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 |
|---|---|---|---|---|---|
1,919 | [
"https://robotics.stackexchange.com/questions/1919",
"https://robotics.stackexchange.com",
"https://robotics.stackexchange.com/users/2033/"
] | I need to actuate 3 or 4 Cnc-like Nema 23 (~1N.m torque) stepper motors, I would like some cable solution to connect easily the motor to the motor driver.
I have not yet bought anything, I have searched various robotic stores and ebay, but did not yet found a triple (motor, cables, driver) which would be "plug and pl... | Don't worry about the cables; plan to make them yourself. I recommend using one of the wide variety of crimp-on connectors available to you, depending on what you can find for your ESC and steppers connections.
Here are a handful of connectors representing part of what we stock in our lab:
<img src="https://i.stack.i... | Stepper motors usually have 4 to 6 <em>wires</em>, often bundled up into one <em>cable</em>.
There might be some pro industrial solution out there that's 100% plug and play, but I would consider myself lucky if I found connectorized motors and drivers for which I could make up cables.
Making up cables is not rocket s... | https://robotics.stackexchange.com |
76,121 | [
"https://dsp.stackexchange.com/questions/76121",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/35309/"
] | I have <span class="math-container">$N$</span> observations <span class="math-container">$\{y_i\}_{i=1..N}$</span>
<span class="math-container">$$y_i = h_i x_i + w_i$$</span>
where <span class="math-container">$\{x_i\}_i$</span> are the realizations of uniform distributed QPSK symbol <span class="math-container">$x$</s... | Consider the distance between the two interleaved codewords formed from codewords <span class="math-container">$\mathbf c_1, \mathbf c_2, \ldots, \mathbf c_t \in C$</span> and <span class="math-container">$\,\mathbf 0, \mathbf c_2, \ldots, \mathbf c_t \in C$</span> where <span class="math-container">$\mathbf c_1$</span... | You may have misunderstood "minimum distance". It's the smallest Hamming distance of all the <em>pairs</em> of codewords in the code. The Hamming distance is the same for any pair of codewords regardless of how you order the bits/symbols so the minimum distance of the code is unchanged.
| https://dsp.stackexchange.com |
1,360,834 | [
"https://math.stackexchange.com/questions/1360834",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/228196/"
] | I have a function $f(x)$ and I want to prove that $x^*>y$ where $x^*$ is the number that satisfies $f'(x^*) =0$ and $y$ is just an arbitrary constant. So what I did is that I assume $x^*>y$ and show second derivative $f"(x^*)|_{x^*>y}<0$, then I show that $f'(y)>0$, representing that $x^*>y$. Thus the... | <blockquote>
I have a function f(x) and I want to prove that x∗>y where x∗ is the number that satisfies f′(x∗)=0 and y is just an arbitrary constant. So what I did is that I assume x∗>y and show second derivative f"(x∗)|x∗>y<0, then I show that f′(y)>0, representing that x∗>y. Thus the first assumption is true and... | First of all, never assume what you are trying to prove. It is possible that it makes sense in your mind to assume something and then test to show it is true, but this is not how formal math is done so most instructors wouldn't accept it.
Second, I am a bit confused about what you are trying to show. This is my guess ... | https://math.stackexchange.com |
313,361 | [
"https://physics.stackexchange.com/questions/313361",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/128927/"
] | If i make a simple pendulum using a ball filled with water and then puncture the ball with needle making a small hole. The pendulum is then made to oscillate . The water will flow through the hole . My question is that what will be the effect on time period of the pendulum? What if i replace the water with some other ... | The time period of a simple pendulum does not depend upon the mass of the bob, so it doesn't matter whether the material is solid, or a shell filled with a liquid. The time period for small oscillations is given by $T=2\pi(\sqrt{\frac{L}{g}})$ which is independent of the mass.
However, when we start draining the liqui... | The time period of the oscillation does not depend on the mass of the blob,
As,
$T\approx{\frac{1}{2\pi}}\sqrt{\frac{L}{g}}$
| https://physics.stackexchange.com |
947,776 | [
"https://math.stackexchange.com/questions/947776",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/177196/"
] | Consider the two linear spaces:
$l^{2} = \left\{x = (x_1, x_2, . . . ) : \sum_{k=1}^{\infty} |x_k|^{2} < \infty\right\}$ with norm $||x||_{2} = (\sum_{k=1}^{\infty} |x_k|^{2})^{\frac{1}{2}}$, and $l^{\infty} = \left\{x = (x_1, x_2, . . . ) : \sup_{k} |x_{k}| < \infty\right\}$ with norm $||x||_{\infty} = \sup_{k} ... | b) I might have misunderstood the problem but I don't think that $T$ necessarily even maps $\ell_2$ into $\ell_2$.
Just take $a=x=(1,1/2,1/3,\dots)$, i.e., $a_k=x_k=\frac1k$.
Clearly $a,x\in\ell_2$.
We have $(Tx)_n=\sum_{k=1}^n \frac1{k^2} = 1+\frac1{2^2}+\dots+\frac1{n^2}$. We have $\lim\limits_{n\to\infty} (Tx)_n... | Hint for the first part:
a) First use Hölder's inequality to show that $T_a(x) \in l_\infty$. Then it's sufficient to show that $T$ is bounded for it to be continous.
$$\|T_a\| = \sup\left\{\|T_a(x)\|_\infty:\|x\|=1\right\}=\sup\left\{\left\lvert \sum_{j=1}^Na_jx_j \right\rvert\,\,\text{or}\,\, \left\lvert\sum_{j=1}^... | https://math.stackexchange.com |
124,942 | [
"https://mathoverflow.net/questions/124942",
"https://mathoverflow.net",
"https://mathoverflow.net/users/32324/"
] | We have an $L^2$ function $u$ defined on $\mathbb{R^2}$ with compact support such that $u \in H^{2/3}$ (H stands for Sobolev spaces, as always), $\partial_y u \in L^2$, and $(x\partial_y - y\partial_x)u \in L^2$. I want to conclude that $u$ lies in some higher order $L^p$ space, that is, for some $p > 2$. Ideally, I... | Let $\phi:C\to[0,\infty]$ be defined for $F\in C$ as the norm of $F$ in $W^{1,1}$ if $F$ is absolutely continuous, and $+\infty$ otherwise. Then $\phi$ is lower semi-continuous for the topology of uniform convergence and $W^{1,1}=\{\phi<\infty\}$ is Borel measurable.
| I think Bill Johnson's answer ``just use the definition" is correct, but that he did not write what he wanted to write. A function $f\in\mathcal C([0,1],\mathbb R^d)$ is absolutely continuous if and only if the following holds:
$$\forall p\in\mathbb N\; \exists q\in\mathbb N\; \forall x_1,\dots ,x_N,y_1,\dots ,y_N\in\m... | https://mathoverflow.net |
101,227 | [
"https://physics.stackexchange.com/questions/101227",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/34081/"
] | <img src="https://i.stack.imgur.com/B1KNO.png" alt="enter image description here">
The book's logic is that there is no induced EMF because flux is constant as it passes through the magnetic field. Which makes sense, but this seems counter-intuitive to what I previously learned.
If i'm not mistaken, if an electron i... | The force on each electron is $qv\times B$, which is toward the left. This clearly doesn't induce them to flow in a circle. (Remember that if electrons flow to the left, current flows to the right.)
The entire ring is not deflected to the right because there is an equal force on the nuclei toward the right.
| There is an EMF induced in this ring!!
When an electron or proton move in a magnetic field, it doesn't matter if the field is constant or not (relative to all of the ring or cable or coil...etc), THERE WILL ALWAYS BE an EMF = Electro Motive Force acting upon the particles!
See faraday wheel, or homopolar generator: ... | https://physics.stackexchange.com |
74,245 | [
"https://electronics.stackexchange.com/questions/74245",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/25762/"
] | Consider an asynchronous FIFO interface (e.g. FT245, FT2232H or comparable). Since there is no clock, how do you decide who deserved to be called master? Or do you call it controller instead?
| I'm not familiar with the specifics of the devices you referenced, but in a general async FIFO there is a "pushing" side and a "popping" side.
The side which pushes into the FIFO is usually called master, and the side that pops from it is a target.
I don't understand why do you have difficulties in identifying the ma... | In a typical asynchronous FIFO design, both sides of the FIFO can act as slaves to the external device connecting them. It may be necessary for either side to wait for the other, but such waiting need not be "precise". If the device that wants to fetch data polls the FIFO and finds that none is available, it need not... | https://electronics.stackexchange.com |
479,628 | [
"https://stats.stackexchange.com/questions/479628",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/270432/"
] | I've got some problem:
Check the relationship between income tax income and migration in Polish "powiats" (an administrative unit in Poland, something like a county in the US). I have data in this regard for individual years.
I think a good model would be:
Tax_income ~ balance_of_migration + year + (1|powiat)... | This is an interesting question. Indeed, understanding random effects <span class="math-container">$u_i$</span> as draws from a larger population is well in line with the underlying model, which treats them as a random variable, e.g.:
<span class="math-container">$u_i \sim_{iid} Normal(0, \sigma^2)$</span>
But there ar... | Yes I think your intuition is correct and you can fit random intercepts. There are several criteria for assessing if a factor should be treated as random and being a sample from a population is only one. Often different criteria point in different directions and it is a matter of judgement. In this case if you really w... | https://stats.stackexchange.com |
2,288 | [
"https://mechanics.stackexchange.com/questions/2288",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/230/"
] | Tonight while driving my 2003 Kia Sedona with the defroster on, suddenly I heard this very loud rumbling which sounded like it was coming from the engine compartment on the passenger side. My first thought was my exhaust had broken, but as soon as I turned off the heat blower to listen, the sound went away.
I also sm... | Sounds like a failing A/C compressor. Assuming the car has A/C... A/C compressor is used with the defroster to dry the air.
| That sounds like the bearings in the fan have failed, or something is caught in the fan, stopping it from turning.
I've got no idea where the heater fan would be on a Sedona, but chances are it'll be behind the dash somewhere - If you're lucky it might be on the engine bay side of the bulkhead, but if it is inside it... | https://mechanics.stackexchange.com |
34,265 | [
"https://softwareengineering.stackexchange.com/questions/34265",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/12547/"
] | During our interview process we give the candidate a programming test as well as a programming design problem.
Our HR department has told us to stop administering the programming test due to legal concerns. This would really hinder our ability to screen candidates.
Are there other HR policies that prevent you from h... | Two things I've seen at my current job:
<b>Inappropriate job titles</b>
We wanted to hire a senior level system admin to do manage our development/production environment. The job also required implementing new security measures for HIPAA. Anyway, the job title was 'Director of Hosted Systems and Security' because h... | There's screening. In many places, HR screens all resumes and applications, and may screen out good people. There's a possibly apocryphal story about Mitch Kapor, who founded Lotus (unless I'm getting my companies wrong): he took the resumes of the first fifty Lotus employees, changed the specifics so they wouldn't ... | https://softwareengineering.stackexchange.com |
675,627 | [
"https://math.stackexchange.com/questions/675627",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/128629/"
] | Let's say I have a puzzle, which says it has <code>1000</code> pieces. I also know it's a <code>4:3</code> ratio picture that I'm trying to put together. <strong>How do I calculate the width & height in number of pieces?</strong>
I know that if I knew the puzzle was <code>1000</code> pieces wide, it would have a <... | Here is a description of why Bran van der Meer's answer works:
Consider
$$
wh=1000
$$
and
$$
\frac wh=\frac43
$$
Multiply these to get
$$
w^2=1000\times\frac43
$$
Divide them to get
$$
h^2=1000\times\frac34
$$
| Hint: If the number of pieces in the short direction is $x$, the number in the long direction is $\frac 43x$. The total number of pieces is then ??? Now solve for $x$
| https://math.stackexchange.com |
5,419 | [
"https://mechanics.stackexchange.com/questions/5419",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/2781/"
] | I have an Australia-built Ford Falcon which has a 4.0L multi-point fuel injected straight 6 and is manual. I rebuilt the head in the car myself last year (blown head gasket) and it has been running fine since.
It’s always had a funny slight flat spot under acceleration. Around 2,500 RPM it struggles to move past until... | The problem was finally fixed.
After reading the manual I suspected a piece called the TFI Module which is attached to the distributor ($30 piece). All of my symptoms were pointing to this part except the manual (and forums) said usually you need to let the car cool down for up to 5 minutes before it will restart.
Ne... | There is a laundry list of things that can cause these problems.
<ol>
<li>Grounding issues.</li>
<li>Alternator</li>
<li>Battery Connections</li>
<li>Computer</li>
<li>Faulty wiring</li>
</ol>
To be honest it can be a nightmare trying to sort these things out, and you are better off taking it to a automotive electri... | https://mechanics.stackexchange.com |
330,469 | [
"https://math.stackexchange.com/questions/330469",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/65182/"
] | I have a function, $f(x) = \sec x + \csc x$ on the interval $x \in (0, \pi/2)$.
The derivative of it is, $f'(x) = \csc^2(x) \sec^2(x) \left(\sin^3 x + \cos^3 x\right)$
Of course, when I tried to solve for $f'(x) = 0$, I realize that $f'(x)=0$ does not exist.
However, the question insists that the there is a minimum... | We should have $$\begin{align}f'(x) &= \frac{\sin x}{\cos^2 x}-\frac{\cos x}{\sin^2x}\\ &= \frac1{\cos^2x\sin^2x}\bigl(\sin^3x-\cos^3x\bigr)\\ &= \frac1{\cos^2x\sin^2x}(\sin x-\cos x)\bigl(\sin^2x+\sin x\cos x+\cos^2x\bigr)\\ &= \frac1{\cos^2x\sin^2x}(\sin x-\cos x)\left(1+\frac12\sin 2x\right).\end{ali... | The derivative is $0$ at $x=\frac{\pi}{4}$ as the derivative goes from $-$ to $+$ you really have a minimum.
The correct derivative is
$$-\cot(x) \csc(x) + \sec(x) \tan(x)$$
<img src="https://i.stack.imgur.com/Cs6PS.png" alt="enter image description here">
| https://math.stackexchange.com |
3,744,453 | [
"https://math.stackexchange.com/questions/3744453",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/621937/"
] | If <span class="math-container">$P$</span> is a doubly stochastic matrix i.e. <span class="math-container">$P=(p_{ij})_{1\leq i,j \leq n}$</span> is s.t. the row sums <span class="math-container">$\sum_j p_{ij}=1$</span> for all <span class="math-container">$i$</span> and <span class="math-container">$\sum_i p_{ij}=1$<... | Take <span class="math-container">$f(x)=\sin^{2}x$</span> for a counter-example. Here <span class="math-container">$L=\pi$</span> and the integral is strictly positive.
| You cannot draw any conclusion about <span class="math-container">$I$</span> if <span class="math-container">$f$</span> is an even function.
But if <span class="math-container">$g(x)$</span> is a periodic odd integrable function and <span class="math-container">$L$</span> is the minimum period, then
<span class="math-c... | https://math.stackexchange.com |
1,727,853 | [
"https://math.stackexchange.com/questions/1727853",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/245761/"
] | How to get from: $$\cos^3(x)-3\cos(x)\sin^2(x)$$
To: $$4\cos^3(x)-3\cos(x)$$
?
| $$3\sin^2(x)=3-3\cos^2(x)$$ so
$$\cos^3(x)-3\cos(x)\sin^2(x)=\cos(x)\big (\cos^2(x)-3+3\cos^2(x)\big )=4\cos^3(x)-3\cos(x)$$
| We can simplify it further to $\cos (3x)$.
Indeed,
$$(\cos x + i \sin x)^3 = (e^{ix})^3 = e^{3 i x} = \cos 3x + i \sin 3x$$
and the result follows by comparing the real component of the left and right sides.
| https://math.stackexchange.com |
20,542 | [
"https://stats.stackexchange.com/questions/20542",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/4705/"
] | I am using the lm() and princomp() functions in R to perform regressions on foreign exchange time series. I would like to weight the regressions (and PCA) such that 50% of the influence on the regression comes from the past 3 months, 25% from the previous 3 months, etc, but in a smooth fashion. Both functions take such... | To be general, let us consider a time series with arbitrary steps. Therefore, let $k$ be the number of steps in three months and write $\omega_1$ for the weight for the immediately preceding time, $\omega_2$ for the weight preceding it, and so on, so that the sequence of weights
$$(\omega_1, \omega_2, \ldots, \omega... | Let us assume that you want such a decaying weight to take place over intervals of T time periods, e.g., 91 days in your example (there are 91 days in three months, so you want the weight on day 91 to equal 1/2 the weight on day 0.) Solve $\exp\{-\lambda T\} = 0.5$ to get $\lambda = -\log(0.5)/T$. In your case, $\lam... | https://stats.stackexchange.com |
240,504 | [
"https://softwareengineering.stackexchange.com/questions/240504",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/126663/"
] | By accident I found that with a polymorphic type using dynamic_cast back to the derived class will throw a 'bad cast' error if the pointer is no longer valid. Is this undefined behavior or could this be a way to check for valid pointers without using smart pointers?
I'm not really against using smart pointers as they... | dynamic-cast needs to deference the pointer to find the proper offset it should be at, dereferencing an invalid pointer is undefined behavior,
one thing you might see:
<pre><code>class Derived2 : public Base
{
public:
virtual ~Derived2(){}
};
int main()
{
Storage store;
{
Derived d;
st... | This is, indeed, undefined behaviour.
<code>dynamic_cast</code> needs to look up the run-time type for your pointer, which will generally require dereferencing it, and this is already UB.
Even if an implementation somehow avoid dereferencing the pointer - say by storing a lookup table with the type of each live objec... | https://softwareengineering.stackexchange.com |
229,876 | [
"https://math.stackexchange.com/questions/229876",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | Showing that $(\mathbb{Z}/p^a \mathbb{Z})^*$ has order $p^{a-1}(p-1)$ where $p$ is prime.
This for a class on elementary number theory, so this question caught me off-guard having only minor experience with group theory. I have a feeling I'll need to use Lagrange for this one, but not sure where to start exactly. Any ... | $\mathbb Z/p^r\mathbb Z$ has $p^r$ elements.
$(\mathbb Z/p^r\mathbb Z)^\times$ is a subset, having removed all the non-invertible elements: that is exactly the multiples of $p$. Those are $p,2p,3p,\ldots,p^{r-1}p$ so there are $p^{r-1}$ of them.
So there are $p^r - p^{r-1} = p^{r-1}(p-1)$ units in $\mathbb Z/p^r\math... | This is just the totient function
| https://math.stackexchange.com |
56,840 | [
"https://dba.stackexchange.com/questions/56840",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/28020/"
] | Suppose I have tables a (with column a1) and b (with columns b1 and b2) and I perform a left outer join
<pre><code>SELECT *
FROM a LEFT OUTER JOIN b
ON a.a1 = b.b1
</code></pre>
Then b1 and b2 will be NULL where a value of a1 has no matching value of b1.
Can I provide a default value for b2, instead of NULL? Note t... | <pre><code>SELECT a.a1,b.b1,
CASE WHEN b.b1 is NULL THEN 5 ELSE b.b2 END AS b2
FROM a LEFT OUTER JOIN b
ON a.a1 = b.b1
</code></pre>
| I find COALESCE to be very useful in that case. It will return the first non NULL value from a list:
<pre><code>SELECT
a.a1,
b.b1,
COALESCE (b.b2, 100) AS b2
FROM a
LEFT OUTER JOIN b
ON (a.a1 = b.b1);
</code></pre>
After @ypercube correctly identified an issue where my answer did not match the question. here is hi... | https://dba.stackexchange.com |
152,358 | [
"https://security.stackexchange.com/questions/152358",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/-1/"
] | I'm in the process of enabling SELinux on my CentOS 7.3 server. I have it set in permissive mode and monitoring the audit log for denials. I'm looking to implement the minimal set of rules that will allow all that is needed on the server and no more.
So this morning I noticed that httpd would have been blocked from co... | As Enigma says:
<blockquote>
The first option [httpd_can_network_relay] is used in an reverse proxy
scenario in which your httpd is relaying requests to some backend
httpd in behalf of the client.
</blockquote>
As for <code>httpd_can_network_connect</code>, it allows httpd to make network connections, including... | The first option [<code>httpd_can_network_relay</code>] is used in an reverse proxy scenario in which your httpd is relaying requests to some backend httpd in behalf of the client.
The second one [<code>httpd_can_network_connect</code>] allows httpd modules and scripts to make outgoing connections to ports which are ... | https://security.stackexchange.com |
222,765 | [
"https://math.stackexchange.com/questions/222765",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/30657/"
] | In my microeconomics book, I read that when we have $1+\dfrac{1}{\eta}$ where $\eta$ is an elasticity coefficient, we can write $1-\dfrac{
1}{|\eta|}$ "to avoid ambiguities stemming from the negative sign of the elasticity".
What does this mean? Is it always legitimate to perform such a transformation?
| If the elasticity coefficient $\eta$ is negative, then $|\eta|=-\eta$. The ambiguity arises because some people may suppress the negative sign and write it as a positive number instead. In this case, using $1+\frac{1}{\eta}$ becomes ambiguous.
For example, if $\eta=-2$ but people write it as $\eta=2$, then $1+\frac{1}... | No, such a calculation is not always allowed...I'm guessing it relates to the constant itself.
| https://math.stackexchange.com |
167,650 | [
"https://softwareengineering.stackexchange.com/questions/167650",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/67012/"
] | It is easy to define the <code>Equals</code> operation in ways that are not commutative. When providing equality against other types, there are obviously situations (in most languages) were equality not being commutative is unavoidable. However, within one's own inheritance hierarchy where the root base class defines... | Non-symmetric<sup>1</sup> <code>equals</code> breaks the contract of the operation, spelled out clearly in the documentation (I'm quoting from the Java's documentation, but it remains essentially the same in other languages)
<blockquote>
The equals method implements an equivalence relation on non-null object referen... | I've never seen <code>=</code> defined as "is-a". <code>=</code> means "is equal to", not "is derived from," even in type hierarchies, and "is derived from" is generally implemented with a different operator or function. Where have you seen it implemented the way you describe? Because that's counter-intuitive and ju... | https://softwareengineering.stackexchange.com |
507,982 | [
"https://physics.stackexchange.com/questions/507982",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/244146/"
] | I'm trying understand the physics of simple inductor-capacitor circuits such that there is just an inductor L and a cacpacitor C and a switch.
Imagine first that the capacitor is fully charged and the switch is then closed.
I do not understand why the current increases from an initial low value as the charge differen... | The term "imaginary" for referring to the so-called "imaginary numbers" (which, by the way, are only a one-dimensional sliver of the full complex numbers that are actually used here) is a historical artifact that has caused way more confusion than it should, now. Let me say one thing that is crucially important here:
... | I believe that you can do all of physics <em>apart</em> from QM without using complex numbers: complex numbers are a convenience (generally because <span class="math-container">$e^{ix} = \cos x + i \sin x$</span>), but they are only a convenience.
However if you want to do QM you either end up using complex numbers or... | https://physics.stackexchange.com |
308,356 | [
"https://dba.stackexchange.com/questions/308356",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/249128/"
] | <strong>tldr:</strong> Is it possible to create a constraint that enforces: only allow update to column if its current value is NULL.
I’m using Postgresql to update a column from NULL to a foreign key when the user takes an action.
Ideally, it works like this:
<ol>
<li>User does action.</li>
<li>If column is NULL, do a... | Just tell the router that any connection on port 12345 of 12.345.678.90 should be sent to 192.168.1.100 and the port number on which the other SQL server is listening.
Anyone on the outside wanting to connect to the other SQL server should connect to 12.345.678.90:12345.
| You can't connect directly to other server, unless you change your network configuration.<br />
Still you can access the data on local server using your server visible from internet. To do this you can create a database link between these two servers (assuming that network configuration will allow to connect from publi... | https://dba.stackexchange.com |
39,915 | [
"https://engineering.stackexchange.com/questions/39915",
"https://engineering.stackexchange.com",
"https://engineering.stackexchange.com/users/30401/"
] | Why is the atmospheric pressure given as the value of length of mercury column in barometer. Since the density for the both the cases are different.
Is it that by experiment we found the value to be same or it has some scientific relation with mercury and atmosphere
| <blockquote>
Since the density for the both the cases are different.
</blockquote>
The density of mercury will remain the same. The density of the air will vary with pressure and that's exactly what you want to measure.
The air pressure pushes the mercury up the tube and, since the top of the tube is a vacuum, there is... | Most labs have a mercury barometer in them to give the atmospheric pressure.
Usually taken at the start and end of a test - especially when dealing with engines, also ambient temperature and humidity are taken as well.
The "lab standard" mercury barometers also have a temperature correction chart to correct t... | https://engineering.stackexchange.com |
18,391 | [
"https://stats.stackexchange.com/questions/18391",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/5405/"
] | Is there a method to understand if two lines are (more or less) parallel? I have two lines generated from linear regressions and I would like to understand if they are parallel. In other words, I would like to get the different of the slopes of those two lines.
Is there an R function to calculate this?
<em>EDIT:</em>... | I wonder if I am missing something obvious, but couldn't you do this statistically using ANCOVA? An important issue is that the slopes in the two regressions are estimated <em>with</em> error. They are estimates of the slopes in the populations at large. If the concern is whether the two regression lines are parallel o... | The first question is actually from geometry. If you have two lines of the form:
$$y=a_1x+b_1$$
$$y=a_2x+b_2$$
then they are parallel if $a_1=a_2$. So if the slopes are equal then then the lines are parallel.
For the second question, use the fact that $\tan \alpha=a_1$, where $\alpha$ is the angle the line makes wi... | https://stats.stackexchange.com |
517,078 | [
"https://physics.stackexchange.com/questions/517078",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/141461/"
] | According to Newton's third law, the forces in an action-reaction pair must have the same magnitude and opposite directions. But do they have to be the same <em>kind</em> of force (gravitational, electromagnetic, strong, weak)?
| We're talking about the "two sides" of the SAME force, so it must be yes, they are of the same type.
For example, if I attract you gravitationally, then you are also attracting me gravitationally. There is only one physical source of the force (in this case, gravity) and it is pulling us both equally and oppositely. ... | If you get down to the fundamental layer, what we perceive as Force on a macroscopic scale is really just exchange of field bosons. So when two particles interact via a boson exchange, it is the same force because it is mediated by the <em>same boson</em>.
The only way you could see asymmetric forces would be if the b... | https://physics.stackexchange.com |
251,022 | [
"https://dba.stackexchange.com/questions/251022",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/187245/"
] | I want to create a function that will return a table as a result. Now, I have written quite a big SQL that works, but as I need to push dates trough that SQL to the beginning, I need a funtion that will do that. Now, as an example, to see if I can get it working, I've created this so far:
<pre><code>create or replace ... | That will not work like that. <code>test_table</code> is a table of <code>test_row</code> objects, not 2 columns, so you need to create <code>test_row</code> objects from the 2 columns, like below:
<pre><code>create or replace FUNCTION test_function
(
p_year_start_in integer,
p_year_end_in integer
) RETURN t... | Can you please try to use the proper alias syntax?
from T_D_DATES as td
But I don't really see the advantage of receiving a temporary table made up of select results over JUST receiving said select results....
So in my eyes the whole test function (if you want some kind of stored procedure or similar) could be short... | https://dba.stackexchange.com |
600,347 | [
"https://physics.stackexchange.com/questions/600347",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/93729/"
] | In Sean Carroll's <em>spacetime and geometry</em> chapter 5 Carroll states the following
<blockquote>
In addition we always have another constant of the motion for geodesics: the geodesic equation (together with metric compatibility) implies that the quantity<span class="math-container">$$\epsilon=-g_{\mu\nu}\frac{dx^\... | You don't need any knowledge of tensor calculus to understand this.
<strong>What Does <span class="math-container">$g_{\mu\nu}\frac{dx^\mu}{d\lambda}\frac{dx^
\nu}{d\lambda}$</span> Represent?</strong>
For a curve <span class="math-container">$x^\mu(\lambda)$</span> in a manifold, the quantity <span class="math-conta... | This is simply a <em>choice</em> of parametrization of the geodesic <span class="math-container">$x^\mu(\lambda)$</span>. If we were in a Euclidean-signature manifold, <span class="math-container">$\lambda$</span> would be proportional to the arc length along the curve. Here, for a timelike geodesic, it would be propo... | https://physics.stackexchange.com |
256,393 | [
"https://physics.stackexchange.com/questions/256393",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/47059/"
] | I'm trying to find the covariant derivative of a covariant derivative, i.e. $\nabla_a (\nabla_b V_c)$.
This is something I've taken for granted a lot in calculations, namely I though that by the Leibniz rule we just have:
$$\nabla_a (\nabla_b V_c) = \partial_a(\nabla_b V_c) - \Gamma_{ab}^{d}\nabla_c V_d - \Gamma_{ac... | <strong>Easy way</strong>
Let me first state the straight-forward way to do this computation.
$$
\langle \nabla_a \nabla_b V, \partial_c\rangle =
\partial_a \langle \nabla_b V, \partial_c \rangle - \langle \nabla_aV, \nabla_a \partial_c\rangle = \partial_a (\nabla_bV)_c - (\nabla_bV)_d \Gamma_{ac}^d
$$
First equality ... | The term $\nabla_b V_c$ is a (0,2) tensor writing in the abstract index notation, when writing in full basis form it reads
\begin{equation}
\nabla_b V_c \;dx^b\otimes dx^c\;,
\end{equation}
Now the status of $\nabla_b V_c $ is a components it <em>is a <strong>scalar</strong> function</em> while $dx^b\otimes dx^c$ is ... | https://physics.stackexchange.com |
598,535 | [
"https://electronics.stackexchange.com/questions/598535",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/301734/"
] | I am confused over this and I’m hoping for clarification.
By Ohm's law we can compute the resistance of a circuit as V/I. In the case of two sinusoids we have sin(x) / cos(x) or something like that. Doesn’t this give a sinusoidal resistance? I don’t understand this because resistance is a set value. A 100 ohm resistor ... | Ohm's Law, in its standard formulation, applies to specifically those circuit elements which we consider resistors (or ohmic) - the voltage and current are instantaneously proportional at all times with the relation <span class="math-container">\$V(t) = I(t)R\$</span>.
This excludes an element with out-of-phase voltage... | Ohm's Law always holds, but you must apply it properly. Ohm's Law describes the relationship between voltage, current, and resistance for an <strong>ideal</strong> resistor. The "voltage" <strong>must</strong> be the voltage across the resistor and the current <strong>must</strong> be the current through the ... | https://electronics.stackexchange.com |
896,202 | [
"https://math.stackexchange.com/questions/896202",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/160881/"
] | If function $f$ defined in an pocked neighbourhood of $x_0$, $N*\lambda(x_0)$ and positive there (meaning $f(x)>0$ to all $x\in N*\lambda(x_0)$).
If :
$$\lim_{x \to x_0} f(x)=0$$
Then:
$$\lim_{x \to x_0} \frac{1}{f(x)}=\infty$$
We need to proof that (The proof as shown in the book):
All $N>0$ exist $\del... | Since $f(x) \to 0$ as $x \to x_0$ we know that for any $0 < \epsilon$ there is a $\delta$ for which $| f(x) - 0 | < \epsilon$ for all $x$ such that $|x-x_0| < \delta$. This is the definition of a limit.
Here we choose our $\epsilon$ to be $1/N$ for a natural number $N$. Then a corresponding $\delta$ was chose... | I think there's a typo: You say "applies $f(x)>N$", but you want actually $\frac{1}{f(x)}>N$.
The reasoning is just in the second last line: Once you know
$0<f(x)<\frac{1}{N}$
you conclude by taking the whole inequality $f(x)<\frac{1}{N}$ to the power -1. And all the quantities are positive so the sta... | https://math.stackexchange.com |
123,044 | [
"https://security.stackexchange.com/questions/123044",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/56001/"
] | 2-factor authentication means to enter MOBILE app (using username and password) and there you get code to enter your account from PC browser, right?
But if hacker has my password, then he can enter in the mobile app without 2-step login?
| 2FA means to have to two factors for authentication, preferable one is a physical factor ("know" and "have"). The idea is that this way it is not enough to get the password due to hacking, data leakage, phishing etc but that the attacker must additionally have access to the physical device.
There are lots of physical ... | <blockquote>
2-factor authentication means to enter MOBILE app (using username and password) and then from that app you get codes to enter your account from a browser on a PC browser.
</blockquote>
No, no it doesn't.
There are several kinds of 2FA. One is by getting a text message. You don't enter your <code>site.c... | https://security.stackexchange.com |
137,568 | [
"https://softwareengineering.stackexchange.com/questions/137568",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/35414/"
] | We have a PHP intranet site that we run with warning/error logging on (as it's not under heavy load and there's a number of bugs/bad code left to find). Because the site is relatively unstable and in active development, I've been clearing the production error logs when each new version releases so I get a clean look at... | If you are using them to eliminate the bugs being reported, then they are doing their job.
They are useful NOW in the identification of current bugs, but if they are all fixed then I'm not sure there is much value in keeping the old logs once issues from them have been dealt with.
| I'd save them and stick them in a zip file or some other archive. Archiving should compress them, so they won't take a log of space, and they'll still be easily accessible if you want/need to review them. Name the files after the release/deployment ID so it's easy to correlate them with the version of the software t... | https://softwareengineering.stackexchange.com |
84,930 | [
"https://mathoverflow.net/questions/84930",
"https://mathoverflow.net",
"https://mathoverflow.net/users/20369/"
] | Hi,
Is it true that covariant derivative on any vector bundle over a manifold X comes from some connection on the bundle of linear frames of that vector bundle? This statement is true for the case of tangent bundle, but I am not sure if it is true in general or not. If it is true then can someone please suggest some re... | If $E \to X$ is a (finite-dimensional) vector bundle and $P$ is the principal $GL(n)$ bundle of frames, then there is a one-to-one correspondence between covariant derivatives on $E$ and principal connections on $P$. A good reference for details is proposition 4.4 of Lawson and Michelson's ``Spin Geometry" (unfortunat... | I don't know much about infinite dimensional things. I am not sure about the right answer but may be following may be useful... Once i saw the following book and statement:
See Page 4 of the book "Lectures on closed geodesics"- W. Klingenberg. Where he says:
"Whereas for Euclidean vector bundles over Euclidean mani... | https://mathoverflow.net |
188,588 | [
"https://softwareengineering.stackexchange.com/questions/188588",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/54159/"
] | I don't like singletons, but I have to compromise now (but only as temporary measure).
However I don't want to go completely singleton. I'd rather want to use this pattern:
<pre><code>interface NameThisInterface {
//Returns the currently centralized object.
static function current();
//Centralizes the instanc... | It is a combination of Singelton and some form of "Creator Patter" (probably a Factory (Though your factory is a little on the disorganized side (since we can't see most of the code)).
There is nothing wrong with combining Singelton with other patterns. In fact I personally think that you can <strong>NOT</strong> use ... | Simon's comment hit the nail on the head. What you've got there is <em>almost</em> exactly a global variable with get/set methods.
I say <em>almost</em> since your set method isn't static; this will prevent you setting your global variable to <code>null</code> after it has been assigned a value. You could debate wheth... | https://softwareengineering.stackexchange.com |
282,179 | [
"https://dba.stackexchange.com/questions/282179",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/197568/"
] | We want to be able to delete entries from a MySQL table, based on deletion criteria set in another table. Let me explain with an example.<br />
I have two tables defined as follows:
<pre><code>CREATE TABLE base_tbl
(
id INT UNSIGNED NOT NULL AUTO_INCREMENT,
f1 VARCHAR(8),
f2 VARCHAR(8),
PRIMARY KEY (id)
);
... | A simple solution is using <strong>IN</strong> clause in the following way:
<pre><code>DELETE FROM base_tbl
WHERE (f1, f2) in (select f3, f4 from del_criteria_tbl)
</code></pre>
| You can use a MYSQL specific construction like:
<pre><code>DELETE a.*
FROM base_tbl a
JOIN del_criteria_tbl b
ON a.f1 = b.f3
AND a.f2 = b.f4
</code></pre>
or use a standard variant:
<pre><code>DELETE
FROM base_tbl a
WHERE EXISTS (
SELECT 1
FROM del_criteria_tbl b
WHERE a.f1 = b.f3
AND a.f2 = ... | https://dba.stackexchange.com |
42,002 | [
"https://security.stackexchange.com/questions/42002",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/30521/"
] | A robust outgoing firewall will protect against desktop applications directly connecting to the internet without your permission. However, as the browser is a generally authorized application for internet access, I have begun to see more applications circumvent this by forwarding a URL request to the browser. This is e... | There's actually a simple solution to this: build an application that takes a single command line parameter (the URL) and launches the browser directly with that URL after a confirmation box is shown. Set this application as the default browser.
When the system sees a ShellExecute or CreateProcess with a <code>http://... | Another solution is to build or modify a existing browser, to not allow any type of external URL typing. You could even have it to use a screen keyboard or such to prevent malicious programs from using <code>SendKeys();</code> and you could have scrambling screen keyboard and much other security features, And user conf... | https://security.stackexchange.com |
248,544 | [
"https://electronics.stackexchange.com/questions/248544",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/20711/"
] | Eagle schematic editor is having the following default layers:
<pre><code>91 Nets: Nets
92 Busses: Buses
93 Pins: Connection points for component symbols
with additional information
94 Symbols: Shapes of component symbols
95 Names: Names of component symbols
96 Values: Values/component types
97 Info: General ... | I've never used the Guide layer, but have used the Info layer. Info is where I write comments in the schematic, names of the sheets, and anything else that isn't the parts or the connections between them.
I just flipped thru a recent schematic and found a note indicating the name of the firmware build that will go in... | In the EAGLE reference manual of version 5, the description of layer 98 is more clear:
<pre><code>98 Guide: Guiding lines for symbol alignment
</code></pre>
So, use this layer for all lines and forms you need to align the symbols in your schematic - and not for e.g. guidelines for assembly of the PCB.
The Info la... | https://electronics.stackexchange.com |
92,640 | [
"https://electronics.stackexchange.com/questions/92640",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/33636/"
] | I am working in a large office building with a lot of people. Pretty frequently there is a very weak and low frequency noise that is very irritating. Unfortunately, I have found just one other person that can perceive it, so it must be at the utmost borders of what a human can hear. I have started searching for equipme... | A good quick and dirty sensor for low frequency sound is a large speaker used in reverse. Basically, you use it as a microphone with low impedance output. You can amplify such low frequencies with just about any opamp. Low pass filter the signal with a rolloff in the 100-200 Hz range at each step, with the high pass... | I had a similar problem in a university I worked at. We used a seismic accelerator, a preamplifier, and a laptop. We tracked the noise down to the chillers on the roof, when both compressors were running they sent a 100hz signal with low amplitude straight through the building like a huge tuning fork. Changing the d... | https://electronics.stackexchange.com |
223,990 | [
"https://electronics.stackexchange.com/questions/223990",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/71302/"
] | Lets suppose we have a pn junction diode made of silicon. When it is forward biased we say depletion region shrinks because <em>electrons in the n-region are pushed towards the junction resulting in the decrease of width of positive charges layer</em> also these electrons cross the junction and current starts flowing f... | The distance that minority carriers can move across the other material is called the "diffusion distance". The time that it takes until the minority carrier disappears is called the "minority-carrier recombination lifetime".
The distance / time that minority carriers have is dependent on the number of recombination s... | It's helped me to understand that holes don't "move" per se, in as much as a hole is an atom with a missing electron.
The atoms themselves are fixed in the crystal lattice.
If you have two atoms that are fixed, and one is missing an electron, if an electron moves from the other atom to fill the "hole", the location o... | https://electronics.stackexchange.com |
917,458 | [
"https://math.stackexchange.com/questions/917458",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/173346/"
] | I have two quantities $A$ and $B$ and I consider the fraction
$$\frac{1}{A+B}$$
I would like to write the above expression as a Product, i.e. find functions $F$ and $G$ such that
$$\frac{1}{A+B} = F(A)G(B)$$
for any functions F and G I don't even need them to be explicit, I just need to know if that is possible :)
Than... | $G(1)F(x)=\frac{1}{1+x}$ so we see that $F(x)=\frac{1}{G(1)(x+1)}$ and similarly $G(x)=\frac{1}{F(1)(x+1)}$. Then $$F(A)G(B) = \frac{1}{F(1)G(1)} \frac{1}{(1+A)(1+B)}$$
$F(1)G(1)=\frac{1}{2}$, so this means:
$$\frac{1}{A+B}=F(A)G(B) = \frac{2}{(1+A)(1+B)}$$
or $$(1+A)(1+B)=2(A+B)$$
That means that $1+AB=A+B$ for a... | Suppose this were the case. Then, for any fixed $B$, we let $A\to \infty$. The fraction approaches zero, so $F(A)\to 0$. This is also true if $A\to \infty$. Hence $\lim_{A\to\pm \infty}F(A)=0$, and by symmetry $\lim_{B\to \pm \infty}F(B)=0$.
However, we now let $A,B$ get large not independently. Set $B=-A+1$, a... | https://math.stackexchange.com |
34,970 | [
"https://stats.stackexchange.com/questions/34970",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/6040/"
] | It is well-known (e.g. in the field of compressive sensing) that the $L_1$ norm is "sparsity-inducing," in the sense that if we minimize the functional (for fixed matrix $A$ and vector $\vec{b}$) $$f_{A,\vec{b}}(\vec{x})=\|A\vec{x}-\vec{b}\|_2^2+\lambda\|\vec{x}\|_1$$ for large enough $\lambda>0$, we're likely for m... | A general method for creating sparse solutions is via MAP estimation with a zero mean normal prior with an unknown variance.
$$p(x_i|\sigma_i^2)\sim N(0,\sigma_i^2)$$
If you then assign a prior to $\sigma_i^2$ which has a mode at zero then the posterior mode is usually sparse. The $L_1$ arises from this approach by ... | Two options:
<ol>
<li>Use an $L_0$ penalty on $\vec x$. The obvious drawback is that this is nonconvex and hence difficult to optimize.</li>
<li>Reparameterize, $x_i = \frac{\exp(w_i)}{\sum_j \exp(w_j)}$ and use a penalty on the new (natural) parameter vector, $\|\vec w\|$. This will encourage events to be equally p... | https://stats.stackexchange.com |
23,582 | [
"https://stats.stackexchange.com/questions/23582",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/6897/"
] | The regret in a multi-arm bandit model is given by
$$\underset{j}{\max}\sum_{t=1}^{T}x_j(t) -G_{A}$$
where $$G_A=\sum_{t=1}^{T}x_{it}(t)$$ is the total reward achieved by the learner, based on an action $i$, taken at each time interval $t$, i.e $i_{t}\in {1,2,...,K}$ and $x_j(t)$ is the reward assosciated with actio... | Rescaling is always a good idea. As pointed out by jb. in some cases it won't make a different, but in some cases it will make a significant difference.
Let me add that for PCA related stuff, sometimes subtle differences in the rescaling can make a relatively large difference. Consider evaluating the following altern... | It depends on the classifier.
Some need to have inputs that have the same magnitude, some not.
Models that AFAIK need scaled data:
<ul>
<li>SVM</li>
<li>Most neural networks</li>
<li>Fisher model --- I not really sure, but I think so. </li>
</ul>
Models that don't need it:
<ul>
<li>Decision trees</li>
</ul>
| https://stats.stackexchange.com |
6,363 | [
"https://dba.stackexchange.com/questions/6363",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/3119/"
] | I have a question regarding the logical backup of MySQL databases
that use both MyISAM and InnoDB.
The <code>mysqldump</code> utility supports these two options:
<ul>
<li><strong>--single-transaction</strong> - Creates a consistent snapshot by dumping all tables in a single transaction. Works ONLY for tables stored i... | With mysqldump you can only safely use <code>--single-transaction</code> if all your tables are InnoDB, otherwise your backup is inconsistent.
If you have the requirement for a hybrid backup, then you need the <code>lock-tables</code> on all tables in the backup (default), which will be safe for all engines. It's als... | As an additional idea for your situation, maybe it's helpful to spend some thoughts on setting up a master slave setup, where your backup is made from the slave.
A MySQL db with a mix of InnoDB and MyISAM will always (as far as I know in every configuration and with every backup tool) lock the MyISAM tables for the ... | https://dba.stackexchange.com |
94,066 | [
"https://mathoverflow.net/questions/94066",
"https://mathoverflow.net",
"https://mathoverflow.net/users/12310/"
] | <strong>Can the Yang-Mills or Chern-Simons action functionals be considered as [possibly <em>perfect</em>] Morse functions?</strong> I assume we would be in an equivariant scenario due to considering the configuration spaces with gauge-groups/transformations. Or at least how far away are they from a Morse-Bott function... | The problem with the CS functional is that the Morse indices of its critical points are infinite. In particular, this functional cannot be perfect. The Floer complex does not compute the homology of any particular space (though it might compute the homology of a certain spectrum).
On 4-manifolds the YM functiona... | Let's consider CS functional for concreteness. The problem is that CS is neither Morse nor Morse-Bott (because its critical points are flat connections and character varieties of 3-manifold groups could be rather bad). The trick is to perturb CS to a Morse function. This was done first by Taubes ("Casson's invariant an... | https://mathoverflow.net |
14,894 | [
"https://security.stackexchange.com/questions/14894",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/8571/"
] | I have seen many write-ups on "malware research", what information are they trying to gather?
Is it the origins of the malware? Or its behavior patterns? Or something else?
| Whodunit, what it does, what vulnerability it exploits, what it's communicating with (if anything), how it spreads, if it contains any useful forensic indicators etc etc.
Like watching a mutating biological virus and working out how it acts and how to develop a vaccine, the purpose of reverse engineering malware is to... | Short answer : find what it does (interactions with the infected system : files creation, keylogging, password stealing etc.).
This can be used to get a pattern used later by an AV product (malware signature). It can be used to trace back the author, too.
| https://security.stackexchange.com |
50,700 | [
"https://mathoverflow.net/questions/50700",
"https://mathoverflow.net",
"https://mathoverflow.net/users/11765/"
] | Let $X_0$ be a smooth projective variety over $\mathbb{C}$ and let $\Theta_{X_0}$ be the locally free sheaf of $O_{X_0}$-module corresponding to tangent space of $X_0$.
Goal: To find a sufficient condition on $X_0$ so that it admits a model over $\overline{\mathbb{Q}}$ (the field of algebraic numbers).
By spreading ... | I think your question has a positive answer: The Kodaira-Spencer maps at each point $p \in B$ fit together to give a map of sheaves $\Theta_B \to R^1 f_* \Theta_{X/B}$ and your condition implies that this is map is zero. One may then base change to $C$ and apply Kuranishi's theorem (On the locally complete families of ... | I doubt you will get a result in the generality you want. First off, if $H^0(X_0,\Theta_{X_0}) \ne 0$, then Kodaira-Spencer is not well-behaved. This may not be a show-stopper. I think results like what you want are only possible when there is a coarse moduli space for a class of varieties containing your $X_0$. If the... | https://mathoverflow.net |
16,305 | [
"https://mechanics.stackexchange.com/questions/16305",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/10067/"
] | I kept my old replaced battery in my boot to carry it to recycle store. The battery water/fluid leaked all over the boot liner mat. There is no damage to the mat whatsoever but it is not drying. It's been a month now. I kept it in open sunshine for a day as well.
Please advise.
Thanks,
DS
| It is highly likely that the "battery water" is a strong lead-containing acid that you do not want to be handling without appropriate personal protective equipment. This will also explain why it is not evaporating easily.
The advisable thing to do is have it professionally disposed of, because lead and lead ions are t... | The electrolyte in a common lead-acid car battery is dilute sulfuric acid H2SO4 which does not contain lead. Spilled electrolyte is corrosive and hazardous. You do not want to inhale the fumes and I recommend swabbing the spilled area with household ammonia or sodium bicarbonate (baking soda), either of which will neut... | https://mechanics.stackexchange.com |
165,847 | [
"https://mathoverflow.net/questions/165847",
"https://mathoverflow.net",
"https://mathoverflow.net/users/14024/"
] | Given a formal power series $$y(x)=\sum_{i=0}^{\infty} a_i x^i$$ Is there an algorithm that decides whether there exists a polynomial$$ P(x,y)=p_n(x)y^n+p_{n-1}(x)y^{n-1}+\cdots+p_0(x)=0,p_j(x)\in F[x]$$the series satisfies and if it exists,how to write it down?
| One result in this area is Christol's theorem, which asserts that
an element of $\mathbf{F}_p[[X]]$ is algebraic over $\mathbf{F}_p(X)$
if and only if its sequence of coefficients is a $p$-automatic sequence,
which means that there is a finite state machine for which
the coefficient of $X^n$ is the output of this machi... | No, there is no algorithm. Suppose there were. Consider a polynomial $q$ in $k$ variables.
Let $a_n = 1/n!$ if $n=2^{2^m}$, and $q(x)=0$ for $x$ the $m^{th}$ $k$-tuple of integers in some enumeration.
Let $a_n=0$ otherwise.
If $q$ has only finitely many roots, then the desired polynomial $p$ exists (trivially with... | https://mathoverflow.net |
365,640 | [
"https://softwareengineering.stackexchange.com/questions/365640",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/29899/"
] | We all know that Scrum works very well for a traditional team where team members come into work each week/sprint, agree goals and meet commitments.
However open source projects are often very different:
<ul>
<li>People tend not to work on Open Source projects full time</li>
<li>While some projects have a group of cor... | Open source projects are very diverse. There are some where a Scrum-like approach can work, but not in the general case.
Scrum timeboxes work into fixed-sized sprints. There are many open source projects with a fixed release schedule. But usually, this schedule is about <em>integrating</em> work that has already been ... | Saying Scrum can't work because of those arguments is like saying that Scrum can't work in a company, because they are currently using waterfall. While Scrum and waterfall don't play well with each other (or rather contradict each other) it does not mean that you cannot establish Scrum there, <strong>if</strong> everyo... | https://softwareengineering.stackexchange.com |
156,610 | [
"https://electronics.stackexchange.com/questions/156610",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/68413/"
] | i'm learning now the about Bluetooth (and BLE) and feel I need some explanation what the advantages of using frequency-hopping method. Can anyone please explain me in simple words (with examples, please) what we earn from the frequency-hopping? i know it should minimize interference, but don't understand how and why.
... | Frequency hopping radios have a bunch of possible carrier frequencies that they can use for communicating. The receiving end of the link has to hop in sync with the transmitter and this means being able to predict correctly where to "move" its carrier frequency and when. This is usually accomplished by a "table" held i... | As mentioned by the other posters, frequency hopping is a transmission technique where the transmitter "hops" between different frequencies with the aim of improving the transmission performance.On the receiver side there are two ways the receiver can receive the information - coherently or more commonly incoherently.
... | https://electronics.stackexchange.com |
478,170 | [
"https://physics.stackexchange.com/questions/478170",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/231101/"
] | I just studied SR and I understand how velocity affects time and length; how do they evolve during the process of reaching a certain velocity?
| For the purposes of your question, acceleration is the process of going from one inertial frame to another.
The differences you see between being in one frame and the other occur during acceleration.
If you are 'looking at' events in a chosen inertial frame as you accelerate, you will see lengths changing - decreasin... | At any instant, the time dilation and the length contraction of a moving object just depend on its instantaneous velocity relative to the observer, not on its acceleration.
If the particle is accelerating and you want to know, for example, the time that elapses on “its clock” over a finite interval, then you have to i... | https://physics.stackexchange.com |
124,740 | [
"https://chemistry.stackexchange.com/questions/124740",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/85877/"
] | It was written in my textbook,
<span class="math-container">$$
\mathrm{d}U =
\left(\frac{\partial U}{\partial T}\right)_V \mathrm{d}T
+\left(\frac{\partial U}{\partial V}\right)_T \mathrm{d}V
$$</span>
If the process is isothermal, <span class="math-container">$\mathrm{d}T = 0$</span>. So, the equation reduces to... | The internal energy of an ideal gas is simply <span class="math-container">$$ U = \alpha nRT,$$</span>
where <span class="math-container">$$\alpha = \frac{\text{degrees of freedom}}{2}$$</span>
So, in an isothermal process,
<span class="math-container">$$
\Delta T = 0 \Longrightarrow \Delta U = \alpha nR\Delta T = 0,$$... | <blockquote>
I was told, <span class="math-container">$\mathrm{d}U=0$</span> in isothermal process.
</blockquote>
That is <em>not</em> generally true. It is, however, true for ideal gases, which is probably what you were discussing. No attractive or repulsive forces exist between ideal gas particles. Hence the o... | https://chemistry.stackexchange.com |
188,158 | [
"https://physics.stackexchange.com/questions/188158",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/81512/"
] | When a ball is thrown vertically upwards, what will be the direction of acceleration at the highest point(where velocity is zero)? Upwards, downwards or arbitrary?
| During the toss, while still in your hand, the ball is accelerated upwards.
As soon as the ball leaves your hand, it begins to slow down. Here, scientific and common usage diverge. Technically, the ball experiences a downward force, and its velocity decreases with time, so it can be said to possess a negative (downwa... | The accelaration is only because of the gravity, that is downwards. So when the ball reaches at the top of it's trajectory and it's speed becomes zero, it also has an accelaration pointing down.
After that the ball accelerates until it reaches ground(if there wasn't a ground it would oscillate around the point 0 from ... | https://physics.stackexchange.com |
129,988 | [
"https://mathoverflow.net/questions/129988",
"https://mathoverflow.net",
"https://mathoverflow.net/users/25358/"
] | Let $G$ be a connected, simply-connected, complex semisimple Lie group with Lie algebra $\frak{g}$. Let $\mu:T^*\mathcal{B}\rightarrow\mathcal{N}$ be the Springer resolution of $\mathcal{N}$. If $G=\operatorname{SL}_n(\mathbb{C})$, then the Springer fibers $\mu^{-1}(e)$, $e\in\mathcal{N}$, are known to be connected pro... | The answer is yes. I'm not sure what sources you are working with, but much of this theory originates in the work of Spaltenstein (Lecture Notes in Math. 946, Springer, 1982). While the fibers are connected, they are not irreducible as varieties but the irreducible components are shown to be of equal dimension, etc.... | Yes: this is discussed in Chriss & Ginzburg 'Representation Theory and Complex Geometry', p.161 Remark 3.3.26. In short, the nilpotent cone is normal and we can apply Zariski's Main Theorem to deduce connectedness of Springer fibres. (This works for reductive $G$, and does not depend on working over $\mathbb{C}$; I... | https://mathoverflow.net |
197,643 | [
"https://softwareengineering.stackexchange.com/questions/197643",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/63484/"
] | First, I believe I've seen this question discussed here before, but I cannot find it. My apologies if you do find it.
I'm starting a new project, and trying to figure out why IsResolved and/or ResolvedOn make more sense to me than Resolved. Obviously, when something is named "CompanyName" (for example), I'm fairly con... | First of all, most of the allergic reactions to Hungarian notation stems from the fact that consistent application tends to result in very unnatural looking names. Occasional uses of prefixes or suffixes that suggest a certain type should not be considered to be Hungarian notation unless the name looks very forced.
Wh... | I'd say that Resolved is a boolean, as well as IsResolved. ResolvedOn could be a date.
However, one simply cannot depend on notation or name of a variable to determine its type, especially if one is going through someone else's code. Imagine variables named in different language (we had an exchange student from Mexico... | https://softwareengineering.stackexchange.com |
51,671 | [
"https://chemistry.stackexchange.com/questions/51671",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/17703/"
] | To extract caffeine from coffee, I tried reacting it with baking soda to make tannins more soluble. I then put in salt to push caffeine out of solution. I saturated it with salt and then mixed the solids with acetone to get the caffeine out. There is also some salt in the acetone. Washes with water should remove the sa... | The popular procedure to experimentally isolate caffeine is sublimation.
Slowly heat a teaspoon of coffee or tea from 150°C to 250°C. Let the sublimated caffeine precipitate at a watch glass cooled with ice. The resublimated caffeine forms white needles that can be identified under a microscope.
The sublimation tempe... | I did this in a practical lesson in chemistry. We filtered the coffee (1.2um filter paper), concentrated it 10x using distillation and then extracted the caffeine into an equal volume of chloroform in a separatory funnel. After evaporating the solvent, we were left with fairly white powder.
It was probably impure, cont... | https://chemistry.stackexchange.com |
16,152 | [
"https://dba.stackexchange.com/questions/16152",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/3787/"
] | I have many so-called summary columns throughout my database to count number of rows in other tables. For example, counting the number of comments by a user (a column in the user table). Currently, I update this column by an extra query:
<pre><code>UPDATE user_table SET comments = comments + 1 WHERE user_id='x'
</code... | The major problems with triggers are:
a) They are difficult to debug
b) Trigger cascades can be fatal to your database - take for example if you have an after update trigger in one table which updates another table, which also has an after update trigger, then you will run into lots of issues
A few questions:
a) Ho... | There are several options for summary fields such as you have.
<ul>
<li>Calculate when needed. This is the preferred starting point until usage indicates you need to de-normalize the value. </li>
<li>Use triggers to maintain the de-normalized value in the appropriate table. This trades off the overhead of maintaini... | https://dba.stackexchange.com |
158,456 | [
"https://mathoverflow.net/questions/158456",
"https://mathoverflow.net",
"https://mathoverflow.net/users/47355/"
] | Let $C$ be a quartic plane curve. Suppose that for a given coordinate system
$C=(F_4(x_0,x_1,x_2)=0)$ where there the polynomial $F_4$ factorizes as<br>
$$
F_4(x_0,x_1,x_2) = F_3(x_0,x_1,x_2)F_1(x_0,x_1,x_2)-F_2(x_0,x_1,x_2)^2
$$
If the coordinate system is fixed. How many factorization are there?
More generally, L... | Yes, if the quartic $C : F_4=0$ is smooth then the line $l : f_1=0$
must be one of the $28$ bitangents (because the restriction $F_4|_l$
is the square of $f_2|_l$), and then $f_2$ restricted to $l$
is one of the square roots of $F_4|_l$, and each of the lifts of $f_2|_l$
lets you solve for $f_3$.
Note that writing $F_... | If we are given one particular 'factorization': $F_4 = F_3F_1-F_2^2$, we can proceed like this:- Express $F_2$ as product of irreducible polynomials, say, $F_2 = P_1^{m_1}P_2^{m_2}\cdots P_k^{m_k}$.
Now define $G_1$ as $\prod_j P_j^{a_j} $ with $a_j\leq 2m_j$, and $G_2$ as $F_2^2/G_1$. Now $F_4=0$ is the same vari... | https://mathoverflow.net |
414,833 | [
"https://stats.stackexchange.com/questions/414833",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/40602/"
] | If I am given two repeatable events -
<ul>
<li>event A - that I am told has a probability of 0.5% to produce result X</li>
<li>event B - that I am told has a probability of 6% to produce result X</li>
</ul>
and given that the events have a cost associated with them, can I approximate how many times I would expect to ... | @Dave's answer is right, but I think you can approach it in a simpler way. Think instead of the probability of NOT getting result <span class="math-container">$X$</span>, which is <span class="math-container">$0.995$</span> if you try once, <span class="math-container">$0.995^2$</span> if you try twice.... and <span cl... | Assuming that it's not a problem if X happens more than once, you can just reverse the probability and calculate the answer very simply. If event A has a 0.5% chance of producing result X, that means it has a 99.5% chance of <em>not</em> producing result X. How many times do you need to multiply 99.5% (i.e. 0.995) by... | https://stats.stackexchange.com |
22,934 | [
"https://mechanics.stackexchange.com/questions/22934",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/13472/"
] | I have two cracked motor mounts on my KIA Sadona. I am curious as to what may have caused this. I am wondering if driving longer distances could cause it to happen
<hr>
It is a 2004 Kia Sedona EX. I bought it in April.
| <blockquote>
<ol>
<li>Because it's hard to get in there and see the camshaft seal, I'm skeptical that it can be diagnosed properly without taking the car apart. Could an apparent oil leak (there's some drippage on a hose underneath the left head) be caused by anything else? How should I go about independently diagn... | Post repair answers to
<ol>
<li>On my SOHC model, there is a left-side timing belt cover that can be removed without taking any other components out, so you can easily take a look at the camshaft sprocket and with a flashlight you can see back to the seal. The sprocket is definitely in the way (and definitely cannot ... | https://mechanics.stackexchange.com |
35,689 | [
"https://stats.stackexchange.com/questions/35689",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/12737/"
] | This might sound dumb but if I have $d_i$, where $i=1 \dots n$ observations and I assume they are exponentially distributed, before I use the MLE, should I transform my data to follow an exponential distribution? The reason I ask is because I often see others who normalize their dataset prior to the MLE if they assume... | Of course if the data does not follow an exponential you should not fit the mle for the event rate based on an exponential likelihood. But transforming to exponential is not your only option. Perhaps a more general Gamma distribution is appropriate. Use the best estimator for the appropriate family of distributions i... | Yes. Your data $\bf must$ follow the distribution under which you will make your estimations or under which you will run you ML estimator. If this is not the case your ML will not be ML for that data. You should transform your data or create a data which follows exponential distribution.
| https://stats.stackexchange.com |
96,161 | [
"https://dba.stackexchange.com/questions/96161",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/60666/"
] | <pre><code>DROP PROCEDURE IF EXISTS proc_x;
DELIMITER #
CREATE DEFINER=`root`@`localhost` PROCEDURE proc_x(sequel CHAR)
proc_main:BEGIN
SET @SQL = CONCAT(@sequel);
PREPARE stmt FROM @SQL;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END proc_main #
</code></pre>
Above is a syntax for creating stored procedure in my mysql d... | <pre><code>(sequel CHAR)
CONCAT(@sequel)
</code></pre>
@variables are totally independent of non-@ variables.
Two fixes needed:
<ul>
<li><code>SET @SQL = CONCAT(@sequel);</code> --> <code>SET @SQL = sequel;</code></li>
<li><code>(sequel CHAR)</code> --> <code>(sequel VARCHAR(999))</code></li>
</ul>
| Make the changes in your query as follows:
<pre><code>INSERT INTO table_name (`column1`, `column2`) values ('val1', 'val2');
</code></pre>
<strong>Note that the column names are enclosed in ` (character above tab) and not in quotes</strong>
| https://dba.stackexchange.com |
134,506 | [
"https://cs.stackexchange.com/questions/134506",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/130290/"
] | I am not a computer scientist and this is my first question.
This question is a question in layman terms and I also want the answer in layman terms.
When I searched hyper-computation. There was a list of models of hyper-computers.
Now, my question is that what are the problems with those models and if it is possible to... | The Church–Turing thesis is about physically realizable machines. To the best of our knowledge, hypercomputation models cannot be realized in the physical world. They are a figment of our imagination.
If someone would find a new law of physics which enables solving the halting problem, that would make a big fuss in the... | I'll just address one model. WP says: <em>The Zeno machine performs its first computation step in (say) 1 minute, the second step in ½ minute, the third step in ¼ minute, etc. By summing 1+½+¼+... (a geometric series) we see that the machine performs infinitely many steps in a total of 2 minutes.</em> We can imagine a... | https://cs.stackexchange.com |
129,406 | [
"https://physics.stackexchange.com/questions/129406",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/45557/"
] | A rectangular loop and a circular loop are moving out of a uniform magnetic field to a field-free region with a constant velocity V as shown in the figure. Explain in which loop do you expect the induced emf to be constant during the passage out of the field region. The magnetic field is normal to the loops.
<img src=... | Well if we take $B$ and $v$ as constants here then it is pretty easy, because all you need to do is calculate the change of the magnetic flux through the surface enclosed by either loops, and you know that the induced EMF ($\epsilon$) is given by: $$ \epsilon=-N\frac{d\Phi}{dt} $$
where $\Phi = \mathbf{B}\cdot \mathbf{... | What @Phonon explained seems partially correct to me as it appears to me that the radius of the circle is infact constant and just like the rectangle,for an infinitesimal value dr=vdt.The rate of change of area of the loop with respect to time is infact not a constant value,but it appears to me that when a certain port... | https://physics.stackexchange.com |
24,843 | [
"https://astronomy.stackexchange.com/questions/24843",
"https://astronomy.stackexchange.com",
"https://astronomy.stackexchange.com/users/16048/"
] | I've read that Uranus has a lower surface gravity due to its lower density than Earth. However, does this mean Uranus has a weaker pull on its moons than Earth does for the same reason?
| The "strength of gravity" (ie gravitational acceleration) is determined by the mass of the planet and the distance between the planet and the moon. The formula is
$$GM/r^2.$$
For the Earth (mass= 1 Earth, distance to moon=390000km) the acceleration is $0.003 ms^{-2}$<br>
For Uranus and Titania (mass =14.5 Earth, dis... | Provided that you are the same distance from the centre and you are on or above the surface then the heavier (more mass) planet will have a greater gravitational attraction. It is still possible that the heavier planet has a lower attraction at the surface since it may be bigger. If you can find the mass and radius t... | https://astronomy.stackexchange.com |
317,632 | [
"https://electronics.stackexchange.com/questions/317632",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/156017/"
] | I have multiple (9v 1500mA to 9v 3500mA) LED light fixtures that I have tested chaining a single fan (12v 5.2w and 12v 0.16A fans) and they seem to work under the brief test with enough air flow for my needs.
Anything I should be careful about while doing this and any direction to better understand if there are conce... | There is generally no harm in running a fan or any DC motor on lower voltage.
If the voltage is too low, then the motor will not run (or continue running if already spinning but not start), because there is not enough torque generated to oppose the “cogging” effect of the layout of magnets inside the motor. (This effe... | Your fan will operate. It will make less noise. It will waste less power. It will last longer. Because it turns slower it will shift less air and it will have less back pressure. All you should do is check your heatsink temperature and deduce the junction temperature. If the junction temperature is low then you are goo... | https://electronics.stackexchange.com |
368,297 | [
"https://softwareengineering.stackexchange.com/questions/368297",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/280642/"
] | Due to a new job opportunity I am switching back from WPF to WinForms.
Currently I am playing around a little to get used to it.
Here are my main questions that arose:
<ul>
<li>Should I use the designer in a professional environment? It feels somehow wrong to get that auto-generated clumsy event handlers... Otherwis... | <strong>Do</strong> use the visual designer to layout your GUI. Doing anything else is masochism.
<strong>Don’t</strong> put all of your code in the Form’s code behind.
<strong>Do</strong> use the Model-View-Presenter pattern. The form raises an event, which the presenter listens for, and reacts to by modifying the... | Regarding the first question, as long as the designer meets muy needs, I usually use it. If or when I want to do something more sophisticated (e.g. create or modify controls dynamically, or even initializing them according to complex rules), then I move to code. In any case, I choose the names ( both of controls and of... | https://softwareengineering.stackexchange.com |
53,597 | [
"https://mathoverflow.net/questions/53597",
"https://mathoverflow.net",
"https://mathoverflow.net/users/12570/"
] | Let $Y$ be a connected CW-complex and $X\subset Y$ a connected CW-subcomplex. Suppose that each cell of $X$ is the boundary of a cell of $Y$. Is this enough to conclude that $X$ is contractible in $Y$ (the inclusion map is homotopic to a constant map)? If the answer is no, then which condition could be enough to get th... | If you mean that every cell in $X$ is the image of the boundary of a cell in $Y$ then the answer is no -- consider for example the standard inclusion $S^1 = RP^1 \hookrightarrow RP^2$. The boundary of the $2$-cell is $RP^1$ (but it runs around twice, so $\pi_1 (RP^2) = \mathbb{Z}/2$ and the $1$-cell represents the gene... | The condition "each cell of $X$ is the boundary of a cell in $Y$" is very strong.
In particular, it implies that each cell of $X$ is attached to the rest of $X$ by a constant attaching map (otherwise, it would have no chance of being the boundary of something else)
Thus, $X$ is a wedge of spheres.
I think that, at t... | https://mathoverflow.net |
36,235 | [
"https://physics.stackexchange.com/questions/36235",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/8850/"
] | As the question states, what is our current best machine for translating falling gravitational potential energy, such as a large weight, into launching a smaller projectile vertically? A lever? A trebuchet? What sorts of efficiency levels have been achieved?
What I'm looking for is the best way to translate:
<span clas... | This question was asked four months ago, but none of the existing answers mentions trebuchets.
To my knowledge the trebuchet design is the only design that is purely mechanical. Other projectile throwing devices store elastic energy and on release transfer that to the projectile.
So a trebuchet it is.<br>
(In effect ... | Have an insulated cylinder of gas with a piston. Put the weight on the piston and let it press down. Then lock the piston in place and put the other mass on. Then turn the cylinder on its side and unlock the piston and the other mass will be accelerated. By turning it on the side, all the energy goes into accelerating ... | https://physics.stackexchange.com |
230,362 | [
"https://electronics.stackexchange.com/questions/230362",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/88025/"
] | I want to know how a thermoelectric generator generates electricity. I know that it converts 'heat flow' to electricity.But some people say it consumes heat and converts it into electricity.The rest of the heat is dissipated through the other side. Which is true?
| First you should contact the PCB vendor and try to get them to remake the boards correctly. If this is not possible you could try making the repairs yourself as follows:
If the hole takes a through hole component just be sure to manually solder each pin at both sides. If the hole is a via from an outer surface to th... | If you have defined the holes correctly, then the fab should do a repeat run for free to correct their mistake, and they should do it expedited if you shout loud enough. However, this is not going to get you boards tomorrow.
Do you have a 2-layer, or more-layer board?
If you have a 2-layer board, then you can do a r... | https://electronics.stackexchange.com |
10,457 | [
"https://electronics.stackexchange.com/questions/10457",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/2897/"
] | I have created a wav file (sampling rate of 44.1kHz at 32-bit floating point precision) consisting of 44.1k samples at 0.0 followed by 44.1k at 1.0. After which the file is played out from line-out using Media Player Classic and looped back into line-in. The resulting waveform is captured and inspected using Audacity.
... | This sounds like there is some DC Bias removal taking place. In general with audio you want your average voltage should be 0v. This is because of how speakers work. 0v would mean that the speaker is at rest. In order for the speaker to sound normal you want it to be able to go back to its resting position.
In order to... | Audio codecs in PCs have digital high pass filters (at least the ones I know) with cutoff at some low frequency like 1 Hz, while your signal is only 0.5 Hz, and this is what deforms it. If you use it for some kind of signaling, you will have to move to higher frequencies. I don't think there is a way to disable the fil... | https://electronics.stackexchange.com |
594,261 | [
"https://physics.stackexchange.com/questions/594261",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/747/"
] | As I understand it, the scalar curvature is a function that assigns a real number between <span class="math-container">$]-\infty,\infty[$</span> to each point <span class="math-container">$(x,y,z,t)$</span> of a manifold:
<span class="math-container">$$
R:\mathbb{R}^4\to \mathbb{R}
$$</span>
I am having difficulty pict... | In Riemannian geometry, there are in principle two fundamental quantities: the metric <span class="math-container">$g$</span> and the connection <span class="math-container">$\nabla$</span>. Each one may be chosen independently of the other, or even be left undefined (that is, you can have only the metric, or only the ... | In general, you seem very focused on the scalar curvature. The scalar curvature is just one measure of curvature. A spacetime can be curved and have zero scalar curvature, as in, e.g., a gravitational wave.
<blockquote>
For a given g, can R be any choice of arbitrary, possibly smooth, function...
</blockquote>
No. R ca... | https://physics.stackexchange.com |
349,131 | [
"https://softwareengineering.stackexchange.com/questions/349131",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/227415/"
] | Recently lots of people adopted DI in their projects (I am working with aspnet core). The problem I have is that DI turns my code towards procedural paradigm. For example in OOP way I would do:
<pre><code>class Something
{
Something(Other object, Dependency dep); // created manually or by factory obj
DoStuff(... | As Doc Brown mentioned, using Dependency Injection containers is not Dependency Injection. Dependency Injection is just parameterization, and Dependency Injection containers are just meant to "simplify" <em>wiring up</em> dependencies. Your issue seems to be mostly driven by (perceived) limitations of whatever DI cont... | <blockquote>
The problem I have is that DI turns my code towards procedural paradigm.
</blockquote>
Your example does not show a "procedural approach".
<hr>
<blockquote>
As a result my objects have no methods, they are just bags of properties that are processed by different services. On the other hand I would ha... | https://softwareengineering.stackexchange.com |
626,281 | [
"https://physics.stackexchange.com/questions/626281",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/278990/"
] | Suppose we have solved for the energy eigenstates of some Hamiltonian operator <span class="math-container">$\hat{H}$</span>.
We call the energy eigenstates <span class="math-container">$\psi_n (x)$</span>, where:
<ul>
<li><span class="math-container">$n=1$</span>: <span class="math-container">$\psi_1 (x)$</span> ... | <em>Any</em> function that satisfies the boundary conditions for the problem - even one that doesn’t solve the time-independent Schrodinger equation - can be written as a sum of functions that do solve the TISE.
The solutions <span class="math-container">$\psi_n(x)$</span> are like “basis vectors” in the same way that ... | In general, If
<span class="math-container">$$H|n\rangle =n|n\rangle $$</span>
Then we can expand <span class="math-container">$|\psi\rangle$</span> on this basis
<span class="math-container">$$|\psi\rangle =\sum_n|n\rangle \langle n|\psi\rangle$$</span>
<hr />
Note that in general, I mean you can do the above procedur... | https://physics.stackexchange.com |
210,696 | [
"https://math.stackexchange.com/questions/210696",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/37824/"
] | I was trying to find the values of $r>0$ for which the integral
$$\int^{\pi/2}_{0}\csc^{r}x\,\mathrm{d}x$$
exists. Unfortunately, I'm short on ideas on how this could be done. If this may serve as any guidance, to be crude, I tried punching different values for $r$ in <em>Mathematica</em>, and it looks like we are... | <strong>Hint:</strong> For $x\ne 0$ in our interval,
$$\frac{1}{2}\lt \frac{\sin x}{x}\lt 1,$$ so
$\dfrac{1}{x}\lt \csc x \lt \dfrac{2}{x}$.
| Note that the problem with the integrand is at $x=0$. Using the fact that $\sin(x) \approx x$ as $x \rightarrow0 $, then $\frac{1}{\sin(x)^r} \approx \frac{1}{x^r}$. Now, you can derive some conditions on $r$.
| https://math.stackexchange.com |
422,176 | [
"https://physics.stackexchange.com/questions/422176",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/-1/"
] | While studying the Dirac equation, $$\left(i\gamma^{\mu} \partial_{\mu} - m\right)\psi = 0.$$ I have been finding difficulty understanding the following summarisation of the algebra that the $\gamma$-matrices follow, $$\{\gamma^{\mu},\gamma^{\nu}\} = 2 \eta^{\mu \nu}$$ Where $\eta^{\mu \nu}$ is the inverse Minkowski me... | It can be easier to write everything in terms of explicit indices,
$$\gamma^\mu_{\alpha \beta} \gamma^\nu_{\beta \delta} + \gamma^\nu_{\alpha \beta} \gamma^\mu_{\beta \delta} = 2 \eta^{\mu\nu} \delta_{\alpha \delta}.$$
There is a sum over $\beta$ on the left-hand side. When you plug in explicit values for $\alpha$, $\d... | Let's try to summarize the characteristics of the $\gamma$-matrices and the metric tensor for flat Minkowski space. First our $\eta^{\mu\nu}$ can be represented by the matrix
$$\eta^{\mu\nu} = \begin{pmatrix} 1&0&0&0 \newline 0&-1&0&0\newline 0&0&-1&0\newline 0&0&0&-1 \... | https://physics.stackexchange.com |
74,200 | [
"https://security.stackexchange.com/questions/74200",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/58615/"
] | Do firewall rules have to be symmetric? That is, does a firewall have to block a particular traffic type both inbound (to the protected site) and outbound (from the site)? Why or why not?
I have to answer the above question. What is the meaning of symmetric rules in firewall? can anyone please explain it to me
| <strong>Q:</strong> Do firewall rules have to be symmetric?
<strong>Short and simple answer:</strong> No, they don't HAVE to be. But it doesn't mean that they CAN'T be.
<strong>Why?</strong>
It depends on the traffic that is expected to pass through the firewall, but most of the time the rules won't be symmetric.
F... | I believe you have some terminology mistakes in your question, but I still think I can answer it. To answer your question I'll explain both common types of firewalls, stateful and stateless. Both types of firewalls compare packets against their rulesets. Both work from a set of data often referred as a tuple, which typ... | https://security.stackexchange.com |
271,610 | [
"https://softwareengineering.stackexchange.com/questions/271610",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/166270/"
] | I have been reading a lot about repository implementation. I am confused about which way to implement it for a project I am sure would change its data layer methods because of db migration from MS Sql Server to NoSQL in a couple of years.
<h2>Imp #1</h2>
<ul>
<li>Implement Rep layer as a completely separate layer. Th... | The main reason you would have the interfaces for your repositories in your BLL is to avoid having hard references to the separate DAL but instead have your changing DAL reference the stable BLL.
<strong>To be able to swap out implementations without changing the stable BLL</strong>
On the internet this might not be ... | Sticking to option 1 will allow you to change the least amount of things when you switch from one database to another, thus the change should have the least impact on your application since only the files concerned with the database will need to be changed.
The business layer and any other layer which does not deal wi... | https://softwareengineering.stackexchange.com |
141,608 | [
"https://softwareengineering.stackexchange.com/questions/141608",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/44933/"
] | <h3>Question</h3>
Should the functional requirements in a requirements doc use wording based on verbs?
<h3>Context</h3>
School assignment, working in a team, working through the SDLC. The requirements doc has been done and we are now into design.
<h3>Problem</h3>
The requirements doc has an enumerated list of what... | You can and should use verbs in your requirements. The important thing is to make sure each requirement is:
<ul>
<li><strong>Unambiguous</strong> - each requirement can only mean one thing and can only be interpreted one way.</li>
<li><strong>Atomic</strong> - each requirement cannot be broken down into multiple requ... | "Should the functional requirements in a requirements doc use wording based on verbs?"
The short answer is "yes", but the path to get there is winding.
If the requirements doc is a collection of "shall" statements written as English sentences, you must have a verb phrase. And that verb phrase will the "shall xxx" as... | https://softwareengineering.stackexchange.com |
448,047 | [
"https://stats.stackexchange.com/questions/448047",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/272835/"
] | I have a dataset of males and females and whether they dispersed (yes = 1). For example:
<pre><code>Male 0
Male 0
Male 0
Male 0
Male 0
Female 0
Female 1
Female 0
</code></pre>
I'm interested in performing a randomization to determine if the frequency of dispersal is different between males and females. Howeve... | Y is independent of X given Z, but Z isn't independent of X. Which you're manipulating (or, well, simulating the manipulation, anyway).
An intervention on the value of X changes the probabilities of Zs you're going to observe. That's why we need the first part, summing/integrating over the post-intervention distributi... | To answer your question straight,
<span class="math-container">$$P(y | \textit{do}(X = x)) = \sum_z P(z | x) \sum_{x'} P(y|x', z)P(x').$$</span>
can't be rewritten as
<span class="math-container">$$P(y | \textit{do}(X = x)) = \sum_z P(z | x) \sum_{x'} P(y|z)P(x')$$</span>
because <span class="math-container">$P(y|x', z... | https://stats.stackexchange.com |
380,952 | [
"https://physics.stackexchange.com/questions/380952",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/167744/"
] | I have read in D.J. Griffith's electrodynamics text that if one finds the field <span class="math-container">$H$</span> by Ampère's law, that means that the divergence of <span class="math-container">$H$</span> is zero, because the free current alone determines the answer. Why is that?
| In general, you need to know both the $\textrm{curl}$ and $\textrm{div}$ of a vector field to determine the field. Now you always have $\textrm{div}\textbf{B}=\textrm{div}(\textbf{H+M})=0$ hence $\textrm{div}\textbf{H}=-\textrm{div}\textbf{M}$ always, so if you can find $\textbf{H}$ from symmetry arguments using Ampere... | The definition of the H field is given as follows:
$$\mathbf{H}=\dfrac{\mathbf{B}}{\mu_0}-\mathbf{M}$$
where $\mathbf{B}$ is the total magnetic field in all space and $\mathbf{M}$ is the magnetization vector. Taking the line integral of $\mathbf{H}$ on a closed loop and using Ampere's law, we get,
$$\oint \mathbf{H}\cd... | https://physics.stackexchange.com |
5,815 | [
"https://physics.stackexchange.com/questions/5815",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/1460/"
] | I would like to expand on what I mean by the title of this question to focus the answers.
Normally whenever a theory (e.g. General Relativity) replaces another (e.g. Newtonian Gravity) there is a correspondence requirement in some limit. However there is also normally some experimental area where the new larger theory... | String theory implies new physics in - and only in - the quantum regime. In particular, at distances that are very short - comparable to the string scale or Planck scale - there are new effects. The black holes decay (while they preserve the information), effective actions have higher-derivative terms, e.g. $R^2$, ther... | The higher derivative terms are there just like in semiclassical gravity simply because when you introduce quantum mechanics, any term that is not forbidden by some symmetry principle or other physical criteria must be present, albeit Planck suppressed. So it's not that surprising that string theory predicts them. The ... | https://physics.stackexchange.com |
243,846 | [
"https://physics.stackexchange.com/questions/243846",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/33635/"
] | Clouds are made out of tiny droplets of water. These droplets, to my understanding, act like tiny prisms (which is why there are rainbows after rain). From my line of thinking, if we had a room with a bunch of prisms randomly placed about and had a light enter from one side, the walls would be quite colorful.
Why, t... | <blockquote>
From my line of thinking, if we had a room with a bunch of prisms
randomly placed about and had a light enter from one side, the walls
would be quite colorful.
</blockquote>
That would depend on the size of the prisms. If the prisms are large enough, then they will throw macroscopic patches of colo... | According to the color spectrum, when all colors combine they make white. You are completely correct. The raindrops do act like a prism. When all the colors combine they form white.
| https://physics.stackexchange.com |
44,087 | [
"https://dba.stackexchange.com/questions/44087",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/2542/"
] | I've got a Postgres 8.4 environment where the encoding on all our databases is set to <code>SQL_ASCII</code> - We're <em>finally</em> migrating to Postgres 9.2, and I would like to migrate everything over to <code>UTF8</code> encoding.
Unfortunately the text data in this DB is not clean -- Trying to restore the pg_d... | I suspect your database content may be in iso8859 or cp1252. If it were ascii, you would not run into problems importing it. You may be able to determine the coding by opening your dump with python. The following python3 tries encodings until it succeeds. It can be used to determine the file encoding.
<pre><code>... | Mangled text encoding dumps are a pain to work with.
The usual - and admittedly crude - solution is to run <code>iconv</code> on the SQL-format dump, with the <code>-c</code> flag to tell it to omit characters that are invalid in the target encoding.
This is only viable if the origin database is <em>supposedly</em> i... | https://dba.stackexchange.com |
180,113 | [
"https://dba.stackexchange.com/questions/180113",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/61215/"
] | I have come across the following code (SQL Server 2016, 2008-compatible database):
<pre><code>CREATE PROCEDURE myProc
AS BEGIN
DECLARE @ERRORMESSAGE As VARCHAR(1000)
BEGIN TRY
DELETE FROM Foo -- (statement #1)
DELETE FROM Bar -- (statement #2)
END TRY
BEGIN CATCH
SET @ERRORM... | I'm not sure that this specific <code>TRY ... CATCH</code> does anything particularly useful. It loses many of the details that would have existed where the actual error occurred (error number, severity, state, procedure/trigger, and line number).
The only obvious thing is does do that could be useful is force the err... | IMHO Using a TRY/CATCH block you don't need to check every single operation.
<pre><code>DELETE FROM Foo;
IF @@ERROR <> 0
BEGIN
RAISERROR('Error deleting table Foo', 16, 1);
RETURN -1;
END
OR
DELETE FROM Bar;
IF @@ERROR <> 0
BEGIN
SET @ErrMsg = 'Error';
GOTO ERROR;
END
</code></pre>
| https://dba.stackexchange.com |
50,463 | [
"https://stats.stackexchange.com/questions/50463",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/-1/"
] | In the "Regression to the Mean" chapter of "Thinking, Fast and Slow" by Daniel Kahneman, an example is given and the reader is asked to forecast the sales of individual stores given the overall sales forecast and the sales numbers from the previous year. For instance(the book's example has 4 stores, I use 2 here for si... | I happen to be reading that book. You have not adequately transcribed the key information. It says that "all stores are similar in size and merchandise selection, but their sales differ because of location, competition and random factors." That is key, especially that last bit. Random factors are necessary for regressi... | With so few data points, the answer will be almost entirely dictated by the prior (or implied equivalent). If the author has seen a lot this kind of data before, they may well have good reason to think their answer is more likely to be correct, given their past observations. I think it's a stretch to suggest this is an... | https://stats.stackexchange.com |
132,575 | [
"https://electronics.stackexchange.com/questions/132575",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/36498/"
] | I am doing a project on electrical load prediction using machine learning. Machine learning involves creating models based on past data. For creating these models where can I get data regarding electrical measurements of Wider Chicago substation?
If you provide any links it will be very helpful.
| If you zoom the datasheet, you can see that the diagrams actually recommends the use of a 2.2uH inductor!
<img src="https://i.stack.imgur.com/3TknO.png" alt="enter image description here" />
Although, it seems that Open Parts Library doesn't have these inductos either (but you can buy them cheap from Digikey, Mouser or... | For switch-mode power supplies (SMPS) inductor <strong>current rating</strong> and <strong>current saturation</strong> are key parameters. The inductance value won't matter too much, if you use anything from 1uH to 10uH it will probably work depending on the ripple voltage you can tolerate. I'd stick with 2.2uH just to... | https://electronics.stackexchange.com |
61,180 | [
"https://mathoverflow.net/questions/61180",
"https://mathoverflow.net",
"https://mathoverflow.net/users/12018/"
] | Hello Everyone,
I am not sure if this question is okay for this site, in case its not feel free to close it.
However, I would love to have it answered. Here goes my question.
<blockquote>
A graph $G=(V,E)$ has a perfect matching if and only if for every $U \subseteq V$ the number of connected components with an odd... | One way to come up with the characterization is to just run Edmond's matching algorithm (which is pretty natural). At the end of the algorithm, a matching $M$ and a subset of vertices $U$ has been explicitly constructed that gives equality in the Tutte-Berge formula (which implies Tutte's theorem). Of course, this is... | To make the condition seem natural, let's just try to copy Hall's Condition. So what's the first thing we do? We grab some set of vertices. In Hall's Condition this set is often called $X$ and we take it inside one of the partite sets, but we will call it $U$ and choose it arbitrarily since there there isn't any obviou... | https://mathoverflow.net |
617,167 | [
"https://physics.stackexchange.com/questions/617167",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/169685/"
] | <h2>A Problem from Feynman's Path Integral Book</h2>
<hr />
Let <span class="math-container">$x_i$</span> be coordinates at different time instances, prove that
<span class="math-container">$$
\langle\chi|m\frac{x_{k+1}-x_k}{\epsilon}m\frac{x_k-x_{k-1}}{\epsilon}|\psi\rangle=\int\int \chi^* \hat{p}\hat{p}\psi dxdy=-\hb... | I think your problem is simply not keeping track of the time differences
for the states. That is using the notation that <span class="math-container">$t_k$</span> is the time when
<span class="math-container">$x_k$</span> operates and Feynman and Hibbs notation that they used to write their Eq. 7.95
just above their st... | As the previous answer mentioned, there is a misprint in the problem. So the Eqn(1) in OP should really be written as
<span class="math-container">$$
\langle\chi|m\frac{x_{k+1}-x_k}{\epsilon}m\frac{x_k-x_{k-1}}{\epsilon}|\psi\rangle=-\hbar^2\int\chi^*(x,t)\frac{\partial^2}{\partial x^2}\psi(x,t) dx\tag{1*}.
$$</span>
A... | https://physics.stackexchange.com |
15,722 | [
"https://electronics.stackexchange.com/questions/15722",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/17697/"
] | What is the algorithm used in charging lipo batteries?
From watching my charger, I see that the amperage is close to the maximum specified (i.e. what I keyed in) at the beginning and gets smaller as each cell nears its maximum charge, but I haven't noticed a direct correlation between the cell voltage and input ampera... | Constant current of x amps (x is usually a fraction or multiple of capacity, e.g. 0.5C, 1C, 2C. Usually never any higher than 5C or cell heating results) until 4.2V ±0.5% is reached. This is called the "CC" stage.
Then a constant voltage of 4.2V ±0.5% is applied until charge current drops below a specific current, usu... | 1: trickle charge until approximately 3 V per cell (for really flat cells),
2: constant current charge until 4.2 V per cell,
3: constant voltage charge until full.
actual values may vary dependent on cells - refer to manufacturers datasheet.
This is fine for upto two cell batteries. Greater than two cells and some c... | https://electronics.stackexchange.com |
45,153 | [
"https://stats.stackexchange.com/questions/45153",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/17488/"
] | I have a sample dataset with 31 values. I ran a two-tailed t-test using R to test if the true mean is equal to 10:
<pre><code>t.test(x=data, mu=10, conf.level=0.95)
</code></pre>
Output:
<pre><code>t = 11.244, df = 30, p-value = 2.786e-12
alternative hypothesis: true mean is not equal to 10
95 percent confidence in... | Use <code>pt</code> and make it two-tailed.
<pre><code>> 2*pt(11.244, 30, lower=FALSE)
[1] 2.785806e-12
</code></pre>
| I posted this as a comment but when I wanted to add a bit more in edit, it became too long so I've moved it down here.
<strong>Edit</strong>: Your test statistic and d.f are correct. The other answer notes the issue with the calculation of the tail area in the call to <code>pt()</code>, and the doubling for two-tails,... | https://stats.stackexchange.com |
36,010 | [
"https://dsp.stackexchange.com/questions/36010",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/1684/"
] | Im studying the DSP book by Steven W. Smith. On page 41 he covers the Nyquist frequency. He makes up an example by writing the following.
<blockquote>
... consider an analog signal composed of frequencies between DC and 3kHz.
</blockquote>
What does that mean? My guess is that in this context DC is equivalent to ze... | You are right, DC means zero frequency. The term probably originates from electrical engineers' use of the terms DC and AC.
| When the current is "direct", it is basically a constant with no change over time, therefore having zero frequency. Alternating current on the other hand, changes over time and therefore having nonzero frequency components
| https://dsp.stackexchange.com |
83,018 | [
"https://electronics.stackexchange.com/questions/83018",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/24838/"
] | What is the difference between Buried Photodiode and Pinned Photodiode?
I understand that the P+/N/P structure where the P+ and P layers have the same potential is the Pinned Photodiode.
So what is the buried Photodiode?
| This is a commonly misunderstood misused set of terminologies.
First off these are not PIN Photodiodes - which stands for P - Intrinsic- N. These have large depletion regions for higher internal QE (Quantum Efficiency) and faster response. You can't make an array with this design though.
Pinning, refers to fermi-l... | The first Pinned PD was not invented by Teranishi at Sony. Teranishi was not in Sony. He was in NEC.
The first Pinned PD, in the form of P+NP sensor element on Nsub structure with the N layer floating for complete charge transfer, was invented and filed in Japanese patent by Hagiwara at Sony in 1975, and was used in ... | https://electronics.stackexchange.com |
4,634,281 | [
"https://math.stackexchange.com/questions/4634281",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/1132073/"
] | Solve <span class="math-container">$\dfrac{25-\frac{x}{3}}{x+1}+\dfrac{16x+4\frac{1}{5}}{3x+2}=5+\dfrac{23}{x+1}$</span>
<span class="math-container">$\Rightarrow \dfrac{25}{x+1}-\dfrac{x}{3(x+1)}+\dfrac{16x}{3x+2}+\dfrac{21}{5(3x+2)}=5+\dfrac{23}{x+1} \ \ \ ...(1)$</span>
<span class="math-container">$\Rightarrow \dfr... | <span class="math-container">$\dfrac{25-\frac{x}{3}}{x+1}+\dfrac{16x+4\frac{1}{5}}{3x+2}=5+\dfrac{23}{x+1}$</span>
Multiplicate <span class="math-container">$15(x+1)(3x+2)$</span> to each sides
<span class="math-container">$\Rightarrow 5(75-x)(3x+2)+3(80x+21)(x+1)=75(x+1)(3x+2)+345(3x+2)$</span>
<span class="math-conta... | I would start simplifying in the following way:
<ol>
<li>Move the last term from the right hand side to the beginning
<span class="math-container">$$\frac{25-23-\frac x3}{x+1}+\frac{16x+\frac{21}5}{3x+2}=5$$</span></li>
<li>Notice that some fraction can be simplified <span class="math-container">$$\frac{ax+b}{cx+d}=\fr... | https://math.stackexchange.com |
151,573 | [
"https://cs.stackexchange.com/questions/151573",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/46385/"
] | I'm reading CLRS and there is something I don't understand regarding counting the number of parenthesization, in the Matrix-chain multiplication chapter, the book says:
<blockquote>
Denote the number of alternative parenthesizations of a sequence of <span class="math-container">$n$</span> matrices by <span class="math-... | The split in a product is between the two outermost pairs of parantheses. For example, in <span class="math-container">$((a*b)*(c*d))*(e*f)$</span>, the split is between the <span class="math-container">$d$</span> and <span class="math-container">$e$</span> because the last multiplication that is performed is between t... | Suppose you have <span class="math-container">$n$</span> matrices <span class="math-container">$M_1, M_2, …, M_n$</span>.
The product of all matrices is <span class="math-container">$M_1\times M_2\times…\times M_n$</span> and since the matrix product is associative, there is no need for parentheses from a mathematical ... | https://cs.stackexchange.com |
129,806 | [
"https://stats.stackexchange.com/questions/129806",
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] | This is a self-study question. The name of the book is called: Applied Statistics and Probability for Engineers by Montgomery and Runger. This problem is on page 73. It's exercise 3-41.
The entire problem is listed as the following:
Given the following cumulative distribution function:
$$
F(x)=\left\{\begin{matrix}... | The <em>cumulative</em> probability distribution function $F_X(x)$ tells us <em>how much</em>
probability mass there is to the <em>left</em> of $x$ or <em>at</em> $x$ for each $x$
on the real line. (The choice of notation, though almost universally used
is truly dreadful for use in a classroom setting! How on earth doe... | Since this question relates specifically to a <em>discrete</em> random variable (as your book says...), I will answer it as such. Dilip Sarwate's answer provides the general result for a discrete random variable.
With those health warnings, the cumulative distribution function (CDF), $F$, between $x=-10$ and $x=30$ do... | https://stats.stackexchange.com |
189,062 | [
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] | So I have a least squares model which tries to identify changes in income due to different levels of experience, education, etc.
But in my model the dependent variable is 'income' and among the independent variables i have 'experience' en the square of 'experience'. When asking the summary of the model, both variables... | The point is that experience ($E$) and experience-squared ($E^2$) are not 'independent'. You seem to reason as if they are: you say that the coeffcient of $E$ is positive while the one of $E^2$ is negative and the latter seems strange to you.
So your reasoning seems to be that, if $E^2$ increases by one unit then th... | Depending on the magnitudes of the coefficients (which you did not provide) the fitted curve may be ever-increasing over the range of your data. The answer comes not in examining the two coefficients but in plotting the fitted curve. And if a quadratic fit is adequate (often it is not as good as using a regression sp... | https://stats.stackexchange.com |
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