qid int64 1 4.65M | metadata listlengths 3 3 | prompt stringlengths 31 25.8k | chosen stringlengths 17 28.2k | rejected stringlengths 19 40.5k | domain stringclasses 28
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|---|---|---|---|---|---|
84,207 | [
"https://chemistry.stackexchange.com/questions/84207",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/42344/"
] | The formula for the work done by adiabatic expansion of gas is
<span class="math-container">$$W = C_V\,\mathrm dT.$$</span>
Why the <em>volumetric</em> heat capacity <span class="math-container">$C_V$</span> is constant and not the heat capacity at constant pressure <span class="math-container">$C_p?$</span> If the sys... | Pretty simple, <span class="math-container">$\Delta U = nC_V\Delta T$</span> no matter what the process is. This is called a <em>state function</em>, i.e. it is independent of the path taken and only depends on the final and initial conditions here initial and final temperature.
However, work done is path dependent, so... | Part 1. Let's consider first law of thermodynamics.
$\delta Q = dU + \delta W$ (classical sign convention)
or
$\delta Q = dU - \delta W$ (IUPAC sign convention)
In the case of adiabatic process, $\delta Q = 0$.
$0 = dU - \delta W$
$dU = \delta W$
$dU = C_VdT$ (by definition)
$C_VdT = \delta W$
After integratio... | https://chemistry.stackexchange.com |
137,319 | [
"https://electronics.stackexchange.com/questions/137319",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/35894/"
] | Now I have ten accelerator sensors (ADXL345) connected to only one PIC16F887 using I2C.
The problem is ADXL345 only has 2 I2C slave addresses configurable. So it brings conflict.
I know NXP does provide a I2C multiplexer that work around this kind of issue, but I have ten sensors. So it seems of no help.
| Ten ADXLs would be 5 pairs with the ALT_ADDRESS pin.
NXP do 2, 4 and 8 channel I2C switches, so you could use an 8-channel switch, which would allow you up to 16 ADXLs.
Alternatively, you could roll your own switch in an ASIC or CPLD.
A third option would be to ditch I2C and use the SPI interface on the ADXL. Thi... | While it would probably be possible to kludge together some additional hardware that would only let one I2C device see each transaction, doing that without relying upon any undocumented behaviors may more work than using SPI. It might be possible to use a one-of-ten selector to drive the address-select wires on the de... | https://electronics.stackexchange.com |
63,591 | [
"https://cs.stackexchange.com/questions/63591",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/40972/"
] | I know that sorting the array take O(logn) time. But why finding the minimum of elements take O(n) time, which is more expensive? If I sort the array, I simply output the first element and this would be my minimum.
| Sorting the array using comparison-based method takes $\Omega(n \log n)$. So sorting is obviously more expensive than finding minimum in $\mathcal O(n)$.
In fact finding minimum or sorting takes $\Omega(n)$ to at least read the whole array. For finding minimum it is actually $\Theta(n)$.
If it were true that sorting ... | I think there is no possible way a sorting algorithm can be less than O(n). Best performance we generally get is O(nlogn).
Also, In case of finding the minimum in an array It should be O(n) at least, unless you have more data about the numbers in array. Because you need to go through all the numbers in array at leas... | https://cs.stackexchange.com |
1,800,056 | [
"https://math.stackexchange.com/questions/1800056",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/229023/"
] | How do we solve this given $f'(0)=-1$. It does not look separable. I can integrate both sides but end up with a functional equation with is not helpful.
| I will assume that $f:\mathbb{R}\to\mathbb{R}$ is supposed to be $C^1$, so $f'$ exists and is continuous everywhere. Now note that for any $x\in \mathbb{R}$, $f'(x)=f'(x/2)=f'(x/4)=f'(x/8)=\cdots$. But $x/2^n$ converges to $0$ as $n\to\infty$, so continuity of $f'$ now implies $$-1=f'(0)=\lim_{n\to \infty}f'(x/2^n)=f... | Suppose $f$ is differentiable everywhere on $\mathbb R$ and $f'(x) = f'(x/2)$ for all $x$.
Now $$\dfrac{d}{dx} \left(f(x) - 2 f(x/2)\right) = f'(x) - f'(x/2) = 0$$
so $f(x) - 2 f(x/2)$ must be constant, say $f(x) - 2 f(x/2) = c$. $f$ is continous at $0$ since it is differentiable there, and taking the limit as $x \to... | https://math.stackexchange.com |
38,178 | [
"https://quant.stackexchange.com/questions/38178",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/30652/"
] | I'm studying the pricing of a Double-Barrier binary option on the price of $S$. By this I mean an option that pays $X$ at maturity $T$ if the lower ($H1$) or upper barriers ($H2$) are not hit during the lifetime of the option.
I was told that the valuation could be done by subtracting an up-and-in-cash(at expiry)-or-... | Assume that $H_1 < S_0 < H_2$. let
\begin{align*}
\tau_1 = \inf\{t: \, t>0 \text{ and } S_t \le H_1 \},
\end{align*}
and
\begin{align*}
\tau_2 = \inf\{t: \, t>0 \text{ and } S_t \ge H_2 \}.
\end{align*}
Then, the option payoff is defined by
\begin{align*}
X\, \mathbb{I}_{\{\tau_1 >T\}} \mathbb{I}_{\{\ta... | If you go short a knock-in call on the upper barrier and at the same time go long a knock-out call on the lower barrier, you basically replicate the lower barrier in the double barrier option scenario. But if your long short portfolio hits the upper barrier, profit is locked in at a constant level similiar to a bull sp... | https://quant.stackexchange.com |
415,394 | [
"https://softwareengineering.stackexchange.com/questions/415394",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/182037/"
] | When writing a library (eg. for dealing with JPEG or PNG files), the first thing library author needs to provide is an abstraction over FILE* (to allow also reading from plain memory, pipe, socket, http...).
Thus something like this:
<pre><code>#include <stdbool.h>
#include <stddef.h>
typedef long long off... | Assuming that you really need this counter, does it really have to be constantly updated <em>every</em> time a user sends a message? Could you use a more "granular" counter which increments every time the user sends 10 messages instead? If so, you just reduced write load by 90%!
Another idea would be to keep ... | I'd recommend to first start by doing the simplest thing of just incrementing a simple counter. For most applications, this is probably going to be sufficient and simple to implement.
If you really need to scale this though, the number of database writes are not the issue. The problem with implementing a shared counter... | https://softwareengineering.stackexchange.com |
17,885 | [
"https://physics.stackexchange.com/questions/17885",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/5245/"
] | I need to make a camera lens that can easily withstand temperatures of about 1000 Fahrenheit.
What should I make the lens out of?
| Fused silica should be ok at this temp. Sapphire will also be fine. It seems unlikely that the correct design is to have your lenses at this temperature though, can you share more about your problem?
| Anything which has an index of refraction greater than that of the surrounding material (air, I suppose) can be used to make a lens. You have to adjust the shape of the lens to accomodate the index of refraction of whatever material you use, but there are known formulas for doing that.
| https://physics.stackexchange.com |
121,754 | [
"https://mathoverflow.net/questions/121754",
"https://mathoverflow.net",
"https://mathoverflow.net/users/23015/"
] | Say, we have a semimagic square $X$, that is, an $n\times n$ square matrix with entries from natural numbers, such that each row and column of it sums up to the same natural number $s$.
Let $M$ be a set with $s$ elements.
To each $X_{ij}$, we assign a subset $M_{ij}$ of $M$ with $X_{ij}$ elements.
<strong> Question: <... | Dylan, I am afraid that you are missing something. It is true that each element of the group gives a map of spectral sequences. But look at the fibers and my comment about isotropy groups above. If you construct the Serre spectral sequence in the usual way, there is a non-equivariantly inconsequential choice of base... | <strong>Personal comment:</strong> I feel that the two previous answers may be together creating some confusion on the subject of the question. I wish to address that with my answer, which would be more suited as a comment, were it not so long. I certainly hope my contribution will not result in even more confusion.
$\... | https://mathoverflow.net |
58,793 | [
"https://quant.stackexchange.com/questions/58793",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/50515/"
] | I am stuck on how a firm can calculate whether a project can generate positive NPV. In this scenario I will be assuming an efficient market hypothesis.
My lecturer said that if a project has IRR 12%, and the cost of capital is 10%, then you will generate positive NPV.
Though, I don't see why. Since regardless of the pe... | <blockquote>
My lecturer said that if a project has IRR 12%, and the cost of capital is 10%, then you will generate positive NPV.
</blockquote>
Let's take a concrete example: <br>
<pre><code>Project Borrow (at 10%) Invest (at 12%) Net cashflow
Period 0 + $100 (inflow) - $100 (outflow) $0
Period 1 - $110 (... | I think the confusion arises from the definition of break-even point. Given a set of cash flows the IRR is the rate such that negative and positive cash flows balance out. If you were to use this rate for discounting, the NPV would be <span class="math-container">$0$</span>. However, there is no reason to use the IRR f... | https://quant.stackexchange.com |
26,667 | [
"https://electronics.stackexchange.com/questions/26667",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/2968/"
] | I have a board design using a Atmel AT90USB1286 (VQFN64) and I placed vias on the back of the chip for routing purposes. Upon inspection, I found that the AT90USB1286 Chip has a thermal pad on the back of the chip and the vias were getting shorted to them.
Is there a way to prevent the shorting of the vias and still ... | Typically thermal pads on chips need a connection to copper to dissipate the heat. Normally these thermal pads have recommended shapes and can often be larger than the IC itself. The IC's footprint probably needs changing.
To salvage the prototypes, a hack would be to cover the vias before placing the IC.
You co... | You shouldn't be putting tracks and vias under a QFN device, like that. The pad should be soldered, otherwise the chip could shear off if the board is subjected to mechanical shock. With many devices, it should be grounded.
From the data sheet:
"The large center pad underneath the MLF packages is made of metal and in... | https://electronics.stackexchange.com |
317,042 | [
"https://dba.stackexchange.com/questions/317042",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/166982/"
] | Recently I setup Forward Data Wrapper. Here is what I do:
<pre><code>CREATE SERVER foreign_server FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host '192.168.125.1', port '5432', dbname 'template1 ');
IMPORT FOREIGN SCHEMA public LIMIT TO (tbl_test) FROM SERVER foreign_server INTO public;
</code></pre>
Create a user on ... | That is not supported. You can either create a user mapping for the user itself or for <code>PUBLIC</code>, that is, for everybody. These are your choices. A user mapping created for a role will not be used for a member of that role. That is working as designed.
| you have to change of role of group before;
set rote demorole1;
select * from tbl_test;
| https://dba.stackexchange.com |
50,948 | [
"https://mechanics.stackexchange.com/questions/50948",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/33324/"
] | Why it is said that DTC (Diagnostic Trouble Code) should not be checked when the engine is running?
| In the two links you provided in comments, it states to plug in your code reader, turn the ignition on, but don't start the engine, then read the codes. It's not telling you "cannot run the engine" while reading the codes, it's just saying not to do it to read the codes at first.
There should be no issue using your c... | I have read my code reader instructions yesterday and it mentions that the DTC's can be read with the ignition on with the engine running or not - handy as I had the engine running at the time...
What it did say was that you should not connect the reader with the engine already running - which I found out after I had ... | https://mechanics.stackexchange.com |
394,363 | [
"https://math.stackexchange.com/questions/394363",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/44695/"
] | I'm trying to solve question 2.2.9(b) in Hatcher's Algebraic Topology.
<strong>Question:</strong> Calculate the homology groups of $X=S^1 \times (S^1 \vee S^1)$.
<strong>My attempt:</strong> I try to use the Mayer-Vietoris sequence.
Let $A=S^1 \times (S^1 \vee \text{small bit}) =S^1\times S^1$ and $B=S^1 \times ( ... | Firstly, I believe that $H_1(\mathrm{Torus})=\mathbb{Z}^2$ hence the sequence should be
$$0 \xrightarrow 0 \mathbb{Z}^2 \xrightarrow f \tilde{H}_2(X) \xrightarrow g \mathbb{Z} \xrightarrow h \mathbb{Z}^2 \oplus \mathbb{Z}^2 \to \tilde{H}_1(X) \to 0.$$
Now, you need to determine at least one map to solve this. In Mayer ... | This solution relies on the long exact reduced homology sequence of a NDR pair (Hatcher's Theorem $2.13$), and provides different approach to the problem from @tsho's solution.
Let us call $$\underbrace{\Huge{\mathsf x} \normalsize\times S^1}_{A}~~\subset~~
\underbrace{\Huge{\propto}\normalsize\times S^1}_{B}~~\subset... | https://math.stackexchange.com |
54,145 | [
"https://dba.stackexchange.com/questions/54145",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/28954/"
] | I happen to be setting up a reporting server at work from scrach. Having very little almost no experience of sql server administration I am finding it really difficult to configure the security settings for the Reports Manager. <br/>
I do understand all the users who will be accessing Reports Manager will need permissi... | Normally in SSRS you would use the SSRS management interface (/ReportsManager IIRC) to set user roles/perms by folder. They are normally inherited by subfolders though you can override that. The role 'browser' is most appropriate for end users as it allows them to run / view reports but not much more. If you grant a us... | <blockquote>
1 – Permission to Access Data from sql server. (for which after some research I have decided to make use of datareader role.
</blockquote>
When you create your report in SSRS you want to set up the data source with an account with adequate privileges within the database to run the report (In your data s... | https://dba.stackexchange.com |
41,085 | [
"https://scicomp.stackexchange.com/questions/41085",
"https://scicomp.stackexchange.com",
"https://scicomp.stackexchange.com/users/42309/"
] | to perform adaptive refinement in the finite element method according to the explicit residual method, the quantity
<span class="math-container">$$\eta_K^2=h_K^2\left\lVert r\right\rVert_{L_2(K)}^2+h_K\left\lVert R\right\rVert_{L_2(\partial K)}^2$$</span>
must be calculated for all elements of the mesh. For the equatio... | Let <span class="math-container">$\mathbf{x} = \mathbf{F}(\hat{\mathbf{x}})$</span> be the mapping from the reference cell to the real cell. Then your solution at a point <span class="math-container">$ \mathbf{x}$</span> is given by
<span class="math-container">$$
u_h( \mathbf{x}) = \sum_j U_j \varphi_j( \mathbf{x}) ... | assume <span class="math-container">$(r,s)$</span> and <span class="math-container">$(x,y)$</span> are the coordinates in the reference and actual element, respectively. the shape functions for mapping and interpolation are defined as
<span class="math-container">$$
\mathbf{M}=
\begin{bmatrix}
M_1(r,s) & M_2(r,s) &... | https://scicomp.stackexchange.com |
2,713,749 | [
"https://math.stackexchange.com/questions/2713749",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/373378/"
] | A=(3, 1, 1), (2, -4, 10), (-1, 3, -7)
and b=(2, -2, 5)
I found that A*A^T is not invertible so Ax=0 and that therefore e is equal to b, but the answer is not correct. I just can't find where I made a mistake
| $$\lim_{x\to\infty} (c/x)^x = 0$$
So your limit becomes 0.
If you meant this instead:
$$\lim_{x\to\infty} 1-(1-\frac{c}{x})^x$$
Then it is
$$\lim_{x\to\infty} 1-((1-\frac{c}{x})^{\frac{x}{-c}})^{-c}$$
$$=1-e^{-c}$$
| <strong>HINT</strong>
The limit indicated in the OP is well defined for $c\ge 0$, and it suffices to prove that
$$\left(\frac c x \right)^x\to 0$$
indeed for $x>2c \implies \frac c x<\frac12$ and
$$0\le \left(\frac c x \right)^x\le \left(\frac12 \right)^x \to 0$$
For the following
$$\lim_{x\to\infty} 1-\le... | https://math.stackexchange.com |
685,066 | [
"https://physics.stackexchange.com/questions/685066",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/323663/"
] | From the little I know about physics and conduction specifically, it seems to be that a system in which the heat transfer is done solely by conduction, cannot reach thermal equilibrium.
I'll preface this by stating what I think I know. Convection is the transfer of heat via atoms or molecules, meaning this is only pres... | In my judgment, your concept of convection is a bit flawed. What happens with convection in fluids is that the conduction that is present in both fluids and solids (via collisions of molecules) is enhanced in the case of fluids by allowing colder regions of fluid to be brought into closer proximity to hotter regions o... | Given enough time, the thermal energies of neighbouring atoms/molecules will eventually become equal (or as close to equal as you like) and the system will reach thermal equilibrium. This just takes a longer time if the only available method of heat transfer is conduction (which is why oven gloves work).
| https://physics.stackexchange.com |
190,712 | [
"https://physics.stackexchange.com/questions/190712",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/4435/"
] | using the eigenvstates of the Harmonic oscillator could we give a meaning to the Hamiltonian
$$ H=\log(a.a^{+}+1) $$
here $ a$ and $ a^{+}$ are the creation/anihilation operators with commutation rules $ [a,a^{+}]=1$ are the energies of the Hamiltonian $$ E_{n}=\log(n+1) $$ for $ n\ge 0$. The idea is that the partiti... | The coefficient of static friction and the normal force together allow you to calculate the maximum force that friction can apply, not the actual force.
A block sitting on a shelf with no external horizontal forces will have a friction force of zero. If you apply a force from the side that is less than the maximum fr... | The thing is that you cannot measure friction between the tires of the car and the road and then calculate its speed.There are two kind of friction. The first is the static friction and the other is kinetic friction. the maximum static friction is measured by
$f_s \le f_{smax}\left( = \mu_s mg\right) $
In order for ... | https://physics.stackexchange.com |
421,865 | [
"https://mathoverflow.net/questions/421865",
"https://mathoverflow.net",
"https://mathoverflow.net/users/169890/"
] | It is known that genus one fibred knots are two trefoils and the figure-eight knot. Is there any characterization of the knot <span class="math-container">$5_2$</span>? Specifically, is there any other genus one knot that shares the same Alexander polynomial <span class="math-container">$2t^2-3t+2$</span> with <span cl... | Ian Agol, in the comments says:
<blockquote>
Yes, there should be plenty. Think of the Seifert surface for the 5_2
knot as a disk with two strips (1-handles) attached. By tying knots
into the strips (with zero framing so as not to change the linking
form), you can obtain many knots with a genus 1 Sefert surface with
th... | For the prime knot example, the first in the tables is <span class="math-container">$15n43522$</span> with diagram:
<img src="https://i.stack.imgur.com/1ioLlm.png" alt="knot diagram" />
| https://mathoverflow.net |
209,289 | [
"https://softwareengineering.stackexchange.com/questions/209289",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/100079/"
] | Is state another name for field or is it something different?
For example:
<pre><code>private int telNumber;
</code></pre>
can I say about telNumber that it is a state or it is a field?
| Let's say I have the following class:
<pre><code>public class Foo
{
private int _bar;
private string _baz;
public Foo(int bar, string baz)
{
_bar = bar;
_baz = baz;
}
public void ChangeBar(int newBar)
{
_bar = newBar;
}
}
</code></pre>
I can create an instance... | <strong>In layman's words:</strong>
<ul>
<li>No. State is not another name for a field.</li>
<li>The state of an object is the combination of the values of all its fields.</li>
<li>If any field gets changed, it is said that the object's state (as a whole) has changed.</li>
</ul>
| https://softwareengineering.stackexchange.com |
363,228 | [
"https://stats.stackexchange.com/questions/363228",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/189617/"
] | On the Wikipedia article on the ARMA model, its derivation is simplified as a combination of the AR and MA models:
<h3>AR</h3>
<blockquote>
$$
X_t = c + \sum_{i=1}^p \varphi_i X_{t-i} + \varepsilon_t
$$
</blockquote>
<h3>MA</h3>
<blockquote>
$$
X_t = \mu + \varepsilon_t + \sum_{i=1}^q \theta_i \varepsilon_{t-i}... | It's much easier to do this "derivation" with lag operator:
$$Lz_t=z_{t-1}$$ and
$$\phi(L)=1+\sum_{i=1}^q\phi_iL^i$$
$$\theta(L)=1-\sum_{i=1}^q\theta_iL^i$$
This way MA(q) is $$x_t=\phi(L)\varepsilon_t=c+\varepsilon_t+\sum_{i=1}^q\phi_i\varepsilon_{t-i}$$ and AR(p) is $$\theta(L)x_t=x_t-\sum_{i=1}^p\theta_ix_{t-i}=c$... | No, there is no way to explain this discrepancy because it does not make sense to sum together an AR and MA model in the way you have, because $X_t$ cannot be both an AR model and an MA model at the same time. If you write down
$$
X_t = \mu + \varepsilon_t + \sum_{i=1}^q \theta_i \varepsilon_{t-i}
$$
with $q$ finite, ... | https://stats.stackexchange.com |
23,120 | [
"https://softwareengineering.stackexchange.com/questions/23120",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/1326/"
] | If you were looking into a job applicant's background and discovered that he or she has 1200 followers on Twitter and averages 50 tweets per day (more than half of which are during business hours), would it affect your hiring decision? How and why?
Personally, I'd be a little worried about the candidate's ability to f... | There's a good chance you shouldn't hire this person. If you've got to the stage of considering whether their Twitter profile is good enough reason to reject them, then I suspect you have some fairly serious doubts.
| I would not hold it against them, but I might bring it up during an interview question. Perhaps say you noticed they average X tweets per day during business hours and that your ideal candidate focuses on the job while at work. See what their reaction/excuse to that is.
Personally, if I know my boss is watching my tw... | https://softwareengineering.stackexchange.com |
229,046 | [
"https://mathoverflow.net/questions/229046",
"https://mathoverflow.net",
"https://mathoverflow.net/users/2481/"
] | Does there exist a prime $p$ and a smooth genus 2 curve $C / \mathbf{F}_p$ such that the characteristic polynomial of Frobenius on the Tate module of $J(C)$ is given by $(T^2 - p)^2$?
More generally, for a curve of arbitrary genus, it possible that both $\sqrt{p}$ and $-\sqrt{p}$ can occur as eigenvalues of the Froben... | In its action on the Tate module, $\operatorname{Frob}_q$ is an element of $GSP_{2g}(\mathbb Q_\ell)$ whose action on the symplectic form is multiplication by $q$. This is due to the Weil pairing, or Poincare duality for etale cohomology.
Every such matrix has eigenvalues $\lambda_1, \dots, \lambda_{2g}$ whose with $\... | After thinking about this a bit more, I realised that no such example exists. Let $q = p^{2}$ (or $p^{2f}$ for any integer $f$), let $C$ be a curve over $\mathbf{F}_q$, and let $J = J(C)$. The Frobenius must be compatible with the Weil pairing $T_\ell J \times T_\ell J \to \mathbf{Z}_\ell(1)$, and Frobenius acts as mul... | https://mathoverflow.net |
1,280,770 | [
"https://math.stackexchange.com/questions/1280770",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/162343/"
] | I want to figure out why the set of zeros of the function $g:\mathbb{R}^{2} \to \mathbb{R}$ defined as $g(x,y) = x^2 - y^2$ is not a differentiable manifold. So what I want to use is the following result:
Let $A \subset \mathbb{R} $ open, $p<n$, $g:A \to \mathbb{R}^p$ of class $C^1$ such that $g'(x)$ has rank $p$ ... | Here is a "non-topological" proof.
The set $M$ is simply the union of two lines $M = L_1 \cup L_2$ where $L_1$ is the line $y=x$ and $L_2$ is the line $y=-x$.
Arguing by contradiction, let's assume that $M$ is a differentiable manifold. It's dimension must equal $1$, because $M-\{(0,0)\}$ clearly <em>is</em> a diffe... | Take any neighborhood of the origin, we want to show it isn't diffeomorphic to the open ball $B^n$. The standard arguments from topology are:
<ol>
<li>If we delete the origin, we disconnect the neighborhood. No point in $B^n$ has this property for $n \geq 2$.</li>
<li>If we delete the origin, there are at least 4 co... | https://math.stackexchange.com |
16,138 | [
"https://scicomp.stackexchange.com/questions/16138",
"https://scicomp.stackexchange.com",
"https://scicomp.stackexchange.com/users/11587/"
] | We know that principle component analysis (PCA) is a eigenvalue problem. Let $A$ be the covariance matrix of $X$, PCA aims to find the eigenvalue of $A$:
$\max v'Av$, subject to $v'v=1$
Multiple PCs can be found by deflation:
Let $\hat{X}=X-\sum_{i=1}^k v_iv_i'X$ (or equivalently $\hat{A}=(I-\sum_{i=1}^k v_iv_i')A(... | Numerical diffusion arises from a first-order finite difference approximation to the spatial derivative $\partial u/\partial x$. To see how this is the case, examine the Taylor series expansion for $u_{i+1}$:
$$
u(x_{i+1}) = u(x_{i}) + \left.\frac{\partial u}{\partial x}\right|_{x_i} (\Delta x) + \frac{1}{2} \left. \f... | Although this equation does not have diffusion, the numerical algorithm for solving the advection equation inherently has a certain diffusion. In fact, there is no numerical method without numerical diffusion. Some algorithms have more numerical diffusion, some less, and there are methods to constrain the spread of dis... | https://scicomp.stackexchange.com |
74,732 | [
"https://datascience.stackexchange.com/questions/74732",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/96722/"
] | I have a two class classification problem and my neural network prior to training predicts with an accuracy of 80%. After training i have an accuracy of 75%. Can you tell me how this is possible?
| If your class imbalance is roughly 4:1, then a network that always predicts the more frequent class would have an accuracy of 80%. (That's why accuracy alone is an insufficient metric: for binary classification, you should use ROC and PRC curves. There are also more holistic scalar metrics such as MCC)
Let's say your p... | It is possible that your model does worse than a random guess for a number of reasons. Some of those could be
<ul>
<li><strong>noisy data</strong>: your model focuses to noise in the dataset rather than features that have predictive value. May be worth inspecting your dataset and look for relevant issues and class imb... | https://datascience.stackexchange.com |
636 | [
"https://mathoverflow.net/questions/636",
"https://mathoverflow.net",
"https://mathoverflow.net/users/126667/"
] | Let Cat denote the 1-category of small categories. The functor Mor : Cat -> Set which assigns to a category its set of morphisms (aka Hom([• -> •], -)) does not commute with most colimits. Does it commute with quotients by free group actions? In other words, if C is a small category and G is a group acting... | OK, I found a "high-tech" argument which is maybe more convincing.
The external input to the argument is that if X -> Z <- Y is a diagram of G-sets with G acting freely on Z (and hence on X and Y) then (X x<sub>Z</sub> Y)/G = (X/G) x<sub>(Z/G)</sub> (Y/G). This is pretty clear because we can always write our origi... | The answer is yes, provided your action has the following data that you didn't specify:
<ul>
<li>For any g in G and any f: a -> b, an assignment gf: ga -> gb, in a way that strictly respects multiplication in G (including 1f = f).</li>
</ul>
If you don't say what G does on morphisms, I don't see how the question make... | https://mathoverflow.net |
657,409 | [
"https://electronics.stackexchange.com/questions/657409",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/183358/"
] | I have 2 pack x ~900 Wh (each consisting of 24s2p LG M50LT cells). I’m planning to buy 3rd pack - but dealer have only packs of Samsung 50E cells. Is it ok to connect them in parallel to increase total capacity? They are nearly the same capacity, but 50E cells have 500 cycles, M50LT have 1000 cycles. Also existing pack... | That is not unusual for a FET with a gate left floating. You can expect anything to happen when gate is left unconnected.
Don't leave the gates unconnected.
The gate may have accumulated charge so there is enough gate voltage to turn the FET on.
So basically, contrary to what you say, the FET is not without gate voltag... | <blockquote>
<em>Are both of my mosfets damaged? or am I missing something crucial?</em>
</blockquote>
A floating gate will inevitably acquire charge because it's basically just a capacitor. Once a volt or two has become charged on the gate with respect to source, the MOSFET will begin to switch on. A few more volts of... | https://electronics.stackexchange.com |
81,794 | [
"https://security.stackexchange.com/questions/81794",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/27600/"
] | I hope this question is direct enough for the stackexchange format, my apologies if it is not, please feel free to downvote to close.
I'm interested in pursuing a CREST certification, however it looks to be a a chicken and egg problem - without certification it seems unlikely i'll be able to land professional experien... | You can pass the CREST exam on skill and knowledge alone. It is however very unlikely that someone without practical experience will be able to do so in the limited time allocated for the exam.
| Wireghoul's answer is spot on. There is no formal requirement for industry experience, but in practice, the only way to get good enough is to do it for a job.
A number of pen testing companies may offer you a junior position without certification. They would typically be looking for an IT background - either a degree,... | https://security.stackexchange.com |
209,192 | [
"https://mathoverflow.net/questions/209192",
"https://mathoverflow.net",
"https://mathoverflow.net/users/48006/"
] | Let $n\in \mathbb N$ be a natural number, $x_1,\cdots.x_n$ be formal variables. Consider the following $n\times n$-matrix $M_n:=diag\{x_1^2+\cdots+x_n^2,\cdots,x_1^2+\cdots+x_n^2\}$, can we find a solution $A=(a_{ij})$ such that
<ul>
<li>(Square root): $A^2=M_n$.</li>
<li>(Polynomial): $a_{ij}\in \mathbb F[x_1,\cdots... | As Will noted, if $n>1$, one must have $n$ even for there to be any solutions. (This holds even if the $a_{ij}$ are allowed to be formal power series in the $x_i$, since ${x_1}^2+\cdots+{x_n}^2$ is not a square even in this larger ring when $n>1$.) (Of course, the $n=1$ case is trivial, so we can set that aside... | As Will notes, this does not really work for odd $n \gt 1$ as the determinant of $M$ is not a square. On the other hand, if $n$ is even then the determinant of $M$ is a square.
Note that your solution to $A^2=M$ for $n=2$ has $A$ symmetric and thus is also a solution of $AA^t=M$. I think $AA^t=M$ is the more promisin... | https://mathoverflow.net |
213,246 | [
"https://stats.stackexchange.com/questions/213246",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/71334/"
] | I have two different feature vectors of completely different scale, which are to be used as training data for machine learning algorithm.
When I concatenate them,
should I scale and normalize them separately first and then concatenate?
Or should I concatenate them first and scale-normalize them together?
| This is too broad question and must be answered carefully. What are your features? If you have categorical features, you are better to encode it in one-hot manner and then you may circumvent scaling problems. Also, it depends on a classifier you want to use downstream : SVM will not tolerate badly scaled and uncentered... | As mentioned above you could train two separate classifiers on both the data. This is useful because say you have more faith in the CNN features than histogram based features then you give that classifier's prediction more weight in the final output. Once you concatenate the features you are unable to use make use of t... | https://stats.stackexchange.com |
1,660 | [
"https://dsp.stackexchange.com/questions/1660",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/1024/"
] | I am filtering an audio signal into various sub-bands in C and would like to decimate the signal by 2 in the lower frequency bands to reduce processing overhead. I get that this involves low-pass filtering the signal and then throwing away every other sample, but how do I go about designing the low-pass filter optimall... | You can determine the "best" filter by analyzing what type of errors is created and how much of each error your specific application can live with.
If your original content is already perfectly band limited, than you don't need a low pass filter at all and can directly throw away the extra samples.
The decimation low... | Every type of filter has its characteristics. For instance, Butterworth filter needs a higher order than Chevyshev, which implies that it is likely that it would computationally cost more. Other characteristics, such as the ripple in the stop filter (which implies different attenuation in those bands), the phase disto... | https://dsp.stackexchange.com |
110,426 | [
"https://softwareengineering.stackexchange.com/questions/110426",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/37312/"
] | What licensing issues arise if I install and use Microsoft Office software (in this case Visio) as part of my web service backend?
My company's flagship piece of software can convert Microsoft Visio files for use in their environment, but of course requires a local install of Visio to decode the files. The system I'm ... | If you have a single license per each web server that should be fine. Technically, your software/the website user identity is using the software.
So long as you don't distribute Visio through the site you are OK.
If you look through technet/MSDN, there are many articles about automating office on the server side, so ... | On the Help menu, click About Visio, Click View the Microsoft Software License Terms.
Not all Microsoft Office software is authorized for use in a sever environment. For example, Excel is not authorized for use on a server. There are also restrictions for use in Terminal Services environments.
| https://softwareengineering.stackexchange.com |
443,526 | [
"https://stats.stackexchange.com/questions/443526",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/269637/"
] | I wish to find the distribution of <span class="math-container">$\hat{y}=Hy$</span> where <span class="math-container">$H$</span> is the hat matrix <span class="math-container">$X(X'X)^{-1}X'$</span> in which a dash represents the transpose. Also, <span class="math-container">$\epsilon$</span> is <span class="math-con... | Assumptions:
<ol>
<li>We are talking about a linear regression model</li>
<li><span class="math-container">$X$</span> is not random</li>
</ol>
Then
<span class="math-container">$$y = X\beta + \epsilon$$</span>
<span class="math-container">$$\hat{y} = Hy = X(X'X)^{-1}X'X\beta + H\epsilon$$</span>
<span class="math-... | You have the assumption that the errors are multivariate normal <span class="math-container">$\epsilon \sim \mathcal MVN(0,\Sigma)$</span>, where the errors is the complete vector <span class="math-container">$\epsilon = (\epsilon_1,...,\epsilon_N)$</span> assuming that there are <span class="math-container">$N$</span>... | https://stats.stackexchange.com |
204,709 | [
"https://electronics.stackexchange.com/questions/204709",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/43513/"
] | Assume we have an antenna in receiving mode, connected to a matched load. The equivalent circuit is a voltage source in series with the antenna feedpoint impedance and the load impedance.
As we know, the feedpoint impedance is made up of a reactive component, an ohmic resistance and a radiation resistance. Now, since ... | Yes, it is effectively radiated back by the antenna. If the antenna is receiving from a point source, but its radiation pattern is not pointing only at the source (e.g. is isotropic), this radiation will go in 'all' directions. Thus the antenna will redirect (reflect) the incoming signal back in a broader beam.
| No, in general, there is no such effect as reradiation. This heavily depends on the antenna type. The radiation resistance is a model for the radiation. Due to reciprocity, one should still match to this resistance in the receive case, but the simple model as you describe it (<code>a voltage source in series with the a... | https://electronics.stackexchange.com |
4,600 | [
"https://physics.stackexchange.com/questions/4600",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/1758/"
] | Today I saw the derivation of the wave equation in class, and I did not understand the following step.
We are modeling a uniform-density string as being made up of tiny masses spaced a small amount $h$ apart connected by springs obeying Hooke's law. Let $y_i(t)$ be the vertical position of the $i$'th such particle. T... | Schrödinger equation is a linear partial differential equation, so sure, you can use the usual formalism of Green's functions to solve it.
First let's recall how the stuff works.
Suppose $L$ is the linear operator and $D$ are the boundary conditions and we want to solve equations $Lu = f$ and $Du = 0$ for $u$. Using t... | In more 'down-to-earth' QM, you use Green's functions to find the density of states. I'm deprived of my books so at a loss for giving a good reference, but the idea is to calculate $$G(x,x';E) = \langle x, (E - H)^{-1} x' \rangle,$$ where $H$ is the system's Hamiltonian. You can then define a spectral function ... | https://physics.stackexchange.com |
89,747 | [
"https://physics.stackexchange.com/questions/89747",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/35582/"
] | Given is a mechanical multiple degree of freedom system described by the following matrices and equation:
<ul>
<li>mass matrix ${\bf{m}} = \left[\begin{matrix} m & 0 & 0 \\ 0 & m & 0 \\ 0 & 0 & m/2 \end{matrix}\right]$,</li>
<li>stiffness matrix ${\bf{k}} = \left[\begin{matrix} 2k & -k &... | The determinant is fairly easy to calculate. You know already, essentially, the eigenvalues of the stiffness matrix; more accurately, you know the eigenvalues of the matrix $\mathbf{m}^{-1}\mathbf{k}$, because the $\omega_i$ are zeros of the equation
$$0=\det(\mathbf{m}^{-1}\mathbf{k}-\omega^2).$$
(The more aesthetical... | I suspect you are overwhelmed by the plethora of variables that obscure the fundamental cyclometric symmetry of the problem. You may scale all but one of them out of the symmetric matrix <strong>M</strong>, part of whose (symmetric) inverse you are seeking, really, by redefining
$${\bf{M}} \equiv {\bf{k}}-\omega^2 {... | https://physics.stackexchange.com |
341,345 | [
"https://physics.stackexchange.com/questions/341345",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/158190/"
] | I've been trying to derive it myself but I can't seem to find something really decent. I'm looking for a simple, yet rigorous derivation. I'm specifically looking at phase transitions.
| Okay, I've found a decent and simple derivation.
Suppose a system is in thermal contact with its environment which we suppose acts as a resevoir of energy - which means it can absorb or release energy without changing its temperature. We call $S_{tot}$ the total entropy of the system + environment, and any variables ... | <span class="math-container">$G=H-T S$</span>
<span class="math-container">$\Rightarrow \quad G=U+P V-T S$</span>
at constant <span class="math-container">$P, T$</span> we have then
<span class="math-container">$\Delta G=\Delta U+P \Delta V-T \Delta S$</span>
but <span class="math-container">$\triangle U=\triangle Q-P... | https://physics.stackexchange.com |
36,023 | [
"https://physics.stackexchange.com/questions/36023",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/11062/"
] | I watched a video of "Glass cutting" which uses a <strong>Water Jet Cutter</strong>. It was said that Cutting glass simply by machines would eventually crack it... So, They're using grains of sand (by placing a sand paper beneath) to cut glass using a Water Jet. It also has enough force to cut through <em>steel</em>.
... | Cutting glass with a water jet uses grains of hard material (such as sand but occasioanlly harder minerals or diamond) to grind away at the glass one particle at a time. The water jet is just used to carry the sand to the glass at high speed and remove it and the eroded glass. As the poster says, it also cools the glas... | To complete Martin's answer: the result of Laser cutting is non smooth because of the melted leftover on both side of the cut. You can experience it with the nylon sticker on clothes. They are laser cut and their rim is hardened up to the itching point...
| https://physics.stackexchange.com |
66,011 | [
"https://physics.stackexchange.com/questions/66011",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/23103/"
] | Why does water have several different solid phase but only one liquid and gas phase? Is there any meaning? or any reason behind it?
Or is it just the way the nature behaves?
| Solid phases differ by the arrangement of the molecules. Molecules in solids stay at the same place so you can have different geometrical arrangements (different phases). In liquids and gases, molecules always move, so you cannot define a fixed arrangement.
| Some other liquids can have multiple phases though. Liquid helium can undergo a phase transition between a superfluid and a non-superfluid phase.
| https://physics.stackexchange.com |
24,637 | [
"https://datascience.stackexchange.com/questions/24637",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/28917/"
] | I have a time series data about daily usage of a computer program, here is an example
<ul>
<li>2017-11-10: <strong>0</strong></li>
<li>2017-11-09: <strong>14</strong></li>
<li>2017-11-08: <strong>0</strong></li>
<li>2017-11-07: <strong>6</strong></li>
<li>2017-11-06: <strong>102</strong></li>
<li>2017-11-05: <strong>0... | I think you have a few options:
<ul>
<li>If you have a pre-set rule to exclude outliers, such as a hard-threshold at 100 which you know the data shouldn't exceed, then something as simple as <code>x = [e for e in x if e < 100]</code> will do.</li>
<li>If you have a parametric belief, such as any observation that fa... | One solution is using <code>mean</code> and <code>variance</code> to detect outlires in your time-series. For example:
<pre><code>>> data=np.array([0,0,102,6,0,14,0])
>> c = 1
>> abs(data - np.mean(data)) < c * np.std(data)
Output: array([ True, True, False, True, True, True, True], dtype=boo... | https://datascience.stackexchange.com |
87,236 | [
"https://softwareengineering.stackexchange.com/questions/87236",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/1668/"
] | I'm starting to get into writing automated Acceptance tests and I'm quite confused where to write these tests against, specifically what layer in the app. Most examples I've seen are Acceptance tests written against the <strong>Domain</strong> but how about tests like:
<blockquote>
Given Incorrect Data When the user... | Typically, an acceptance test is written against a requirement. Regardless of how you capture requirements, you should have a set of things that your system must be able to do, expressed as a list in an Software Requirements Specification (SRS) or as a collection of user stories, among other options. Each requirement s... | Who is really supposed to accept your delivery ? Answer on that question will help you determine how to structure your Acceptance tests.
Better still - involve them (customers, customer representatives, stakeholders, system tests etc) in writing/defining acceptance tests.
| https://softwareengineering.stackexchange.com |
582,889 | [
"https://physics.stackexchange.com/questions/582889",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/256066/"
] | Griffiths example 3.8 says
<blockquote>
An uncharged metal sphere of radius <span class="math-container">$R$</span> is placed in an otherwise uniform electric field <span class="math-container">$\mathbf{E}=E_{0} \hat{\mathbf{z}} .$</span> The field will push positive charge to the "northern" surface of the sp... | The only difference between positive and negative charges is the sign of the force that they experience in an external field. The external field is along <span class="math-container">$\mathbf{\hat{z}}$</span>, and so the positive charges will be pushed "up", and exactly the same thing will happen to the negat... | Imagine that, before considering the charge that's on the sphere, you flip the universe in the z-direction. This leaves the sphere the same but flips the electric field because it depends on <span class="math-container">$
\mathbf{\hat z}$</span>. Now flip all the postive charges with all the negative charges (this is c... | https://physics.stackexchange.com |
427,265 | [
"https://physics.stackexchange.com/questions/427265",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/205949/"
] | My teacher gives me an equation for the position of a particle to analyse the equation by the dimensional analysis method:
<blockquote>
The position of a particle moving under uniform acceleration is some function of time and the acceleration. Suppose we write this position:
$$ s = ka^mt^n $$
</blockquote>
I r... | Draw yourself a picture of a cylindrical bar magnet, with the field lines emerging from one end, looping around, and entering the opposite end to connect up with the emerging lines again. the poles of the magnet are those <em>zones near the ends</em> where the field lines begin to diverge as they near the physical end ... | The north pole is where the field lines leave and the south pole is where they enter. For a current loop the two coincide: it is the surface or any surface enclosed by the loop. For a piece of magnetised iron it is conventionally the material surface.
| https://physics.stackexchange.com |
570,402 | [
"https://electronics.stackexchange.com/questions/570402",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/285829/"
] | In searching for the correct replacement LED for a project, I've noticed that, more often than not, high power LEDs seem to be primarily categorized by their wattage. Why is this the standard?
It would seem that voltage and amperage would be more appropriate for classification and searching, since multiple <strong>vol... | I somewhat disagree with the other answers. The power class of the diode is actually important, since low power diodes can be used with cheap FR4 PCB, higher power can be used on metal core PCBs, and very high power COBs typically are directly bounded to metal heatsinks. Since you usually decide (based on budget and ma... | Impossible to answer conclusively, but a good reason is that a lighting designer will select an "LED" (or array/cluster of LED's) that provides the illumination required, THEN design a circuit to drive it. You're kinda doing it backwards.
| https://electronics.stackexchange.com |
1,095,271 | [
"https://math.stackexchange.com/questions/1095271",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/206293/"
] | I have this question that's bugging my mind:
<strong><em>"Discuss by giving suitable examples the role of mathematical relations (Unary, binary and ternary) in computing."</em></strong>
I'm sure it's a very simple question, but the way it's been phrased is just killing me.
Our lecturer is not a competent English spea... | The key here is to transform this into a quadratic equation.
$$2^{2x+1} - 2^{x+4} = 2^3 - 2^x$$ can be rewritten as $$2 \cdot 2^{2x} - 2^4 \cdot 2^x = 2^3 -2^x.$$
Temporarily we will express $2^x=u$ and we find that the equation turns into $$2u^2 - 16u = 8 - u.$$ Finally writing this as $$2u^2 - 15u - 8 = 0$$ we can ... | $$\Longleftrightarrow 2^x(2^{x+1}+1)=2^3(2^{x+1}+1)$$
| https://math.stackexchange.com |
148,688 | [
"https://softwareengineering.stackexchange.com/questions/148688",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/31740/"
] | At our company we are not using a source control so we do backup manually.
My habit is like this: I backup only scripts where I removed code snippets, and those scripts where I only added code snippets I don't backup.
Is it reasonable to backup each time when you change something, or there are some conditions for fil... | When you start asking questions like "should I backup everything or only parts", then it is time to introduce source control (heck, you should introduce source control before such questions arise). Now. No excuses. Checkin whenever you have finished a small feature slice, and backup the repo daily.
| <ol>
<li>you should back up everything, experiencing data loss shouldn't cost you more than a day of lost work.</li>
<li>Get source control and use it, there are good free solutions, and after having used it I can't imagine not having it.</li>
</ol>
| https://softwareengineering.stackexchange.com |
424,398 | [
"https://physics.stackexchange.com/questions/424398",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/181869/"
] | My doubt is very basic and fundamental, by Newton's second law we can say that $F=\frac{dp}{dt}$.
Hence, there can also be possible cases when $F=\frac{dm}{dt}v$, when the body is moving with constant velocity in the presence of a force! Then what is the effect of that force as a whole, what is it doing? We have always... | Yes such a situation is possible, but you are no longer considering point mechanics (where $m$ is by definition constant), but the mechanics of a system consisting of multiple point particles. In other words: to arrive at such an equation with changing mass, you have to analyse a system of point masses, for each of whi... | This is the idea behind a rocket. Very simplified, while the rocket looses fuel mass, the exhaust produces thrust
| https://physics.stackexchange.com |
8,947 | [
"https://earthscience.stackexchange.com/questions/8947",
"https://earthscience.stackexchange.com",
"https://earthscience.stackexchange.com/users/6841/"
] | I am looking for any academic reference (i.e. articles) regarding the time of concentration for large and small basins.
E.g. I would expect that a large basin (area > 1,500 km^2) will flood (or reach its max peak flow) later in time (e.g. 1-2 days later) when compared to a small basin if the same amount of rain will f... | A quick literature search seems to confirm Gordons estimation, even at the scale of a whole bucket:
<blockquote>
[T]he annual mean pH, based upon
samples collected weekly during 1970-1971 and weighted proportionally to
the amount of water and pH during each period of precipitation, was 4.03
at the Hubbard Broo... | Raindrops gain a small amount of acidity as they fall through carbon dioxide in the air, but that's not what this question is about. Raindrops commonly start off as ice crystals which have to nucleate around something, usually an aerosol particle such as soot, clay, bacteria, sulphur dioxide , dimethyl sulphide, etc. I... | https://earthscience.stackexchange.com |
20,613 | [
"https://dba.stackexchange.com/questions/20613",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/5606/"
] | <ul>
<li>I've created a Credential <strong>Cred1</strong> (DOMAIN\USER1 with the secret = domain password)</li>
<li>I've created a SQL Agent job with owner to NT AUTHORITY\SYSTEM</li>
<li>The job needs to proxy as credential <strong>Cred1</strong></li>
</ul>
So when I run the job it fails with the message that NT AUTH... | Expand SQL Agent<br>
Expand Proxies<br>
Find your proxy, right-click on it and choose Properties<br>
Go to the Principals tab<br>
Add the NT AUTHORITY\SYSTEM account
This will give that account permission to use the proxy.
| Check <code>select * from msdb.dbo.sysproxies</code> if your proxy is enabled, we had a similar problem and we recreated the proxy with enabled = 1.
That seem to have solved the problem with this error.
| https://dba.stackexchange.com |
588,762 | [
"https://physics.stackexchange.com/questions/588762",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/261424/"
] | <strong>The questions:</strong>
Our Prof wrote the following expression that confused me,
<span class="math-container">\begin{align}
\Lambda^0_{\,\,\,i}&=\eta^{00}\eta_{ij}\Lambda_{0}^{\,\,\,j}\\
&=-\eta_{ij}\Lambda_{0}^{\,\,\,j}\\
&=v_i\gamma
\end{align}</span>
Now, as far as I understand, this could only ... | There's no sign discrepancy at all. Start from <span class="math-container">$\Lambda^0_{\:i}=\eta^{0\mu}\Lambda_{\mu i}=\eta^{0\mu}\eta_{i\nu}\Lambda_\mu^{\:\nu}$</span>. Since <span class="math-container">$\eta$</span> doesn't mix space with time in Cartesian coordinates (a fact I'll use hereafter without comment), th... | I don't assume this will be a complete answer, just several suggestions that I hope could be helpful.
So you are using the <span class="math-container">$(-,+,+,+)$</span> metric, I'm using the opposite <span class="math-container">$(+,-,-,-)$</span>, sorry. Consider that a generic Lorentz transformation is subjected to... | https://physics.stackexchange.com |
84,054 | [
"https://cs.stackexchange.com/questions/84054",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/80356/"
] | Are languages whose sets are empty recursively enumerable or recursive? For example, consider the language $L$ $\subseteq$ {0,1}*:
<blockquote>
$L$ = { $w$ | $w$ consists of only 0's} $\cap$ { $w$ | $w$ consists of only 1's }
</blockquote>
I believe this language creates an empty set. There are many different ways ... | The empty language is recursive. The TM accepting this language just returns $0$/REJECT on any input. Similarly, the language $\Sigma^*$ is also recursive. Just return $1$/ACCEPT on any input.
Basically, it does not matter how a language is defined. What makes them recursive/RE is the fact that we can effectively de... | I strongly believe the answer from fade2black is wrong. The empty language is not recursive, in fact it is not even recursive enumerable. The question you have to ask yourself is given any encoding of a turing machine M, can we determine if its language is the empty language.
The way you can formally prove this would b... | https://cs.stackexchange.com |
145,387 | [
"https://softwareengineering.stackexchange.com/questions/145387",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/38148/"
] | I had an argument yesterday with one of my colleagues. He (a business-analyst, previously a programmer) thinks that he should be aware of the technology used to implement the system, so he can take better design decisions. In my opinion (I am a programmer), an analysis should not be coupled in any way to the technology... | There are certainly cases where it makes sense for a business analyst to understand the technology at least well enough to understand where it makes sense to question a business user about how important a particular feature would be. For example, if the business is accustomed to the behavior of a fat client applicatio... | If the technology that will be used is known it should be taken into consideration by analysts when creating the design. Different technologies do things differently and a design that doesn't take into account those differences is going to have problems.
However, business analysts shouldn't care about what technology ... | https://softwareengineering.stackexchange.com |
17,494 | [
"https://engineering.stackexchange.com/questions/17494",
"https://engineering.stackexchange.com",
"https://engineering.stackexchange.com/users/13240/"
] | Suspension often requires different ranges of behavior. Small amplitude and high sensitivity for vibrations and small sharp bumps but also high amplitude and low sensitivity for larger smoother road deformation.
So my question is whether or not it's possible to use two suspension system on top of the other.
I was think... | There are manufacturers of progressive springs - soft for the first part of the movement then harder for the rest of the travel.
There are also shock absorber manufacturers that make various types of shock - depends on the applications you want and what is available "off the shelf" ie fairly standard or you can go to ... | The short answer is 'yes,' since each absorber has its own frequency response curve.<br>
Now, the ideal setup is an active absorber with, say, piezo-driven adjusters which respond to the initial impact profile. There is a prototype out there somewhere of an active sensor which is designed to recognize road bumps and e... | https://engineering.stackexchange.com |
36,063 | [
"https://scicomp.stackexchange.com/questions/36063",
"https://scicomp.stackexchange.com",
"https://scicomp.stackexchange.com/users/34261/"
] | I have the following problem:
<span class="math-container">$$\min_{x\in \mathbb{R}^n}\|Ax-b\|_1$$</span>
where the matrix <span class="math-container">$A$</span> is large and sparse. I am looking for methods/code that can minimize this efficiently. References are very welcome.
| If you have a good LP solver, then the linear programming approach often works well. You don’t want to implement your own simplex or interior point code for LP though.
A specialized variant of the simplex method due to Barrodale and Roberts is a popular approach to this problem, but it takes some effort to implement th... | Thanks to Wolfgang Bangerth for pointing out that this can be rewritten as a linear problem. My reformulation would be:
<span class="math-container">$$\min_{x\in\mathbb{R}^N}\|Ax-b\|_1 \implies$$</span>
<span class="math-container">$$\min_{(x,y)\in\mathbb{R}^{N+M}}\sum_{i=1}^N y_i, $$</span>
<span class="math-container... | https://scicomp.stackexchange.com |
355,520 | [
"https://electronics.stackexchange.com/questions/355520",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/91384/"
] | In electronics the Nyquist sampling rate shows you the maximum frequency that could be sampled without aliasing. So if the human eye is able to see 24fps does it mean that you can see all the information below 12fps?
| The firing of neurons in the eye is completely asynchronous, so for all intents and purposes, the process we call "seeing" must be considered continuous-time, not sampled. Nyquist does not apply.
If things move too fast, they simply appear blurry. And "too fast" varies from person to person; there is no well-defined c... | The terminology used when applying to the human eye is the Critical Fusion Frequency, also called the Flicker Fusion Threshold. This is the frequency at which a point of flashing light merges visually into a single solid point of light. The precise frequency differs depending on what area of the retina is being stimula... | https://electronics.stackexchange.com |
39,775 | [
"https://biology.stackexchange.com/questions/39775",
"https://biology.stackexchange.com",
"https://biology.stackexchange.com/users/19507/"
] | I had the following question on an exam of mine and I got the points for the question, but I am wondering if someone could please explain why it is correct, or rather, <em>if</em> it is correct at all:
<blockquote>
As living organisms grow, they increase in complexity and decrease in entropy. How does this relate to... | You are correct. Anything within the universe is part of the closed system <em>of the</em> universe. Therefore, they must also increase the entropy of the universe. You are correct, they do release a lot of heat. Plus, while making ATP, the major source of cellular energy, they convert a complex molecule, glucose, to s... | I would say D.) too.
<blockquote>
The resulting increase in entropy of the universe is greater than the
decrease in entropy associated with the growth.
</blockquote>
We can argue that the cells and the universe together are a closed system and entropy increases in a closed system by time. So the sum of their entr... | https://biology.stackexchange.com |
4,306,035 | [
"https://math.stackexchange.com/questions/4306035",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/986293/"
] | Reduce to first order and solve <span class="math-container">$yy'' = 3y'^2$</span>
Dividing both sides by <span class="math-container">$y$</span> and re-arranging <span class="math-container">$y'' - 3y' = 0$</span>. This is clearly an homogenous equation, a solution might be <span class="math-container">$e^{3x}$</span... | The algebra is wrong, it would be <span class="math-container">$y'' - 3{(y')^2 \over y} = 0$</span>.
It's better to write it as <span class="math-container">${y'' \over y'} = 3{y' \over y}$</span> and view <span class="math-container">${y'' \over y'}$</span> as the derivative of <span class="math-container">$\ln |y'|$<... | How do you cancel <span class="math-container">$y$</span>? Observe instead that the equation can be rewritten as
<span class="math-container">$$
\frac{yy''-(y')^2}{(y')^2}=2
$$</span>
and that the left-hand side is the derivative of <span class="math-container">$-y/y'$</span>.
Now <span class="math-container">$y=(c_1-2... | https://math.stackexchange.com |
197,364 | [
"https://math.stackexchange.com/questions/197364",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/32860/"
] | Say you have an arbitrary ring with three elements, $\{0,1,c\}$. Why does it have to be that $c^2=1$? If we don't assume that $c$ is invertible, what goes wrong if $c^2=0$ or $c^2=c$?
| Notice that we must have $c+1=0$, since $c+1=1$ implies $c=0$ and $c+1=c$ implies $1=0$, both contradictions. Thus we have $c=-1$. Therefore, $c^2=(-1)^2=1$.
| As a different way to see it, a ring with three elements is in particular a group with three elements. Now the only group of order $3$ is $\mathbb Z / 3 \mathbb Z$, so clearly your $c$ must be $[2]$ and thus $c^2=[2]^2=[1]$.
| https://math.stackexchange.com |
199,749 | [
"https://stats.stackexchange.com/questions/199749",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/107333/"
] | I am currently working on a classification problem, and I am observing that the linear classifiers are outperforming the non-linear classifiers. This is very unintuitive to me. What could be the cause of this?
<ul>
<li>training data? </li>
<li>number of features? feature types?</li>
</ul>
I hypothesize its the lack o... | If you are truly interested in all-or-nothing classification you can fool the proportion classified correctly with a variety of bogus models. If you are interested in prediction instead, and use proper accuracy scoring rules, you'll see that what outperforms other methods in a variety of situations is additive models ... | If a problem is nonlinear and its class boundaries cannot be approximated well with linear hyperplanes, then nonlinear classifiers are often more accurate than linear classifiers. If a problem is linear, it is best to use a simpler linear classifier.
The performance depends on the problem.
| https://stats.stackexchange.com |
57,837 | [
"https://stats.stackexchange.com/questions/57837",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/-1/"
] | I have a database with 500 records. I want to split these records to 75% and 25% *randomly*in order to use the different datasets for training and testing to machine learning algorithms. Does anyone knows how to do that? For example using an sql query
| You did not provide SQL product so I will give one example. You need a random function from your DBMS. For example oracle provides dbms_random.value, SQL server NEWID or RAND(). MySql provides RAND().
Assume that you have a table named DATA which holds your 500 records.
<pre><code>WITH TEMP
(
SELECT ROW_NUMBER()... | A sequential algorithm using conditional probabilities can also be used. To demonstrate a simple case, suppose you want to sample 1 item out of 4. Generate a uniform[0,1] random number $U_1$. If $U_1 < {1 \over 4}$, select the first element. If not, generate $U_2$. If $U_2 < {1 \over 3}$, select the second elemen... | https://stats.stackexchange.com |
3,219,606 | [
"https://math.stackexchange.com/questions/3219606",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/349094/"
] | Let us say <span class="math-container">$S_1=2xy^2+3xy$</span>, and <span class="math-container">$S_2=3y^2+7x+7y+8$</span>. Then, can we say that <span class="math-container">$S_1\ge S_2$</span> if <span class="math-container">$y\le x-2$</span> and <span class="math-container">$x,y\in\mathbb{N}$</span>?
I think yes, b... | <blockquote>
can we say that <span class="math-container">$S_1\ge S_2$</span> if <span class="math-container">$y\le x-2$</span> and <span class="math-container">$x,y\in\mathbb{N}$</span>?
</blockquote>
No, we cannot.
If <span class="math-container">$y=1$</span>, then <span class="math-container">$S_1-S_2=-2x-18\lt ... | I will only answer the continous parts of this question.
In <span class="math-container">$\mathbb R^2$</span> neighbourhoods, calculate Hessian of <span class="math-container">$S_1-S_2$</span> in the points where <span class="math-container">$\nabla(S_1-S_2)$</span> is zero vector. Hessian needs to be definite pos/neg... | https://math.stackexchange.com |
13,325 | [
"https://mechanics.stackexchange.com/questions/13325",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/7623/"
] | I recently changed my oil, and I didn't have quite enough oil to top it off so I used HD-30 oil for my last .7 out of 4 quarts total. My car normally takes 5W-30 and after a little research I figured that it would be OK. Unfortunately, my car has a hard time starting up after sitting for a while now. I have to give it ... | While I agree with Allan that you should clean out the oil and put what is recommended into the system, I think the problem is not the oil. I think you might have a leaking injector causing your hard start/black smoke on start-up issue. This is the reason I asked about the color of the smoke, as it makes a huge differe... | HDEO is allowed only if it is SM/SN rated on the container, HDEO is better because it can take the rigors of Diesel use.
| https://mechanics.stackexchange.com |
19,434 | [
"https://cstheory.stackexchange.com/questions/19434",
"https://cstheory.stackexchange.com",
"https://cstheory.stackexchange.com/users/18032/"
] | I have the following Equivalent DNF problem:
Input:Two DNF formulas, $F_1$ and $F_2$,with variables $a_1,a_2,...a_n.$
Output: $1$ if $F_1$ and $F_2$ are equivalent, $0$ otherwise.
$F_1$ and $F_2$ are equivalent if for all $(a_1,a_2,...a_n)∈\{0,1\}^n,F_1(a_1,a_2,...a_n)= F_2(a_1,a_2,...a_n).$
Is the DNF-Equivalence... | A special case of DNF equivalence is DNF tautology: Given a DNF formula $F$, is it satisfied for all assignments? This can be seen by setting $F_1 = F$ and $F_2$ to be a trivial tautology. CNF non-satisfiability is co-NP-complete. Negating the input formula turns a CNF formula into a DNF formula and vice versa and non-... | The DNF-Equivalence problem is a special case of the Formula Equivalence problem that is coNP-complete. If you assume that the DNF-Equivalence is $\mathsf{NP-hard} $ then, by definition, you have that every problem in $\mathsf{NP} $ reduces to the DNF-Equivalence problem that in turn implies $\mathsf{NP} \subseteq \mat... | https://cstheory.stackexchange.com |
78,824 | [
"https://mathoverflow.net/questions/78824",
"https://mathoverflow.net",
"https://mathoverflow.net/users/4298/"
] | Let's consider closed simply connected manifold $M^n$ and a $a\in H^k(M)$ and $a*\in H^{n-k}(M)$ is the dual to $a$.
Is it true that dual to $a$ is realisable as a immersed sphere or $ a*=bc $ for some $b,c\in H^*(M)$ ?
Edit: it is more natural to ask about possibility to decompose dual to $a$ as a product, see exam... | I will construct a closed simply-connected $8$-manifold $M$ and an $a\in H^3(M;\Bbb Z)$ such that the Poincare dual $b$ of $a$ is not realizable by a map $S^5\to M$, and a Hom-dual element in $H^5(M;\Bbb Z/2)$ to the $\bmod 2$ reduction of $b$ is not a nontrivial product.
Let $K$ be the suspension over $\Bbb C P^2$. ... | Consider $M=SU(4)$. Rationally, the cohomology ring $H^\ast(M;\mathbb{Q})$ is the product $H^\ast(S^3;\mathbb{Q})\otimes H^\ast(S^5;\mathbb{Q})\otimes H^\ast(S^7;\mathbb{Q})$. Consider $a$ to the product of the generators of $S^3$ and $S^5$. The dual is the generator of $S^7$ which doesn't split. However, $\pi_8(SU(4))... | https://mathoverflow.net |
478,046 | [
"https://electronics.stackexchange.com/questions/478046",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/182852/"
] | How would one determine the maximum current output you could achieve from an improved Howland source given a specific op amp? In other words, what do I need to look at in the op amp's datasheet? (I am using a TL074)
| The op-amp has to supply all the output current plus a bit for the feedback. So you want to look at the current that the op-amp can supply. In the case of the TL074 it can supply 10V into 2K with a +/-15V supply, so that's 5mA, guaranteed.
It can probably supply quite a bit more current but there is nothing guarantee... | In addition to "paper" design, you should definitely breadboard it and do measurement in your desired application. The output current capacity will depend somewhat on the voltage drop in the driver stage. You must also take into account thermal limits: if the output stage will dissipate too much, the op-amp m... | https://electronics.stackexchange.com |
192,048 | [
"https://mathoverflow.net/questions/192048",
"https://mathoverflow.net",
"https://mathoverflow.net/users/12481/"
] | Let $f(x,y)=0$ be irreducible elliptic curve over the rationals.
Are there $f$ for which:
Both $x,y$ are arbitrary large powers infinitely often,
i.e. infinitely many rational points $(u^k,v^m)$ with
both $k,m$ arbitrary large?
<hr>
For $x,y$ squares (asked in previous revision) this is possible.
Take $f(x,y)=x^{6}... | The method <strong>joro</strong> suggests works with little change for any $(k,m)$.
Fix an elliptic curve $E: P(X,Y)=1$ of positive rank.
Some simple examples are
$F(X,Y) = Y^2 - X^3 + 2X$ and $F(X,Y) = Y^2 - X^3 - 2$,
each with generator $(X,Y)=(-1,1)$.
Let $u$ and $v$ be "random" rational functions on $E$
that gene... | Partial answer, someone possibly might hopefully complete it.
Solutions exists for $k=m \in \{2,3,4,5\}$.
The case $k=4$ was found with rational surface and $5$ using
very similar approach.
Fix $k,m$ large.
Consider the rational surface given by parametrization:
$x=u^k, y=v^m, z=h(u,v)$ where $h(u,v)=0$ is genus $1... | https://mathoverflow.net |
286,252 | [
"https://softwareengineering.stackexchange.com/questions/286252",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/182970/"
] | Java SE 8 comes with a new mechanism for dates, introducing <code>LocalDate</code>, <code>LocalTime</code> and <code>LocalDateTime</code> classes to represent instants of time. To manipulate such instants, a set of methods are given: <code>LocalDate.plusDays(...)</code>, <code>LocalDate.minusDays(...)</code> and so on.... | <blockquote>
The only reason I can think of is that dates are immutable objects, so by calling plusDays you're not adding days to the original object but creating a new one with new properties, but that's very vary subtle.
</blockquote>
This is exactly the reason. Imagine you had some kind of api for manipulating ra... | In .NET the naming is different although the result is exactly same. Instead of:
<pre><code>tomorrow = LocalDateTime.plusDays(1);
</code></pre>
there is:
<pre><code>tomorrow = DateTime.Now.AddDays(1);
</code></pre>
This only means that differences between understanding of "plus" and "add" ended up as matter of pers... | https://softwareengineering.stackexchange.com |
671,163 | [
"https://physics.stackexchange.com/questions/671163",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/316261/"
] | I'm troubled about the distinction between what I am tentatively calling physical forces and component forces. Consider an object of mass <span class="math-container">$m$</span> on an inclined plane that is at angle <span class="math-container">$\theta$</span> to the horizontal. The physical force of gravity creates a ... | An ohmmeter is
<ul>
<li>supplying power to a circuit and</li>
<li>comparing the voltage across two legs of the circuit (a reference leg and a test leg)</li>
</ul>
If the resistance differs too much, then it will be difficult to compare the voltages. You want to be able to tell the difference between a high resistance a... | I suppose an 'ideal' ohmmeter could be modeled in ideal circuit theory as an ideal <span class="math-container">$1\,\mathrm{A}$</span> DC current source in parallel with an ideal voltmeter.
A resistance <span class="math-container">$R$</span> connected to this parallel combination would have a voltage across of <span c... | https://physics.stackexchange.com |
83,621 | [
"https://physics.stackexchange.com/questions/83621",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/23742/"
] | My highschool textbook states that Zener diodes are a special type of diode which is made of highly doped p and n junctions, and which can survive reverse bias - unlike normal diodes, which get damaged due to the excess heat produced in that mode. How can heavy doping help Zener diodes avoid damage in reverse bias?
I... | The key difference between a Zener diode and a normal diode is that the Zener diode has a low breakdown voltage - typically in the few volts range. The breakdown voltage is low because the heavy doping means the depletion layer is very thin, and even at a low voltage the field strength over this thin depletion layer is... | E=V/L<br />
So,thin depletion layer means more internal electric field so less external voltage will be required for the breakdown.
It also accounts for the less barrier region .
| https://physics.stackexchange.com |
117,233 | [
"https://mathoverflow.net/questions/117233",
"https://mathoverflow.net",
"https://mathoverflow.net/users/17822/"
] | <ol>
<li>To formulate Lebesgue differentiation theorem one needs a metric and a measure. Apart from the Euclidean spaces i.e. $\mathbb R^d$, the theorem holds true for homogeneous groups (e.g. Heisenberg group) and spaces of homogeneous type. (E. Stein: Harmonic Analysis; S. Krantz: Panorama of Harmonic analysis). The... | Briefly, to understand $p$-phenomena in characteristic $p$ you need to replace the etale site by the fppf site. For example, to understand the $p$-torsion in the Brauer group you need the $p$-Kummer sequence and the cohomology of $\mu_{p^n}$, and the study of the $p$-torsion in the Tate-Shafarevich group entails the st... | The cohomology of $\mu_{p^n}$ and $\mathscr{A}[p^n]$ for an Abelian scheme $\mathscr{A}$ in characteristic $p > 0$ can also be studied using <em>syntomic</em> cohomology.
| https://mathoverflow.net |
27,449 | [
"https://dba.stackexchange.com/questions/27449",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/886/"
] | My question is, I have a table which contains a column called myDateColumn (for example) which is of the type date:
I can confirm <code>desc myTable</code> contains this line:
<pre><code>myDateColumn NOT NULL DATE
</code></pre>
However, when I try to select all the data that is between certain dates, for exa... | Your belief that this should not be necessary is correct based on your assumptions. Normally this means the assumptions should be rechecked (unless there is data corruption or a bug). If you run the following you should get the same results from the first query as the second:
<pre><code>DROP TABLE t1;
CREATE TABLE t... | All dates in Oracle have a time component.
I'm pretty sure you want all rows between the 13th and the 15th <em>included</em>. However, when you write:
<pre><code>BETWEEN to_date('13-02-2011', 'DD-MM-YYYY') AND TO_DATE('15-02-2011', 'DD-MM-YYYY')
</code></pre>
you select only all date up to the <code>15-02-2011</code... | https://dba.stackexchange.com |
5,521 | [
"https://security.stackexchange.com/questions/5521",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/363/"
] | I am about to start an evaluation process for firewalls. I have experience with Checkpoint and Juniper, but I don't have any information on Palo Alto networks, other than their marketing stuff.
So I would like to hear from IT analysts/network administration the pros and cons of Palo Alto in areas such as:
<ul>
<li>A... | Let me state up front, that I am a partner of Palo Alto Networks as well as Check Point and Juniper. Over the years we have had a lot of success with all three manufacturers. Palo Alto Networks has built a network security device that is technically different from everything else on the market. If you clear away the ma... | My experience with Palo Alto is limited so please take what I say with a grain of salt (and I'm sure people here can correct me as necessary)...
Palo Alto is a completely different firewall paradigm than Check Point, Juniper, or almost any other firewall. A traditional firewall defines traffic flow based on source IP... | https://security.stackexchange.com |
287,636 | [
"https://softwareengineering.stackexchange.com/questions/287636",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/91691/"
] | I am writing a finite state machine in C++, designed as a library.
Furthermore, I have a GUI implemented as a separate project which needs to update the user interface according to state changes happening in the library.
The way I implement the communication now, is that the GUI is calling a library function which r... | The simplest method is to pass a callback function to the state machine that gets called when a state changes (or other event happens).
Then when it gets called you can update the gui in that function (or forward the message to the gui thread if the state machine runs in a separate thread).
The signature would be somet... | GUI libraries (like for example Qt) are using some kind of subscriber-publisher design pattern, which is event based. That means, on an event, all subscribers are going to be notified.
So, on a state change, publish the event. Every object that subscribed to the event, is going to execute a callback.
| https://softwareengineering.stackexchange.com |
442,278 | [
"https://physics.stackexchange.com/questions/442278",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/209383/"
] | In Numerical Relativity by Thomas W. Baumgarte and Stuart L. Shapiro.
There are bunch of superscript <span class="math-container">$(4)$</span> over <span class="math-container">$T,\Gamma, R$</span><br>
i.e. <span class="math-container">$^{(4)}\Gamma^a_{bc}$</span> _ <span class="math-container">$^{(4)}R_{abcd}$</s... | As @G.Smith says, it refers to 4-dimensionality.<BR>
In the first paragraph on page 27, it says
"Four-dimensional objects associated with <span class="math-container">$g_{ab}$</span> are denoted with
a superscript <span class="math-container">${}^{(4)}$</span> in front of the symbol,..."
| It most likely indicates the number of dimensions. For example, <span class="math-container">$^{(4)}R$</span> indicates a 4-dimensional Ricci scalar.
| https://physics.stackexchange.com |
437,509 | [
"https://stats.stackexchange.com/questions/437509",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/223371/"
] | I have the following problem and questions provided:
<blockquote>
Assume the state of the weather in Sydney on any particular day can be modelled using a state space S={1≡sunny,2≡overcast,3≡wet}. Also assume that the likelihood of any state depends only on the state on the previous day. A sunny day is followed by a ... | For your part (b), I don't see a contradiction.
A Markov chain is said to be irreducible if it is possible to get to any state from any state. In fact, when you compute <span class="math-container">$P^{100}$</span>, you have shown explicitly that each entries are positive. It is indeed irreducible.
For part (c), you... | By taking <span class="math-container">$\mathbf{P}^{100}$</span> you get (correct to to seven places) the limiting matirix <span class="math-container">$\lim_{n\rightarrow\infty}\mathbf{P}^{n},$</span> of which
all three rows are <span class="math-container">$\lambda = (0.4901961,\, 0.3137255,\, 0.1960784).$</span>
I... | https://stats.stackexchange.com |
146,804 | [
"https://stats.stackexchange.com/questions/146804",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/103833/"
] | I have a question about two different methods from different libraries which seems doing same job. I am trying to make linear regression model.
Here is the code which I using statsmodel library with OLS :
<pre><code>X_train, X_test, y_train, y_test = cross_validation.train_test_split(x, y, test_size=0.3, random_state... | First in terms of usage. You can get the prediction in statsmodels in a very similar way as in scikit-learn, except that we use the results instance returned by <code>fit</code>
<code>predictions = results.predict(X_test)</code>
Given the predictions, we can calculate statistics that are based on the prediction error... | In the OLS model you are using the training data to fit and predict.
With the LinearRegression model you are using training data to fit and test data to predict, therefore different results in R2 scores.
If you would take test data in OLS model, you should have same results and lower value
| https://stats.stackexchange.com |
345,406 | [
"https://softwareengineering.stackexchange.com/questions/345406",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/267366/"
] | For context, I am running a REST API built with Node.js. Because of callbacks and some complex DB calls, I have a chain of functions that are async but also unique, so it's tough to reduce redundancy. I came up with the idea to call my own endpoints (different endpoints) from within the code itself so I can reduce redu... | <ol>
<li>There is little use of actually doing an HTTP request.
When the underlying application processes the GET request from your example, it probably calls the business layer which does some input checking, and then calls the data access layer which retrieves the puppy.
In a similar way, the POST request could, in... | Yes its bad(tm).
The extra overhead of making the http call although needless probably wont be much of a factor.
I think The real dangers are:
<ul>
<li>The accidental introduction of endless loops. ie Post calls get and Get calls Post or similar scenarios.</li>
<li>Unindented caching effects</li>
<li>http server is... | https://softwareengineering.stackexchange.com |
421,086 | [
"https://physics.stackexchange.com/questions/421086",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/203162/"
] | I know that operator for $p = {h\over i} {d\over dx}$.
so $p = {h\over i} {d\psi\over dx}$ where $\psi$ is the wave function.
So, $T$ (kinetic energy) $ = {p^2 \over 2m} = {-h^2\over 2m} {d\psi \over dx}^2$,
so the operator for $T$ should be $-{h^2\over 2m} \left({d\over dx}\right)^2$ and not $d^2\over dx^2$.
Please ... | Momentum operator is defined as:
$$\hat{p}=\frac{\hbar}{i}\frac{d}{dx}$$
This means that if you want to get the momentum from the wavefunction $\psi$, you must apply the momentum operator to the wavefunction. Let's define the wavefunction as:
$$\psi=A\exp\left[\frac{i}{\hbar}\left(px-Et\right)\right]$$
It is easy t... | We have $$p^2\psi=\hat{p}^2\psi=\hat{p}\hat{p}\psi=\frac{\hbar}{i}\frac{d}{dx}\left(\frac{\hbar}{i}\frac{d\psi}{dx}\right)=-\hbar^2\frac{d^2\psi}{dx^2}.$$
| https://physics.stackexchange.com |
2,296,446 | [
"https://math.stackexchange.com/questions/2296446",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/340233/"
] | The probability of a man hitting a target is 3/4. He tries 5 times. What is the probability that the target will be hit at least 3 times ?
| The answer for your question is shortly "No", because $$\left( \frac { 2x-1 }{ 2x+1 } \right) ^{ 2 }>0\quad \Rightarrow x\in R-\left\{ \pm \frac { 1 }{ 2 } \right\} \\ \\ \frac { 2x-1 }{ 2x+1 } >0\quad \Rightarrow x\in \left( -\infty ;-\frac { 1 }{ 2 } \right) \cup \left( \frac { 1 }{ 2 } ;+\infty \right) $$
| No, it's not equivalent to $\frac{2x-1}{2x+1}>0$. Instead, note that the square of any real number $z$ is non-negative, and is zero iff $z=0$; thus $\left(\frac{2x-1}{2x+1}\right)^2>0$ implies $\frac{2x-1}{2x+1}\ne0$, which is equivalent to $2x-1\ne0$. But $2x+1\ne0$ too because division by zero is not allowed, s... | https://math.stackexchange.com |
137,442 | [
"https://electronics.stackexchange.com/questions/137442",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/57709/"
] | I need to check if a micro-switch can do the below (Honeywell 11SM1-H2):
"the contacts shall switch an inductive load below 53V DC / 0.4A with a time constant of 40ms"
I'm not sure I understand what is being asked. My understanding is this: the load has a time constant of 40ms, meaning that L/R = 0.04. R = 133 Ohms... | No, transforms aren't "necessary", but they do make some types of calculations much simpler and more convenient.
It is possible to do all computation and analisys of a signal in either the time domain or the frequency domain. However, some operations are much simpler and more intuitive in one than the other.
This ca... | All transforms are tools to make analysis easier. They are tools that engineers, scientists, and mathematicians have developed over the years to help make their jobs easier or to help them gain a greater understanding of the phenomenon they are looking at. The Laplace transform, for example, makes solving differentia... | https://electronics.stackexchange.com |
3,471,317 | [
"https://math.stackexchange.com/questions/3471317",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/457450/"
] | As the title suggests, I am considering the question of whether <span class="math-container">$\text{PSL}_2(\mathbb{Z})$</span> contains any cyclic subgroups of finite index. For this recall that <span class="math-container">$\text{PSL}_2(\mathbb{Z})$</span> is the quotient of <span class="math-container">$\text{SL}_2(\... | <span class="math-container">$\frac{2^{2^{n}+1}+1}{2^{n}+1}= \frac {2^{nk} +1}{2^n+1}$</span>
Let <span class="math-container">$a = 2^n$</span> then we have <span class="math-container">$\frac{a^k + 1}{a+1}$</span>. If <span class="math-container">$k$</span> is odd then <span class="math-container">$\frac{a^k + 1}{a+... | So let's suppose we have positive integers <span class="math-container">$n,k$</span> such that <span class="math-container">$2^n + 1 = nk$</span>. Then
<span class="math-container">\begin{align}
2^{2^n+1} + 1
&= 2^{nk} + 1 \\
&= (nk - 1)^k + 1 \\
&= \Biggl(\sum_{m = 0}^{k} \binom{k}{m}(nk)^{k-m}(-1)^{m}\Big... | https://math.stackexchange.com |
250,810 | [
"https://electronics.stackexchange.com/questions/250810",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/117580/"
] | I have few questions on software timers and interrupts on a microcontroller. Just for information, I use a dsPIC33E microcontroller.
The final goal is to implement a serial communication protocol: RS485 with Modbus. I managed to transmit and receive a message, and now I have to do a message processing part.
Since I n... | You do not need special "software interrupts". Have a look at the following code. I neglected some technical stuff like variable declaration for clarity.
Use your ISR to just count some timer (tick). In the main() you are waiting to sync to this timer. Use a dedicated timer for each task you have to process. All these... | After some Googling and discussion, I realized that I tried to achieve prioritized tasks, which is basically an RTOS (real-time operating system). Although an RTOS can be implemented on a microcontroller, it requires a huge amount of resources (RAM etc.).
The other option is to use a scheduler as Batuu explained. In t... | https://electronics.stackexchange.com |
1,283,992 | [
"https://math.stackexchange.com/questions/1283992",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/151615/"
] | we have the following subspace over $\mathbb{R}$ $$M = \{ A \in M^{{n\times n}} | A = -\overline{A} \}$$
I found that it is a subspae and now I need to find the linear span of it.
how can I calculate the linear span of a subspace?
| Think of it this way: those column matrices you're talking about are really just <em>coordinates</em> of some vector in a vector space. They're not necessarily the vector you are discussing, they're just a representation of that vector in an easier vector space.
So what are coordinates? There is a theorem in linear ... | To start with, a vector doesn't have a dimension. A vector space has a dimension. In particular, every vector is member of a one-dimensional vector space, which is spanned by it (unless it is the zero vector, then is spans the zero-dimensional vector space consisting only of the zero vector).
Now what do those three n... | https://math.stackexchange.com |
114,301 | [
"https://stats.stackexchange.com/questions/114301",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/37540/"
] | I am studying now t-scores. As far as I understand, t-scores are used when we don't know true population parameters (such as: standard deviation and population mean) and cant use z-scores. Here is formula that is in books and in Internet for calculating t-score:
$$t= \frac{\bar{X}-\mu }{\frac{S}{\sqrt{n}}}$$
As far as... | <blockquote>
As far as I know μ is used to define true population mean.
</blockquote>
Not quite, and here's the rub. μ <em>represents</em> whatever the true mean is. It's <em>defined</em> by the problem for which this little bit of statistical inference is the analysis, not by the data itself (that would make it ... | There are <em>two different $\mu$'s</em> involved here:
<ol>
<li>the hypothesized mean that you use in the numerator of your
t-statistic for a t-test (sometimes denoted as $\mu_0$), and</li>
<li>the true population mean, $\mu$.</li>
</ol>
The t-test is actually to see whether the true population mean differs from the... | https://stats.stackexchange.com |
112,975 | [
"https://physics.stackexchange.com/questions/112975",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/41066/"
] | Why does a thread/rope swirled like this:
<img src="https://i.stack.imgur.com/xprTt.gif" alt="enter image description here">
become more strong than just the group of single threads without swirling?
Is it because of more contact between them and therefore more friction?
(I don't know the appropriate word for this pr... | We can perhaps note that in the "real" world, you will probably not stress all the thread the same way, and it may that at some point one thread will deal with too much a tension, it will then break.
The friction between the threads will allow for the tension to be more equally shared between all the threads (the bes... | it does not make the collection of threads stronger. if one takes thread or string and wraps it around and between two pegs in a loop, and then wraps the length of the "rope" formed in this way with more string one has created a Selvagee Strap, an old school longshoreman's or wagoners rig for lifting heavy weights on a... | https://physics.stackexchange.com |
14,568 | [
"https://mechanics.stackexchange.com/questions/14568",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/3444/"
] | I just did an oil change on my 1995 Nissan Altima, and noticed that the gasket on my dipstick does not make a seal inside the dipstick tube. The inside diameter of the tube is maybe 3/32 of an inch larger than the dipstick gasket. I don't know if this is how the car came or if this is an aftermarket dipstick. The gas... | It's not a huge deal. The biggest thing you'd be looking for is if there is leakage from there. It's not going to be causing any running issues or any such.
I think the biggest thing would be if the stick was fitting deeper into the hole and giving you a false reading as far as oil level. If it's too low in the hole,... | Your vehicle is designed (early) pre-OBD, so it will not affect the vehicle operation to any great extent, maybe a minor oil seepage. Later vehicles monitoring the air imput to the engine, ie OBD, the dipstick becomes important so it does not leak air. With OBD, a leaking dipstick seal would cause a mis-reading from th... | https://mechanics.stackexchange.com |
80,225 | [
"https://electronics.stackexchange.com/questions/80225",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/8469/"
] | <strong>Objective</strong>
I am trying to change the state of a pin on the PIC24FV16KA304 depending on a button press. Initially the pins are set as inputs, upon a button press the pins change to an output with a low state, another button press will keep the pins as outputs but change the state to high. Finally the 3r... | It seems that you need a pull-down resistor on INT0,(you do not seem to mention you have one), my clue to needing a pull resistor is your problem number one, where your body affects the change of state. This problem arises because you have a high impedance input (INT0) and it is floating, there is no guarantee that you... | The cause of the wrong LED states was because I was not allowing enough time between port change commands.
When writing:
<pre><code> PORTBbits.RB6 = 1;
PORTBbits.RB5 = 1;
</code></pre>
It should actually be:
<pre><code> PORTBbits.RB6 = 1;
Nop(); //skip cycle
PORTBbits.RB5 = 1;
</code></pre>
... | https://electronics.stackexchange.com |
25,667 | [
"https://physics.stackexchange.com/questions/25667",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/-1/"
] | Some suggest that if a human was born and raised in a gravity-less environment their bone structure/strength would be different that for children born on earth. If this is the case do you think this change would be gradual (over several generations) or would it be immediate? Also would these changes be permanent in the... | I don't think that physical differences in skeletal structure would be apparent immediately because while yes there is less force acting on your growing bones, a large portion of that growth occurs while we are sleeping and horizontal. Our bones would have less weight to support but their overall structure is defined b... | I suspect that some direct physical differences would begin to show immediately. Children born an raised in that environment (whether gravity free or just lower gravity) would not have the same forces acting on them. Their bones could grow longer faster as there would be less force for the bones to support.
Permenan... | https://physics.stackexchange.com |
442,392 | [
"https://softwareengineering.stackexchange.com/questions/442392",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/423421/"
] | I'm trying to create repositories in my app. The problem is mainly with 2 entities:
<ul>
<li>Workspace</li>
<li>Member</li>
</ul>
a <code>Workspace</code> has many members, while a <code>Member</code> is only related to one Workspace (so a member exists only within a workspace).
Workspace has the functionality to add m... | <blockquote>
What I am not sure about is whether to have two separated repositories [WorkspaceRepository / MemberRepository]
</blockquote>
As usual, the answer is <em>depends</em>.
Ok, the relationship <code>Workspace</code> <- <code>Member</code> is enforced, programmatically, from <code>Workspace</code>, which act... | I think the question you should ask is: can a <strong>Member</strong> live outside a <strong>Workspace</strong>?
If a <strong>Member</strong> can exist outside a <strong>Workspace</strong>, then I would opt for separate repositories, and would <strong>assign</strong> a Member to a <strong>Workspace</strong>.
If a <stro... | https://softwareengineering.stackexchange.com |
170,483 | [
"https://physics.stackexchange.com/questions/170483",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/75433/"
] | Why may an air passenger experience an electric shock when he touches the door knob of the toilet of an aeroplane flying at a high altitude?
| You get the electric shock in exactly the same way you get it on the ground: walking around, or (as @Glen suggested) by rubbing the blankets you accumulate electric charge on your body. This charge is transferred to the rest of the system (the plane or the earth) when you touch the knob. Glen's link explains how the ch... | As the passenger walk through the carpeted floor of the plane the passenger might get charged up.So when he holds the metal knob he gives path to the charges to flow so he gets an electric charge.
| https://physics.stackexchange.com |
7,900 | [
"https://cs.stackexchange.com/questions/7900",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/4751/"
] | Suppose there are $n$ tasks, which need to be scheduled by a <em>pre-emptive</em> scheduler. Each task $T_i$ has a deadline $d_i$ and a total processing time $t_i$ associated with it. Now, all $n$ tasks are given a priory to the scheduler. The scheduler can run a task for 1 unit of time in one go. After each unit of ti... | The problem <em>minimizing maximal lateness</em> is discussed in the algorithm book by Kleinberg and Tardos under <em>greedy algorithms</em>. The solution is like yours, ordering the tasks by their deadline. Indeed the length of the tasks is not relevant.
The main ingredient of the proof is that any optimal solution $... | Here is a simple proof that the additional criteria that job need not be completed in one go is actually not necessary:
suppose the input is a sequence of tasks $\{(d_i, m_i)\}$, where $d_i$ is the deadline of task $i$ and $m_i$ is the duration of task $i$, for $i$ = 1, 2, ...,$n$. We can transform the input into ano... | https://cs.stackexchange.com |
228,445 | [
"https://mathoverflow.net/questions/228445",
"https://mathoverflow.net",
"https://mathoverflow.net/users/42278/"
] | Let $N$ be a fixed positive integer, and denote by $C(m)$ the number of
permutations on an $N$-element set that have exactly $m$ cycles (counting
$1$-cycles). Then it is in the literature that the polynomial generating
function
$$
\sum_{m=0}^N C(m)x^m = x(x+1) \dots (x+N-1),
$$
the rising factorial.
I would like a ref... | This follows from using generating functions a la Polya. By the cycle index formula, one sees that (using the notation of $C_n(k,j)$ for permutations in $S_n$ with $k$ cycles including $j$ fixed points)
$$
\sum_{n=0}^{\infty} \frac{t^n}{n!} \sum_{k,j} C_n(k,j) y^j x^k
= \exp\Big( tx y +\sum_{j=2}^{\infty} \frac{x t^... | For the reasons apparent below I shall use the notation $C_N(m,j)$, not $C(m,j)$. It is sufficient to prove
$$
\sum_{m=0}^N\sum_{j=0}^m jC_N(m,j)x^m=Nx\cdot x(x+1)\cdots (x+N-2),
$$
as this formula together with your first formula implies what you want to prove. The LHS of this is the same as
$$
\sum_{(\sigma,k)\co... | https://mathoverflow.net |
169,776 | [
"https://stats.stackexchange.com/questions/169776",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/87433/"
] | <em>I hope this question fits the site, if not feel free to flag.</em>
In the place I live there's a simple game of cards that you play on your own. Cards are 40, let's say A1 ... A10 , B1 ... B10 , C1 ... C10 , D1 ... D 10. The game goes as follows:
<ol>
<li>You dispose all of the 40 cards on the table, with their f... | We can ignore the rows and column, and just say that you have 40 positions and 40 cards that each point to a specific position.
For the first draw, you have 40 cards but only 1 card will make you lose the game (the card that points to the position you chose). This gives a probability of 39/40 to proceed to the next ro... | As @Jonas pointed out, in the whole game there's only one losing card, and it is the one you choose at first. If you ever flip that, you lose; if that is the last card you flip, you win.
Then a simpler approach to the question might be interpreting the game this way:
<ul>
<li>you name one card;</li>
<li>you shuffle y... | https://stats.stackexchange.com |
74,592 | [
"https://mathoverflow.net/questions/74592",
"https://mathoverflow.net",
"https://mathoverflow.net/users/8799/"
] | Let me denote $X_n$ the set of transpositions in $n$ elements. Equivalently, $X_n$ is the set of doubletons in $[1,n]\times[1,n]$. The cardinality of $X_n$ is $N=\frac{n(n-1)}{2}$.
If $f:{\mathbb Z}/N{\mathbb Z}\rightarrow X_n$ is a bijection, let us denote
$$r(f):=\min\{|\ell-m|;\ell\ne m\quad\hbox{and}\quad f(\ell)... | Close to $n/2$ is possible. I'll do odd $n$ and leave even $n$ for someone else's pleasure.
Let $m=(n-1)/2$. For $i=0,\ldots,n-1$ and $j=1,\ldots,m$, let $M(i,j)$ be the pair $\{i-j,i+j\}$ (all values taken mod $n$, of course). The solution is
$$ M(0,1).\ldots,M(0,m),M(1,1),\ldots,M(1,m),\ldots,M(n-1,1),\ldots,M(n-... | $R_n \geq n/16$ can be obtained by starting from an arbitrary $f$ and then switching pairs of transpositions to get rid of any overlapping pairs whose images are too close to each other.
Suppose $r(f) < k$, and suppose $f(l)$ overlaps some $f(m)$ with $0 < |l-m| < k$. We want to find some $l'\in{\bf Z}/N{\bf... | https://mathoverflow.net |
131,590 | [
"https://cs.stackexchange.com/questions/131590",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/101486/"
] | Consider the following language:
<span class="math-container">$$L=\{w \in \textstyle\Sigma_1 ^*\mid|w| \text{ is even and 1's can only occur in the second half of $w$}\},$$</span>
where <span class="math-container">$\Sigma_1 = \{0,1\}$</span>.
I need to show that this is not regular. I tried to prove this with the pump... | Two points:
<ul>
<li>the reasoning is incomplete: you need to show that pumping <span class="math-container">$y$</span> in the two first cases also leads to elements outside <span class="math-container">$L$</span></li>
<li>the reasoning does not use <span class="math-container">$ |xy|<d$</span> which entails that on... | Take <span class="math-container">$s = 0^d 1^d$</span>. Given a decomposition <span class="math-container">$s = xyz$</span> such that <span class="math-container">$|xy|\leq d$</span> and <span class="math-container">$y \neq \epsilon$</span>, you can check that <span class="math-container">$xy^0z \notin L$</span>, hence... | https://cs.stackexchange.com |
485,074 | [
"https://physics.stackexchange.com/questions/485074",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/175031/"
] | Definition of box operator in curved space time is <span class="math-container">$g^{\mu \nu}\partial_{\mu}\partial_{\nu}$</span> and in FLRW metric <span class="math-container">$g_{\mu \nu}$</span> is <span class="math-container">$diag(1 ,-a^2(t)$</span> <span class="math-container">$,-a^2(t),-a^2(t) )$</span> so the b... | If you take the scalar field in the curved spacetime the action is,
<span class="math-container">\begin{equation}
S=\int d^4x\sqrt{-g}\Big(g^{\mu\nu}\partial_\mu\phi\partial_\nu\phi-m^2\phi^2\Big)
\end{equation}</span>
Now the trick is that when you integrate by parts the term with the variation <span class="math-conta... | Because the definition of box operator is <span class="math-container">$g^{\mu \nu} \nabla_\mu \nabla_\nu$</span> and not <span class="math-container">$g^{\mu \nu} \partial_\mu \partial_\nu$</span>. You need to use covariant derivatives instead of partial derivatives. You will then get extra contributions from Christof... | https://physics.stackexchange.com |
45,448 | [
"https://security.stackexchange.com/questions/45448",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/2666/"
] | When doing a vulnerability assessment on a large network, it seems general practice to determine which hosts on the network are live.
This can be done in various ways. From what I have read it is good to do some ICMP scans, perhaps use a vulnerability scanner that has a discovery component, and perhaps do some TCP/UDP... | What I typically do for this depends on the time I have for the scan. If I'm looking for relatively fast discovery beyond basic nmap (ICMP + port 80 ACK) I do a discovery scan with a list of common TCP ports something like
<blockquote>
nmap -sP -PS 21,22,23,53,80,443,500,3389 [input range]
</blockquote>
If I've go... | you can use the flag -PE, which performs a ICMP echo
as an example:
<blockquote>
nmap -sn -n -PE 192.168.1.1-255
</blockquote>
or for a cleaner result
<blockquote>
nmap -sn -n -PE 192.168.1.1-255 | grep report | cut -d" " -f5
</blockquote>
| https://security.stackexchange.com |
352,924 | [
"https://softwareengineering.stackexchange.com/questions/352924",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/277786/"
] | Let's say that I have a custom database <code>contacts</code>, and there are two tables inside:
<pre><code>contacts
- user
- group
</code></pre>
A user may belong to many group, and a group can have multiple user. If I add/update/remove a user into the database, group should be updated too (if the user belongs to... | As @Laiv states in his comment you should think about how you want it to be consumed. How does the user of the API want this to work? Will it on the other side of the API be more beneficial to perform one or multiple actions to do this?
We cannot speak for what is best in your case.
With that said though I typically ... | Many things can go wrong between client and server. I would always make sure that one request can never put the DB in some erroneous state. Thus: if <code>group</code> needs to be changed upon a <code>PUT /user</code>, do so. Don't assume the client will be able to request the <code>PUT /group</code> as well!
The resu... | https://softwareengineering.stackexchange.com |
292,316 | [
"https://math.stackexchange.com/questions/292316",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | I have the following situation,
$$E = A X B + C X D$$
where $A,B,C,D,E$ and $X$ are matrices with proper dimensions. I want to obtain an expression like,
$$X = f(A,B,C,D,E)$$
i.e., leave $X$ alone at one side. Is there a way to do this? I am stuck. Thanks in advance!
PS: OK, my situation is more interesting. I have th... | Option 1 is the easier one. "Plug in" $3$ for $x$. Is the result $0$?
In general, if we have the curve $y=f(x)$, to find the $y$-coordinate at the point with $x$-coordinate $a$, we just calculate $f(a)$.
Dividing your polynomial by $x-3$ will work, but it is quite a bit more painful. And the procedure will not wor... | Like the answer below me said plug in 3. Notice f(x) = 3 + 15 + 9 - 27 = 0. So we know x-3 is a factor of this solution. You could also see this would have no remainder,
If we apply synthetic division (i dont know how to properly do it right here but ill give it a shot)
3 [ -1 1 5 3], if we apply synthetic ... | https://math.stackexchange.com |
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