address on your driver's license? No, you need to contact the government and get a new license printed.
Slightly less sassily, a certificate is basically a signed statement from a trusted 3rd party (the CA)... | https://security.stackexchange.com |
64,627 | [
"https://physics.stackexchange.com/questions/64627",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/8082/"
] | Surely, upon an increase in temperature, the atoms within the thermistor would vibrate with more energy and therefore more vigorously, hence making the electrons flowing through the electric circuit more likely to collide with one of the atoms, so increasing resistance.
However, the effect of temperature on a thermist... | Thermistor with this particular temperature behavior are commonly <strong>semiconductors</strong>. In a semi-conductor, there is an energy gap between the (filled) valence and the (empty) conduction band. At zero temperature, no charges are in the conduction band and the resistance should be infinite as the system beha... | Using your playground example....
Imagine if you had to pass a message (electricity) across the playground, when cold you would have to stretch between each fixed person to pass this message. When hot, more people fill the gaps, the message is easier to pass.
Hope this helps :)
| https://physics.stackexchange.com |
139,463 | [
"https://electronics.stackexchange.com/questions/139463",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/58739/"
] | I understand that something that is charged with one coulomb and has a potential difference of one volt across it will have a capacitance of one farad. \$C = \frac{q}{V}\$. I get that because current is the derivative of charge with respect to time, \$I = C\frac{dV}{dt}\$.
This is where I start to get confused. If you... | q is not equal to
\$
\int_{t0}^{t} I(\tau) d\tau + V(t_0)
\$
it has no sense to sum apples (charge) and pears (potential) :)
if you have
\$
V(t)=\frac{1}{C}\int_{t0}^{t} I(\tau) d\tau + V(t_0)
\$
\$ \frac{1}{C} \$ is just multiplying \$\int_{t0}^{t} I(\tau) d\tau \$ and not \$ V(t_0)\$
\$
\int_{t0}^{t} I(\tau) d... | This is really just a math question.
When you take the integral \$ \frac{1}{C}\int_{t_0}^t I(\tau)d\tau \$, you get \$ \frac{Q(t)}{C} - \frac{Q(t_0)}{C} \$ = \$ V(t) - V(t_0)\$. This is bad, because we just want \$ V(t) \$; the simple way of fixing this is to add \$V(t_0)\$ to cancel out the term in the integral.
We ... | https://electronics.stackexchange.com |
70,801 | [
"https://stats.stackexchange.com/questions/70801",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/5931/"
] | I am lost in normalizing, could anyone guide me please.
I have a minimum and maximum values, say -23.89 and 7.54990767, respectively.
If I get a value of 5.6878 how can I scale this value on a scale of 0 to 1.
| If you want to normalize your data, you can do so as you suggest and simply calculate the following:
<span class="math-container">$$z_i=\frac{x_i-\min(x)}{\max(x)-\min(x)}$$</span>
where <span class="math-container">$x=(x_1,...,x_n)$</span> and <span class="math-container">$z_i$</span> is now your <span class="math-c... | The general one-line formula to linearly rescale data values having observed <strong>min</strong> and <strong>max</strong> into a new arbitrary range <strong>min'</strong> to <strong>max'</strong> is
<pre><code> newvalue= (max'-min')/(max-min)*(value-max)+max'
or
newvalue= (max'-min')/(max-min)*(value-min)+min'.
... | https://stats.stackexchange.com |
626,396 | [
"https://physics.stackexchange.com/questions/626396",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/215354/"
] | Suppose we have double-slit experiment. Before the right slit, we add a particle detector that allows the particle to go through. It is well-known that if we carry this out, then the wave interference effect goes away, and the distribution of the particles on the opposite wall looks like the distribution of bullets.
... | Superposition does not mean the particle is in both states at the same time. The state <em>is</em> the superposition. So the fact that you have particles that start in the same state but then get different results upon measurement indicates the state was a superposition of spin states.
Measurements give you single valu... | Say you have an electron which can be spin up or spin down, i.e.
<span class="math-container">$$
| + \rangle, | - \rangle
$$</span>
Say you have some sort of measurement device with three states:
<span class="math-container">$$
| \text{prepared}\rangle, |\text{measure}+\rangle, |\text{measure}-\rangle.
$$</span>
Time e... | https://physics.stackexchange.com |
106 | [
"https://cardano.stackexchange.com/questions/106",
"https://cardano.stackexchange.com",
"https://cardano.stackexchange.com/users/321/"
] | I'm a developer looking into starting my first dApp project. I've been waiting for Cardano to release smart contracts due to Ethereum's insane gas fees and low transaction throughput.
I recently discovered another blockchain (Solana) that already supports smart contracts and has even cheaper transaction fees and very h... | I recently discovered Solana too and found that it has <em><strong>awesome</strong></em> speed. Here are my thoughts from a little review:
<ul>
<li><strong>Decentralized limit</strong>: Hardware recommended requirements for Solana nodes are pretty advanced which limits the amount of nodes that can be run.</li>
<li><str... | <ul>
<li>Research papers and correctness proofs</li>
<li>Transparency</li>
</ul>
The first will be hard for other chains to reproduce, though some of the results are not specific to Cardano / Ouroboros so they could be adopted by other chains.
The second is an on-going process and it would be possible for another chain... | https://cardano.stackexchange.com |
658,866 | [
"https://physics.stackexchange.com/questions/658866",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/220040/"
] | According to physics, our current understanding has space and time as continuous entities. Photographs generally work by letting light modify a surface by striking it and it forms the picture. The question is, do photographs capture very small intervals of time, or do they capture instantaneous moments? If they do capt... | They certainly do not capture instants of time. A camera needs to capture light for a certain period of time in order to create an image. On most cameras, the duration of the period can be adjusted to suit the lighting and the type of image to be captured, by setting what is known as the 'shutter speed'. The shutter sp... | <ol>
<li>The light doesn't travel instantaneously.</li>
<li>In case of Electrochemical photography, the chemical reaction does not occur instantaneously. And in case of digital photographs, the conversion of photosignals to electrical current does not take place instantaneously. It also takes finite time for processing... | https://physics.stackexchange.com |
29,397 | [
"https://electronics.stackexchange.com/questions/29397",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/8324/"
] | I am new with electronics and I have some issues removing DIP ICs from breadboard after testing something with them. I end up bending their legs if I pull them with bare fingers or pliers.
Is there some recommended way to pull them up nicely?
| On a <em>breadboard</em>, take practically any old pointy thing that will fit in 'the gutter', and nudge it carefully under one end of the chip (that's what the gutter is for!) until it just starts to come up. Then move to the other end, and bring that end up a little more. At that point it should be loose enough to ju... | I usually try to leave some room at one end, so I can insert a small screwdriver underneath the IC. Then just lift. Will bend the pins ever so slightly that it doesn't cause problems upon reuse.
Alternatively, Wiha seems to have chiplifters, a bit like a small crowbar:
<img src="https://i.stack.imgur.com/zIZaT.jp... | https://electronics.stackexchange.com |
85,047 | [
"https://mechanics.stackexchange.com/questions/85047",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/53083/"
] | What is the consequence of using a fuel filter with a higher-spec'd bypass valve opening pressure ?
Currently have a UFI 31.833.00 / 6Q0201051C on my 2008 City Golf - rated at 4.0 bar.<br />
Considering a HENGST H155WK02 as replacement, but it is rated at 6.4 bar.
Would this higher pressure rating cause fuel delivery ... | Seeing as my other answer was not correct in this case, I would propose that the reason there is a bypass valve is to ease the load on the pick up fuel pump while there is low demand for fuel by the high pressure pump that is further down the line.
The majority of the time, the pick-up fuel pump will be able to provide... | Under normal operation the bypass valve should be closed at all times, your engine will be getting protected from dirty fuel by the filter.
Without a bypass valve, if the filter becomes clogged with dirt, the fuel pressure to the engine will drop and the engine will become starved of fuel and possibly run lean. This c... | https://mechanics.stackexchange.com |
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