qid int64 1 4.65M | metadata listlengths 3 3 | prompt stringlengths 31 25.8k | chosen stringlengths 17 28.2k | rejected stringlengths 19 40.5k | domain stringclasses 28
values |
|---|---|---|---|---|---|
15,829 | [
"https://softwareengineering.stackexchange.com/questions/15829",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/6775/"
] | For 4 years now, I've been the solo developer for a small company. We have a handful of well establish products in a niche industry. We will be hiring 1-2 developers soon, and that will likely change the way things operate around here.
While I won't have a "real" title, I will be "in charge" of this team. What I want ... | Get your house in order and ensure that things are setup for a collaborative effort.
<ul>
<li><strong>Version Control</strong> - You mentioned that you already have an SVN server, which is great. Make sure that you have established the repo and organized the projects in a standardized way.</li>
<li><strong>Automated B... | <blockquote>
hire someone a lot more competent than you
</blockquote>
... and with experience in large enterprise software project.
They will help you industrialize your processes and you will probably learn a lot from them.
You will teach them your niche market.
If you can't find such a person, I suggest you to ... | https://softwareengineering.stackexchange.com |
3,925,564 | [
"https://math.stackexchange.com/questions/3925564",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/654304/"
] | I have just started on probability theory and I was thinking about this object.
Let <span class="math-container">$(\Omega,\mathcal{F},\mathbb{P})$</span> be a probability space. Let <span class="math-container">$\mathcal{F_1},\mathcal{F_2}$</span> be sub <span class="math-container">$\sigma$</span>-algebras. If we just... | If you don't care about constructing explicit chart maps, you could say that <span class="math-container">$S = F^{-1}(0)$</span> is the zero set of the polynomial equation <span class="math-container">$F(x,y,z) = x^2 + y^2 -z^2 -1$</span>. Since <span class="math-container">$F$</span> is smooth, and the Jacobian <span ... | <span class="math-container">$x^2+y^2=1+z^2\subset \Bbb{R}^3$</span> is isomorphic to the cylinder <span class="math-container">$X^2+Y^2=1\subset \Bbb{R}^3$</span> through
<span class="math-container">$$(x,y,z) \to (\frac{x}{\sqrt{1+z^2}},\frac{y}{\sqrt{1+z^2}},z),\qquad (X,Y,Z)\to (X\sqrt{1+Z^2},Y\sqrt{1+Z^2},Z)$$</s... | https://math.stackexchange.com |
78,616 | [
"https://electronics.stackexchange.com/questions/78616",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/27363/"
] | lets say I loose the acdc adapter of a device, like a laptop or a wireless router, how do you know that another adapter will work with my device? my assumption is that I have to match the same voltage and the amperes have to be equal or higher than the missing adapter. But I heard of people that just used an adapter th... | <blockquote>
But I heard of people that just used an adapter that fit in the device and even with very different voltage specification the device worked, were they lying?.
</blockquote>
No, but the components in the device might wear out faster. There is a certain amount of give in the requirements, but it is genera... | Your assumptions are ok but with the understanding, that not all designs are so tolerant, but good ones are.
Constant current sources are normally used in such devices as LED strings that do not have current limiiting resistors.
| https://electronics.stackexchange.com |
747,530 | [
"https://physics.stackexchange.com/questions/747530",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/356340/"
] | In my book, it is written that:
<blockquote>
v = fλ in a given medium under conditions of fixed temperature and
humidity.
</blockquote>
It is also written that:
<blockquote>
one of the more important properties of sound is that its speed is nearly independent of frequency.
</blockquote>
So does this mean v ∝ λ (f = con... | <blockquote>
<em>if</em> the medium changes, the speed changes and thus the wavelength of the <em>same</em> sound also changes?
</blockquote>
Yes, this is the case. Although, there's a small error in your way of thinking. The frequency <span class="math-container">$f$</span> and the wavelength <span class="math-contain... | Sound travels quicker underwater than in air so I assume that if it is travelling at a different speed underwater then the wavelength changes as compared to air. I guess the speed of sound depends on the density of the medium it is traveling through which is why sound can not travel through a vacuum such as outer space... | https://physics.stackexchange.com |
531,977 | [
"https://physics.stackexchange.com/questions/531977",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/231920/"
] | I am currently reading about the steady magnetic field, and when I came across the vector magnetic potential I came to know that it can be defined in both the regions where current density <span class="math-container">$\mathbf J = 0$</span> and <span class="math-container">$\mathbf J\neq 0$</span> unlike the scalar mag... | Here is an explicit way to see why <span class="math-container">$\vec{A}$</span> is defined at all points.
The vector potential <span class="math-container">$\vec{A}$</span> is given by the following:
<span class="math-container">$$\vec{A}(\vec{r},t)=\int d^3r’ \frac{\mu_0}{4\pi}\frac{\vec{J}(\vec{r}’,t)}{\big| \vec{... | I think you can agree that the magnetic field <span class="math-container">$\mathbf B$</span> can be defined in regions where <span class="math-container">$\mathbf J$</span> is <span class="math-container">$0$</span> (think of the field around a current carrying wire). Since the vector potential <span class="math-conta... | https://physics.stackexchange.com |
88,927 | [
"https://mechanics.stackexchange.com/questions/88927",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/28943/"
] | I am not sure if if this is the right forum to ask this, but even my driving instructor cannot give me a clear answer, hence the question.
Learning driving on a toyota vios, automatic transmission. The interrelated questions I have are
<ul>
<li>Does the accelerator control and ensure gas is flowing through the engine? ... | In short, find a better driving instructor and, if possible, learn to drive a car with manual transmission.
The long (still simplified) answer:
<blockquote>
Does the accelerator control and ensure gas is flowing through the engine? The harder I press, the more gas flows in and the faster is the engine RPM. Is my mental... | Note, most of these answers are very specific to an automatic transmission and also a gasoline (petrol) engine -
The position of the throttle generally controls the volume of air allowed to enter the engine. The air volume will depend on the throttle position, current RPM of the engine and the current speed of turbo i... | https://mechanics.stackexchange.com |
2,511,960 | [
"https://math.stackexchange.com/questions/2511960",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/460456/"
] | Given 6 choices (order, numbers are completely arbitrary). If the choices can be only picked once and are to be picked at random six times, what is the probability that specifically choice 2 and 3 DO NOT occur in succession? In other words, what is probability that choice 1,4,5,6 are picked between choices 2 and 3. Aga... | Treat this as a people seated together problem. There are six people, and two people don't want to sit together. They are sitting in a line.
Here, there are $6$ people.
So, total number of arrangements = $6!$
Now, suppose $2$ and $3$ sit together always. Then, number of arrangements = $2 \times 5!$
So, you want the... | <strong>Hint</strong>. Compute the probability that $2$ and $3$ occur in succession.
| https://math.stackexchange.com |
440,018 | [
"https://softwareengineering.stackexchange.com/questions/440018",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/418310/"
] | There are a few threads about how to handle the version number with semver upon changes in dependencies (usually resulting in a +1 to the patch number). Mine is probably a corner case: <strong>what if you are just updating a development dependency</strong> (e.g., a linter, a test library, a formatter...)?
Usually, from... | Based on the Semantic Versioning specification, any changes to a versioned package must result in the release of a new version of the package.
If the development dependency is not part of the versioned package, then you don't need to do anything with the version. You could optionally use the build metadata portion to i... | If the change in development dependencies is not part of a larger change in the API or internals, don't even increment the version number. This would be considered "work in progress", not a release. Document this in a commit message and a "ReadMe" file.
Semantic versioning does not cover a use case ... | https://softwareengineering.stackexchange.com |
140,480 | [
"https://mathoverflow.net/questions/140480",
"https://mathoverflow.net",
"https://mathoverflow.net/users/22277/"
] | Suppose that $\mathcal{U},\mathcal{V}$ are ultrafilters on sets. Recall that $\mathcal{U}\leq_{RK}\mathcal{V}$ (here we say $\mathcal{U}$ is Rudin-Keisler less than or equal to $\mathcal{V}$) iff for each first order structure $\mathcal{A}$, the ultrapower $\mathcal{A}^{\mathcal{U}}$ is elementarily embeddable in $\mat... | Convention: I'll use my favorite notation $\mathcal U\otimes\mathcal W$ for what is called $\mathcal U\cdot\mathcal W$ in Jonathan Verner's answer and $\mathcal W\cdot\mathcal U$ in the question. It follows from a theorem of Mary Ellen Rudin (in the 1960's if I remember correctly) that any RK-equivalence between two su... | It is relatively consistent with ZFC plus the existence of a measurable cardinal $\kappa$ that all your cancellation laws hold for the $\kappa$-complete ultrafilters on $\kappa$. Your question is more general than this, but the case of measures on a measurable cardinal is surely one of the main contexts in which we use... | https://mathoverflow.net |
36,752 | [
"https://dba.stackexchange.com/questions/36752",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/5791/"
] | As the question states I'm currently internally strugling with a database design question. I work as a software developer (and I tend to do everything database related as well since we're rather small) for a financial company and we're doing a revamp of our current system.
Our system is starting to become outdated and... | You want to store the details of the transaction at the time it happens and not calculate at the time you select the record. This is because the information the calculation is based on (such as tax rate ) is subject to change. So yes, store the tax calculation. This is not an issue of performance, it is an issue of tem... | As I understand, you are going to store some intermediate calculated results along with source data and rely on both in different parts of your system. Personally I do avoid such approach. This is definitely not redundant.
Why do you need to do this? It seemed that somewhere you have performance issues. I think the qu... | https://dba.stackexchange.com |
382,296 | [
"https://electronics.stackexchange.com/questions/382296",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/141747/"
] | I had an accident in the house and one of the LAN (cat. 6) cables including RJ-45 connector has been flooded. This "flooded" cable, along with other (dry) ones, was connected into a working gigabit router with WiFi but it was the only one that was not connected to the PC.
After a few minutes, I took out the plug from t... | All Ethernet connections are supposed to be galvanically isolated with transformers inside the device. This serves to prevent ground loops and protect the components behind the transformer from external voltage and short circuits. Most likely the router is perfectly fine. The main thing I would be worried about is wate... | dish water is unlikely to damage the router, but it could degrade the connections between the gold-plated contacts and the wires behind them.
If you're worried rinse the cable with clean water, pat dry and then rinse again with denatured alcohol then dry again, that should get most of the moisture out. The router wi... | https://electronics.stackexchange.com |
479,389 | [
"https://electronics.stackexchange.com/questions/479389",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/185830/"
] | Am wanting to use this Capacitive push button for a project but am having trouble understanding the application circuit.
I understand that it is a PNP Highside switch and that current will flow to the load when the base is connected to ground. However I don’t understand why the pin on the controller is a output and th... | The controller pin is an output as it has to drive the PNP's input (the PNP's base).
I think you're confused because the current that is needed to activate the PNP flows <strong>into</strong> the controller. That doesn't make it an input. It is still an output as it drives (controls) the circuit connected to it.
Inpu... | The transistor is switching the current that runs the LEDs the sensor itself does not need this connection.
The purpose of R1 is to ensure that the transistor (and therefore the LED) turns fully off whe the controller is not commanding light. without it there's a likelyhood of a small leakage of current and thus a fai... | https://electronics.stackexchange.com |
34,945 | [
"https://mechanics.stackexchange.com/questions/34945",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/21433/"
] | When the engine is hot the starter it will not work. When I run a wire from the battery to the neutral safety switch it starts. Also, after it cools down it starts.
| <blockquote>
... and screw it in by hand, maybe a couple of twists with an oil filter strap wrench.
</blockquote>
Don't over-tighten it. Tighten it by hand then at <em>most</em> about a quarter turn with a strap wrench or by hand (definitely not a couple twists!)
A good indicator is if you can <em>unscrew</em> your... | Oil filter strap wrenches are made to <em>remove</em> filters, not put them on. Oil filters should be made <strong>finger-tight</strong>, not hand-tight and certainly not wrench-tight. The filter will go through expansion-contraction cycles with the heat from the engine, it will tighten itself up, as you've testified t... | https://mechanics.stackexchange.com |
47,859 | [
"https://physics.stackexchange.com/questions/47859",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/16995/"
] | I'm reading Sean Carroll's book on general relativity, and I have a question about the definition of an inertial reference frame. In the first chapter that's dedicated to special relativity, the author describes a way of constructing a reference frame in the following manner:
<blockquote>
"The spatial coordinates (x... | What Sean Carroll refers to is acceleration as indicated by an accelerometer that is right next to the rods, co-moving with the rods.
The readout of an accelerometer is a <em>local measurement</em>. That is important in this stipulation about the rigid rods. The demand is not about being unaccelerated with respect to ... | The standard test to find out if you are in an inertial frame is to surround yourself with some non-interacting particles e.g. in a sphere. If the shape made up by the particles does not change with time then you are in a (locally at least) inertial frame. Curved space can be detected by either the volume and/or the sh... | https://physics.stackexchange.com |
189,653 | [
"https://physics.stackexchange.com/questions/189653",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/46199/"
] | I was reading Schutz, A First Course in General Relativity. On page 9, he argued that the metric tensor is symmetric:
<blockquote>
$$
ds^2~=~\sum_{\alpha,\beta}\eta_{\alpha\beta} ~dx^{\alpha}~dx^{\beta}
$$
$\text{Note that we can suppose}$ $\eta_{\alpha\beta}=\eta_{\beta\alpha}$ $\text{for all}$ $\alpha$ $\text{an... | Assume $\eta_{\alpha\beta} \neq \eta_{\beta\alpha}$. Because it's irrelevant what letter we use for our indices,
$$\eta_{\alpha\beta}dx^{\alpha}dx^{\beta}=\eta_{\beta\alpha}dx^{\beta}dx^{\alpha}.$$ Then $$\eta_{\alpha\beta}dx^{\alpha}dx^{\beta} = \frac{1}{2}(\eta_{\alpha\beta}dx^{\alpha}dx^{\beta} + \eta_{\beta\alpha}... | The metric tensor is created from the spacetime interval equation. On top of that, $[dx^\alpha,dx^\beta]=0$. Suppose we have a 1+1 dimensional spacetime, if you are given an interval equation resembling:
$$ds^2=-a\,dx_0^2+b\,dx_1^2+c\,dx_0dx_1$$
Obviously, $\eta_{0\,0}=-a$ and $\eta_{1\,1}=b$. Now, you <em>can</em> a... | https://physics.stackexchange.com |
1,202,110 | [
"https://math.stackexchange.com/questions/1202110",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/75830/"
] | I am in a first year differential equations course, and in class on Friday, the teacher did a problem from the book that I wasn't quite sure how to solve (yet I'm sure has a possibility of showing up on a test!).
The question I have written in my notes is: "create a differential equation that has $ y = C_1e^{-2x} + C... | Yes, you're on the right track.
Presuming you're looking for a linear d.e. with that general solution, note that since the solution space has three parameters, the d.e. must be third order (also, since $0$ is evidently not a solution, we know immediately that the d.e. cannot be homogeneous).
First, we find the (homog... | Travis answer is nice, but, if you invest some time learning operators it may be easier to understand. Let $D=d/dx$. The operator $(D+2)$ gives $e^{-2x}$ in its kernel; $(D-2)[e^{-2x}] = -2e^{-2x}+2e^{-2x}=0$. Then $(D-3)$ for the $e^{3x}$, but, you have $x$ so repeat. Hence $L = (D+2)(D+3)^2$ gives you the operator wh... | https://math.stackexchange.com |
55,214 | [
"https://mathoverflow.net/questions/55214",
"https://mathoverflow.net",
"https://mathoverflow.net/users/10774/"
] | The first place where the amenability problem for Thompson's group $F$ appears in the literature is, I believe, 1980 in a problems article by Ross Geoghegan. I have heard, however, vague comments to the effect that the problem was considered by other people before this. Does anyone have any knowledge about the existe... | Richard Thompson visited me at Princeton several times I believe in the mid '70's, gave me copies of some handwritten notes about his groups (which I shared with a few people), and raised the amenability question, which we discussed a bit. I don't think Thompson's groups were very widely known at the time.
| I think the story is this. Greenleaf gave a series of lectures on amenability in Berkeley in 1967. He mentioned the Day-von Neumann's problem, in particular. At that time Thompson was a student in Berkeley and has discovered his group already, he also proved that all free subgroups of his group are cyclic. Thus the pro... | https://mathoverflow.net |
181,965 | [
"https://security.stackexchange.com/questions/181965",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/168744/"
] | <strong>General Background:</strong>
I have written an echo server trying to implement an example of BoF in C that utilizes a strcpy() function call like such:
<pre><code>// .... including the corresponding libraries depending on host environment
#include <stdio.h> ... | My guess is that you overwrote other local variables (i or sending_str) and thus caused premature crash.
You will have to look at the generated code to see what’s going on. Depending on settings the compiler can do interesting things to your code, including inlining the called function altogether, thus making your ret... | Voted @manduca for best answer because the answer provided the direction of research and more testing. And after several tests, I was able to conclude the complete answer to my question asked:
<ol>
<li>the type of the function return value had nothing to do with the
problem, it was simply the wrong assumption</li>
<l... | https://security.stackexchange.com |
15,530 | [
"https://dba.stackexchange.com/questions/15530",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/2187/"
] | If I create a foreign key constraint for table 'photos' in phpmyadmin, I later see that the constraint is named 'photos_ibfk_1', and the next constraint is called 'photos_ibfk_2', etc. From this I have gathered that [tablename]_ibfk_constraintIndex is the convention for DB constraints in MySQL. Is this correct? What do... | innodb foreign key. It's just short hand naming convention. You could call it asdfqwerty and the name would still work.
| Although foreign key names can be anything, it's actually a good practice to follow the convention of putting the table name first.
The most important reason for this is that foreign key names must be <strong><em>unique within a database</em></strong> (contrarily to index names, which must only be unique within each t... | https://dba.stackexchange.com |
29,006 | [
"https://stats.stackexchange.com/questions/29006",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/3525/"
] | I studied the standard econometrics textbooks about panel data, but most textbooks only mention the difference between balanced and unbalanced panels. The advantage of having balanced panel data is not usually explained. I would like to know: <strong>What is the advantage of having a balanced panel?</strong> I believe ... | I believe these are largely historical reasons. In the 1940s, one had to conduct analysis of variance with paper and pencil, so having balanced designs led to simple sums for both means and variances. Any imbalance would require inverting matrices 4x4 or larger (I've done it a couple of times on regression exams, and n... | I think whenever you have unbalanced panels, you need to come up with a formal description of why that is the case. You need to worry about self-selection, nonresponse, and attrition, especially if you're interested in population parameters and consistency. For most estimators, the mechanics are largely the same.
| https://stats.stackexchange.com |
1,399,335 | [
"https://math.stackexchange.com/questions/1399335",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | Let $\{a_n\}$ be a decreasing sequence of non-negative real numbers such that $\lim \inf (na_n)=0$ , then is it true that $\lim (na_n)=0$ ?
| Consider the two functions $1/n$ and $1/n^2$. Clearly we dont care of the definitions of $a_1,\ldots,a_{n_0-1}$, as long as they are in decreasing order. Set
$$a_{n_0}=\frac{1}{n_0^2}.$$
Let $n_1$ be the smallest positive integer such that $\frac{1}{n_1}<\frac{1}{n_0^2}$. Then define $a_{n_0+1},\ldots,a_{n_1-1}$ to... | For $n=2^{k^2}$ to $2^{(k+1)^2}-1$, let $a_n=\frac{1}{k2^{k^2}}$. Then our sequence is non-increasing and has the desired property. Indeed $na_n$ has limit $0$ along the subsequence with $n=2^{k^2}$, but $na_n$ can take on arbitrarily large values. One can perturb the example a bit if one wants strictly decreasing.
| https://math.stackexchange.com |
3,323,655 | [
"https://math.stackexchange.com/questions/3323655",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/678826/"
] | <span class="math-container">$$\int_0^\infty\frac{\exp{\left(-\frac {y^2}{4w}-t^2w\right)}}{\sqrt {\pi w}}dw=\frac{\exp(-ty)}t$$</span> for <span class="math-container">$t$</span> and <span class="math-container">$y$</span> positive. This integral is useful in the following context: suppose we are given <span class="ma... | Let <span class="math-container">$F(y,t)$</span> be given by the integral
<span class="math-container">$$F(y,t)=\int_0^\infty \frac{e^{-y^2/4\omega-t^2\omega}}{\sqrt \omega}\,d\omega\tag1$$</span>
First, enforcing the substitution <span class="math-container">$\omega\mapsto\omega^2$</span> reveals
<span class="math... | <span class="math-container">$\newcommand{\bbx}[1]{\,\bbox[15px,border:1px groove navy]{\displaystyle{#1}}\,}
\newcommand{\braces}[1]{\left\lbrace\,{#1}\,\right\rbrace}
\newcommand{\bracks}[1]{\left\lbrack\,{#1}\,\right\rbrack}
\newcommand{\dd}{\mathrm{d}}
\newcommand{\ds}[1]{\displaystyle{#1}}
\newcommand{\expo}[... | https://math.stackexchange.com |
222,568 | [
"https://mathoverflow.net/questions/222568",
"https://mathoverflow.net",
"https://mathoverflow.net/users/54552/"
] | Given $f\in H^1(\mathbb{R}^3)$ and $t>0$, consider the following integral:
$$I_f(t):=\int_{\mathbb{R}^3}\int_0^{+\infty}e^{-s+i\frac{(s+|x|)^2}{t}}f(x)dsdx$$
I need to show that $I_f(t)$ is finite, and to find how fast it grows as $t\rightarrow +\infty$.
If $f$ decays fast enough at infinity so that $f\in L^1(\math... | Here is a more detailed version of my sketch in the comments above.
First of all, let $\rho$ be a compactly supported smooth function (cutoff function) such that $\rho=1$ on $B_1$. Write $f=\rho f+(1-\rho)f$. Since $\rho f$ is compactly supported, it is $L^1$, and your old argument applies to it, so it remains to boun... | We set $h=1/t$ so that $h\rightarrow0_+$. We have with $I_f(t)=J_f(h)$
$$
J_f(h)=\int e^{ih\vert x\vert^2} f(x) g(h,\vert x\vert)dx,
$$
with
$
g(h,y)=\int_0^{+\infty} e^{-s+ih s^2+2ihsy}ds.
$
The function $g$ is $C^\infty$ and bounded and such that $g(0,y)=1.$
If $f$ is in the Schwartz space,
we find that $J_f$ is smoo... | https://mathoverflow.net |
296,525 | [
"https://softwareengineering.stackexchange.com/questions/296525",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/146711/"
] | I'm creating a website CMS in Rails (for fun and learning) and I'm trying to figure out how I'm going to store the data for an "about" page. Due to the nature of this page, the data will only have one instance, it may change and will be persistent so I want to store it in my database but I'm not sure if this is the bes... | I'd store it there, since you have a database anyway and each release can just update the row as needed. I don't see why it matters how many instances you'll have of the class which uses the data. I sometimes create a system table that holds configuration data for the application even though there will only ever be o... | There are number of ways doing it,
<ol>
<li>Sometimes even if you see it as a single record, actually it could be multiple records. For example if you want to keep the history of the entries about records could be a list of records with one marked as active</li>
<li>If you have some text in about page and you want to ... | https://softwareengineering.stackexchange.com |
658,585 | [
"https://physics.stackexchange.com/questions/658585",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/311052/"
] | person rather new to physics here, and I just started on a rather simple book on particle physics, but I was flustered to learn that there are particles without mass enforcing the fundamental forces (Like the gluon or the Higgs boson). There is some Greek term for it but I don't remember. I just can not comprehend this... | This is not a stupid question and you are not alone. Many have struggled with the same concern when first introduced to the idea of a massless particle. In physics today, we use the word mass in association with the properties of a particle that is at rest (not moving). However, it is also true that mass is associat... | As for the title question, massless particles are useful because despite being massless they still carry information. Individually it's a tiny amount but added up it can be any amount needed.
| https://physics.stackexchange.com |
69,311 | [
"https://chemistry.stackexchange.com/questions/69311",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/41791/"
] | Is there any specific rule to what kind of molecules can be eliminated in condensation polymerization? From Wikipedia I found that "small molecules" like $\ce{H2O}$ can be eliminated. Is there a more accurate criteria to what these "small moecules" are?
| As an abstract answer, to me it would be whatever is commercially viable to manufacture and sell. Commercial polymerisation processes eliminate ethylene glycol (in the manufacture of PET) which is probably recycled, methanol in transesterification reactions, ethene or propene in metathesis type, phenol (recycled) in ... | yes , only small molecules such as H2O or CO are eliminated , as such it is no rule . if large and bulky molecules are extracted as waste, it will no longer be called an elimination product but a side-product. i hope you understand now !
| https://chemistry.stackexchange.com |
63,104 | [
"https://mechanics.stackexchange.com/questions/63104",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/35395/"
] | My exhaust pipe in my 2007 maxima has a crack about an inch outside of a flex pipe, and the repair cost is more than the car is worth to me.
The 2 pipes are totally separate, but right next to each other. I was thinking of buying an exhaust coupler that is slightly bigger on one end, and place the smaller part into t... | I figured out that the formula stated in the paper for fuel flow below:
fuelFlow = (MAF * 3600) / (14.7f * 820) ; // l/h
is off by a single 0(needs an extra 0 added to the right of decimal).So we need to divide by 10, so the actual formula for fuelFlow now becomes:
fuelFlow = ((MAF * 3600) / (14.7f * 820)) / 10 ; ... | I used above formulas to create below gnuplot helper functions
<pre><code>vol_eff = 0.8322
f_time(x) = x/1000
f_rpm_to_rps(x) = x/60
f_mbar_to_kpa(x) = x / 1000 * 100
f_celcius_to_kelvin(x) = x + 273.15
f_imap(rpm, press_mbar, temp_c) = f_rpm_to_rps(rpm) * (f_mbar_to_kpa(press_mbar) / f_celcius_to_kelvin(temp_c) / 2)
... | https://mechanics.stackexchange.com |
353,193 | [
"https://mathoverflow.net/questions/353193",
"https://mathoverflow.net",
"https://mathoverflow.net/users/143172/"
] | Let <span class="math-container">$R$</span> be a noncommutative ring with unit, let <span class="math-container">$P$</span> be a projective left <span class="math-container">$R$</span>-module, and denote <span class="math-container">$^{\vee}\!P := \,_R\mathrm{Hom}(P,R)$</span>. One often sees it written that projectivi... | You are right. There is no such a map as the one you are trying to describe.
Here is a map that actually exists. Let <span class="math-container">$R$</span> and <span class="math-container">$S$</span> be two noncommutative rings with units, and let <span class="math-container">$P$</span> be an <span class="math-cont... | Doc, you ain't write no evaluation map. If <span class="math-container">$R$</span> is commutative, you write the trace map. If <span class="math-container">$R$</span> is noncom, god knows what you write. The evaluation map, that is an isomorphism for a finitely generative projective generator and a homomorphism of <spa... | https://mathoverflow.net |
51,631 | [
"https://physics.stackexchange.com/questions/51631",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/20003/"
] | My question is very naive and could sound strange but it seems to me natural in so far as the Planck constant is related to the first quantization (of newtonian particle mechanics/galilean relativity) while the Dirac one shows up in the second quantization (of fields theory/special relativity)... Anyway I think the con... | "First quantization" and "second quantization" are widely used names for the same procedure applied to two different classical systems – classical mechanics and classical field theory. In both cases, the operation introduces Planck's constant and it's always the same constant. For example, the angular momentum $J_z$ is... | Nah, they're the same. Even Planck's constant comes from fields; he was looking at the electromagnetic radiation (which is all field, all the time) getting kicked off by a warm black body.
Also, while most people use $h \nu$ for the energy of a photon, in grad school (physics) we often used $\hbar \omega$. Also, usua... | https://physics.stackexchange.com |
689,989 | [
"https://physics.stackexchange.com/questions/689989",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/317995/"
] | Suppose two waves interfere constructively at some point, in a double slit experiment set-up. This essentially means that the two waves have their peaks and troughs in sync with each other, and there is a phase difference of <span class="math-container">$2n\pi$</span>. However, it is said that, we obtain bright fringes... | A detector, such as your eye, does not respond at the rate of oscillation of the optical or RF frequency, instead it responds to the variation of its peaks or to variation of its average power (energy per cycle). These variations are many orders of magnitude slower than the oscillation rate. For example your eyes can t... | Think of a stone dropped in a pool, the waves will be present forever if the water is ideal and the pool walls perfectly reflecting. If by some coincidence in maybe a 1000 years the surface looks calm, the energy is not lost .... waves will reemerge again.
For light an excited electron creates a photon in the EM field,... | https://physics.stackexchange.com |
189,498 | [
"https://softwareengineering.stackexchange.com/questions/189498",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/81884/"
] | I have one one software system which allows developers to specify an ID or name to create <code>NodeReferences</code>. Both work fine, but ID's are not guaranteed to be the same across different environments. I've tried in documentation and in conversations to stress that ID's should not be hard coded (since they will ... | Hide the <code>int</code> ids in opaque objects/structs;
<pre><code>var id = level.id;
</code></pre>
Here <code>id</code> is such a struct.
This way you can remove the <code>ById(int)</code> method and replace it with <code>ById(Id)</code> and it still lets you keep the cashed <code>Id</code>s for future use.
You ... | Provide a better API:
<pre><code>var node = NodeReference.byName("Reporting", "Default", "Patient", _patientGuid.ToString());
</code></pre>
Or provide utility functions to fetch important nodes
<pre><code>var node = NodeReference.byName( theDefaultPatientNode(), _patientGuid.ToString() );
</code></pre>
Or have the ... | https://softwareengineering.stackexchange.com |
2,238,144 | [
"https://math.stackexchange.com/questions/2238144",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/370916/"
] | Use the convolution theorem to show that
$$\int_{0}^1 u^m(1-u)^ndu = \frac{m!n!}{(m+n+1)!}$$
I don't know where to start on this, do I have you use commutative property of convolution?
| The inner product <strong>can be defined</strong> in basis-independent way. That is $\langle v,w\rangle=|v|*|w|*cos(a)$.
This value can be calculated easily if you have coordinates of the vectors in some orthonormal basis. First you have to prove that inner product is linear, than you need to present each vector as a ... | You are right, that formula is not invariant under change of basis. The inner product is, but how you compute it is not. So when you change basis you need to change the formula to have the same inner product.
| https://math.stackexchange.com |
1,728,390 | [
"https://math.stackexchange.com/questions/1728390",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/296971/"
] | How do I find the value of $$\lim_{x\to\ 0} \frac{a^{\tan\ x} - a^{\sin\ x}}{\tan\ x - \sin\ x}$$
in easy way.
| Let $u=\tan(x)-\sin(x)$. Note that as $x\to 0$, $u\to 0$. Then, we can write
$$\begin{align}
\lim_{x\to 0}\frac{a^{\tan(x)}-a^{\sin(x)}}{\tan(x)-\sin(x)}&=\left(\lim_{x\to 0}\frac{a^{\tan(x)-\sin(x)}-1}{\tan(x)-\sin(x)}\right)\,\left(\lim_{x\to 0}a^{\sin(x)}\right)\\\\
&=\left(\lim_{u\to 0}\frac{a^u-1}{u}\rig... | Observe that
$$ \lim_{x \to 0} \frac{\tan x - \sin x}{x^3/2} = 1
$$
Hence
$$\lim_{x\to\ 0} \frac{a^{\tan x} - a^{\sin x}}{\tan x - \sin x} = \lim_{x\to\ 0} \frac{a^{\tan x} - a^{\sin x}}{\tan x - \sin x}\frac{\tan x - \sin x}{x^3/2} = 2\lim_{x \to 0} \frac{a^{\tan x} - a^{\sin x}}{x^3}
$$
Now l'Hopital only need to be ... | https://math.stackexchange.com |
87,601 | [
"https://mechanics.stackexchange.com/questions/87601",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/1623/"
] | When using a Volkswagen Passat 2014 and going backward, I notice that the backup camera turn off when I start gaining speed (over > ~30 km/h). How can I prevent the backup camera from turning off?
| It's set in the ECU in the "Park/Steer Assist" module and controlled by an adaption channel called "Switch-off speed for parking assist" IIRC and you can change the value there (ironically jwh20 was correct in their initial guess - it's usually 15kph from the factory), the value has both lower and u... | <h1>I want to offer more on the frame challenge, that you just shouldn't.</h1>
What you are doing is bypassing a convenience feature that refuses to support you being more unsafe.
Start by considering what a reversing camera <strong>can't</strong> do for you. It can't give full context. You only see what a small fishey... | https://mechanics.stackexchange.com |
23,027 | [
"https://stats.stackexchange.com/questions/23027",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/9224/"
] | Just trying to see if my reasoning is correct....
If the probability that next saturday will rain is 0.25 and the probability that next sunday will rain is 0.25, what is the probability that during the weekend will rain?
Assuming that A and B are independent events,
P(A∪B)=P(A)+P(B)-P(A∩B)
and
P(A∩B)=P(A)*P(B)
th... | Use the <code>predict()</code> command's <code>newdata</code> argument to obtain mean responses at given covariate value, then generate data using the <code>rnorm()</code>, <code>rbinom()</code>, <code>rpois()</code> function appropriate for your GLM. For Gaussian data you'll also need to specify a dispersion parameter... | I just realized that the <em>stats</em> library in R has a function <strong>simulate</strong> which can simulate from fitted GLMs. I believe this is what you want to do.
| https://stats.stackexchange.com |
135,237 | [
"https://cs.stackexchange.com/questions/135237",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/131674/"
] | If RAM is a short term memory and SSD is a long term memory, why don't microarchitecture of computer nowadays use SSD or another long term memory for saving temporary data like hidden variable for programming?
If it's about speed, then SSD can improve its speed, is it possible that SSD will become faster than RAM at so... | There's two simple reasons, one fundamental and one related to our current technology. First the technical one: volatile storage is (generally) faster than non-volatile storage. It has fewer requirements - it only needs to store the data for a short while until it gets refreshed, so it's not a surprise that it often is... | @orlp is already discussed speed. There's probably more than could be added (e.g., about latency vs. bandwidth), but let's leave that aside for now.
<h3>Write Endurance</h3>
Instead, let's start by considering a completely separate point: write endurance. Most SSDs use Flash memory. Writing to Flash memory slowly wears... | https://cs.stackexchange.com |
133,952 | [
"https://softwareengineering.stackexchange.com/questions/133952",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/25849/"
] | Today I found what looked to be my supervisor's password in some code in version control. The password is to a database. He is very experienced and has explained before how to avoid having passwords in the source code.
How should I handle this situation? Is there a best practice about preserving evidence of a security... | All version control systems I am aware of have some form of "obliterate" capability that allows you to remove data permanently. This is normally not something you want to do lightly as it inevitably involves rewriting history. Most version control systems in fact restrict this ability to administrators or at least to u... | Remove the reference from the code, as it's not safe.
Communicate to him that his current password has become compromised listing the reason.
Have him change it and tell everyone coding this should never happen again.
| https://softwareengineering.stackexchange.com |
136,676 | [
"https://softwareengineering.stackexchange.com/questions/136676",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/46283/"
] | If you are starting up a web, mobile, or standalone application, how well must you know the rest of your collaboration team? Would it be a bad idea to ever work with someone unless I've met him personally? Is it okay to sometimes accept small public contributions such as code snippets and artwork without meeting the co... | Not necessarily. It depends on the personalities of the people involved.
I've seen musicians actually write and perform compositions using Internet tools, never meeting in person. Programmers have the magical ability of distributed source control, so face-to-face meetings are not required at all to achieve something... | <strong>No.</strong>
You need to meet in person, and share some time together to build <strong>trust</strong>.
Once you have trust going on, you can stay 3 timezones apart and hardly even notice.
| https://softwareengineering.stackexchange.com |
41,163 | [
"https://dba.stackexchange.com/questions/41163",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/4490/"
] | I've just migrated my ERP database server from SQL Server 2000 to SQL Server 2008 R2 (both Enterprise Edition) and that migration went fairly smoothly.
However, now that I'm trying to recreate replication from the ERP to the Reporting environment, I have two publications that are trying to create a series of <code>MS... | If replication is automatically creating indexes, then you probably can't avoid it. The only way to do so might be to create a matching index on the subscriber.
Try running the merge agent from the command line with a more verbose logging level and see what information it returns. It might be bumping against somethin... | Make sure that your subscription database is new one. Issue will be resolved if you use new database for subscription. This worked for me.
| https://dba.stackexchange.com |
17,568 | [
"https://softwareengineering.stackexchange.com/questions/17568",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/7226/"
] | Timesheets are something that I've never been fond of, but none the less something that is a requirement within my company. They don't bother me so much, but they seem to really grind some other people's gears. I suppose I have a few questions, and feedback would be great.
<ol>
<li>Are you required to do timesheets, a... | As a manager yes I get the team to do timesheets. Here's why and a few notes on how they're implemented to, hopefully, minimise disruption:
<ol>
<li>As a business much of our work is done on a time and materials basis. Without timesheets that obviously doesn't work. We have 10 clients and a range of different proje... | I don't get the anti-timesheet thing at all. Timesheets are what cause you to get paid. No timesheet, no money. I like getting paid, therefore I like my timesheets. I've never had a timesheet that took more that five minutes a day to fill out. Honestly, there are plenty of much more annoying things in my day than a fiv... | https://softwareengineering.stackexchange.com |
108,950 | [
"https://security.stackexchange.com/questions/108950",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/-1/"
] | The general method used for storing passwords securely involves generating a salt, feeding the salt and password to the password hashing function, and then storing the salt and the output of the password hashing function in the database.
However, instead of generating a random string of characters as a salt, why isn't... | The reason for adding a little salt to a password before hashing it, is to make it much harder to reverse the hashing. Therefore a completely random number is the best solution.
Having to come up with a "derived salt" number which is unique enough so as most of your user passwords have a different hash is going to be ... | I don't think a unique derived salt is any worse than a random unique salt. The only purpose of a salt is to confound discovery of the passwords when the table of hashes is obtained.
For a password to be discovered from a hash, it has to be somewhat guessable in the sense that it found in an existing rainbow table, or... | https://security.stackexchange.com |
444,248 | [
"https://physics.stackexchange.com/questions/444248",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/212841/"
] | I have seen in many textbooks and sources which say that we can't experimentally measure potential energy but we can measure differences in potential energy.
<span class="math-container">$$\Delta U_g=-W_g$$</span>
Choosing zero potential (reference point) at the ground.
Now if I measure change in gravitational pote... | Simply put, potential energy is the energy an object possesses because of its position. Position, or location, is always relative. Therefore there is no such thing as an exact or absolute position in space and consequently no exact potential energy.
Potential energy must be measured relative to something. Suppose a ... | It's because the "zero point" you mentioned is arbitrary, and doesn't have to be zero. I could just as say that <span class="math-container">$GPE(y = 0) = 10 \text{J}$</span> or any other arbitrary number. In that case, even thought I could find <span class="math-container">$GPE$</span> at whatever height the ball reac... | https://physics.stackexchange.com |
640,877 | [
"https://electronics.stackexchange.com/questions/640877",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/325257/"
] | I have 0.35mm laminated steel sheets, and I want to make it bond together to form the stack. What is the method that I can use?
This stack will be used to be the stator for bearingless motor in temple design. So! I need to form a stack and cut it.
| It should be fine, since each SMPS is doing its own rectification and switching on the primary side of the transformer. It's the secondaries where they meet, and they all have DC.
| Yes. It's common practice to feed them from two different UPSes, often on separate phases. Personally, I have one supply on UPS and one directly from grid. I don't know if the UPS tries to match output phase with grid or not but this does not matter for SMPSes.
| https://electronics.stackexchange.com |
65,142 | [
"https://security.stackexchange.com/questions/65142",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/40127/"
] | Fed up with the following definition.
Reflected attacks are those where the injected script is reflected off the web server, such as in an error message, search result, or any other response that includes some or all of the input sent to the server as part of the request. Reflected attacks are delivered to victims... | So let's say you navigate to www.example.com/page?main.html and it puts you on the main page of example.com. Now you navigate to the index, which is located at www.example.com/page?index.html. You start to wonder, what other pages are there?
So you type in www.example.com/page?foo and hit enter, and you get an error p... | <h2>Reflected XSS</h2>
I send a victim a link to <code>http://example.com/page?var=<script>alert('xss')</script></code> and somewhere on the page that value is echoed back to the victim. The value is only on the page if they follow my special link.
The downside of this type is I have to specifically attac... | https://security.stackexchange.com |
34,087 | [
"https://mathoverflow.net/questions/34087",
"https://mathoverflow.net",
"https://mathoverflow.net/users/7872/"
] | Suppose $(x_\alpha)_\alpha$ is an uncountable, linearly independent family of norm one vectors in a Banach space. Can one always select a basic sequence (or at least a minimal system) from this family? I suspect the answer is no but I cannot come up with an example.
Thank you!
| Not a basic sequence. Consider $e_0 \oplus e_\gamma$ in $R\oplus H$ for $H$ a non separable Hilbert space, or, if you want a separable example, make $e_\gamma$ a Hamel basis for a separable Hilbert space.
EDIT: Aug 2. Every separated sequence of unit vectors contains a minimal subsequence with bounded biorthogonal f... | Indeed, such sequences exist. Sequences that have the same closed span as any of their subsequences appear in the literature under the name <em>overcomplete</em> or <em>overfilling</em> (see for example <em>Byorthogonal Systems in Banach Spaces</em>, by Hajek, Montesinos, Vanderwerff and Zizler, Excercise 1.1, page 42... | https://mathoverflow.net |
70,498 | [
"https://stats.stackexchange.com/questions/70498",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/29109/"
] | I have performed a regression in SAS and extracted the estimated correlation matrix of the parameters, which includes the intercept. One of my variable parameters has a strong correlation to the intercept parameter. I have not seen this before and want to be sure I understand what it means (if anything). Can anyone pro... | In simple terms, imagine you fix one parameter, say the intercept, and estimate the slope. The question is: as you vary the fixed parameter, will your slope estimate change? It will to some degree, and the strength/direction of the effect is the correlation of the parameters.
Suppose you have a simple linear regressi... | Just in case, the correlation refers to the <em>estimated parameters</em>, and springs from the fact that they are derived using the same data. It does <em>not</em> imply a correlation between the unknown parameters being estimated, which being constants (in the frequentist approach), cannot have "correlation". In that... | https://stats.stackexchange.com |
65,067 | [
"https://mechanics.stackexchange.com/questions/65067",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/46875/"
] | Handbrake is off all this time.
I read/heard that while in complete stop or idle, when car is in neutral, there is RPM around 800 RPM.
<ol>
<li>Does that mean the flywheel (which is connected to the engine) rotates at 800 revolution per minute while in neutral?</li>
<li>If so, how come it is safe to fully release the... | <blockquote>
<ol>
<li>Does that mean the flywheel (which is connected to the engine) rotates at 800 revolution per minute while in neutral?</li>
</ol>
</blockquote>
Yes. The flywheel is directly connected to the engine so it will rotate at exactly the same speed as the engine.
<blockquote>
<ol start="2">
<... | Because neutral means none of the gears (1 to 5 or 6 plus reverse) are engaged so no drive to the driving wheels...
| https://mechanics.stackexchange.com |
279,112 | [
"https://physics.stackexchange.com/questions/279112",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/113358/"
] | If I'm in a satellite which is moving in circular orbit of radius $r$ around a planet which has no atmosphere, means no air friction. And if I throw a stone from satellite toward planet then it's obvious that it will land on its surface, same as probe landing. But here is some calculation- $$$$ If mass of planet is $M$... | <blockquote>
If I'm in a satellite which is moving in circular orbit of radius $r$ around a planet which has no atmosphere, means no air friction. And if I throw a stone from satellite toward planet then it's obvious that it will land on its surface, same as probe landing.
</blockquote>
This is not the case. What yo... | To get out of orbit and land on an object you need to reduce your orbital height - ultimately to the planets radius. To do this you need to reduce the orbital speed. Orbital height only depends on speed.
One way to do this is to fire a rocket motor toward the direction you are travelling in - to slow you down. The pro... | https://physics.stackexchange.com |
1,664,877 | [
"https://math.stackexchange.com/questions/1664877",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/145510/"
] | How do I prove $2 \binom n2 + 4 \binom n4 + 6 \binom n6 + \cdots = n 2^{n-2}$
I thought about using known $\sum k\binom nk = n2^{n-1} $ and then just dividing it by 2. But is it possible you can help me with writing the full solution, because for the first known theorem you need to derivate $(a+b)^n $and also $a=1, b=... | \begin{align*}
(1+x)^n & = \sum_{k=0}^n \binom{n}{k}x^k\\
\frac{d}{dx}(1+x)^n & = \sum_{k=1}^n k\binom{n}{k}x^{k-1}\\
n(1+x)^{n-1} & = \sum_{k=1}^n k\binom{n}{k}x^{k-1}
\end{align*}
Likewise
\begin{align*}
n(1-x)^{n-1} & = \sum_{k=1}^n (-1)^kk\binom{n}{k}x^{k-1}
\end{align*}
Now add
$$n(1+x)^{n-1} +n(1... | Hint: For an "algebraic" argument, note that $k \binom{n}{k}=n\binom{n-1}{k-1}$.
| https://math.stackexchange.com |
1,929 | [
"https://security.stackexchange.com/questions/1929",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/998/"
] | I see some sites are blocked for IT security at schools, colleges and offices. But, why are some obvious good sites blocked too?
| There are all sorts of security risks - the ones I tend to highlight are:
<ul>
<li>The big social networking sites are a
wonderful target for attackers.
Imagine a group of over a billion
users, most of whom are not
technically savvy, that all use the
same web app (eg Facebook) and have
personal data, links to others e... | In short - because you should be working. And, in general, any truly relevant knowledge available via social sites is equally available elsewhere.
| https://security.stackexchange.com |
21,692 | [
"https://stats.stackexchange.com/questions/21692",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/5397/"
] | Please help!
I have recently been criticized for using pairwise comparisons to explain all three levels of a factor within a negative binomial GLM rather than all levels at once. I was told that it is "long-winded" and "uneccessary". I was under the impression that in GLMs one cannot bulk all levels of a factor togethe... | You could fit the model with and without the categorical variable and do a likelihood ratio test to determine it's overall significance.
| You have probably already solved this, but I had the same problem and found a solution in "A Beginner's Guide to GLM and GLMM with R".
If you use the drop1() function you get one P-value for each factor in your model (not for each level of the factor.
This is a similar approach to the one suggested by B_Miner, but f... | https://stats.stackexchange.com |
284,994 | [
"https://stats.stackexchange.com/questions/284994",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/160061/"
] | Estimating a Vector Error Correction Model (VECM) in R can be done by using the command <code>cajorls</code>. The R output consists of the coefficients of the Error Correction Terms and the values of the coefficients on the lagged variables used in the VECM, together with the "Betas". When using <strong>two</strong> va... | <blockquote>
Although all the coefficients necessary to build the VECM are getting calculated by R, there are no standard errors or respective p-values reported in the R output. In R, is there a command that lets you compute the standard errors of the coefficients on the error correction terms (ECT)?
</blockquote>
A... | You can use package tsDyn for this, function <code>VECM</code>, and <code>summary()</code> on that output:
<pre class="lang-r prettyprint-override"><code>library(tsDyn)
library(vars)
data(Canada)
beta_tsDyn <- VECM(Canada, lag = 1, estim = "ML")
## sd in parenthesis:
summary(beta_tsDyn)
#> #############
#>... | https://stats.stackexchange.com |
390,856 | [
"https://physics.stackexchange.com/questions/390856",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/185234/"
] | <blockquote>
<strong>Question:</strong> Using the fact that if $|\phi\rangle = (AB) |\psi\rangle $ then $\langle \phi| = (AB)^{\dagger}\langle\psi|$ prove that
$$(AB)^{\dagger} = B^{\dagger}A^{\dagger}.$$
</blockquote>
<strong>Attempt:</strong>
I assume that the kets are normalized. Then,
$$\langle\phi|\phi\rangl... | Let's consider the operators $A$ and $B$. We want to show that $(AB)^\dagger=B^\dagger A^\dagger$. Hopefully you'll agree that if we can show $\langle\psi|(AB)^\dagger|\chi\rangle=\langle\psi|B^\dagger A^\dagger|\chi\rangle$ for every $|\chi\rangle$ and $|\psi\rangle$, we've proven the two operators are equal.
So, let... | You're confusing two different concepts:
<ul>
<li>A the Hermitian conjugate of an operator $A$ is the (provably unique) operator $A^\dagger$ such that for all states $\phi,\psi\in\mathcal H$, $$\langle \phi, A\psi\rangle = \langle A^\dagger\phi,\psi\rangle.$$</li>
<li>An operator $U$ is unitary iff $U ^\dagger U=\math... | https://physics.stackexchange.com |
141,507 | [
"https://electronics.stackexchange.com/questions/141507",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/39152/"
] | I have a cheapo soldering station (Duratool D02265). It is fine for what I need. But, I often want to swap the tips round quickly, without waiting for ages for it to cool down!
I was wondering:
Can I dip the end in water (without destroying it!) to cool it down quickly (Just to the top of the nut, so I can undo the n... | Dipping the tip into water does not sound like a good idea.
However, you don't need to anyway. I change tips occasionally on my soldering iron too. Just use a pair of plyers to undo the nut, pull the tip out, and put in the holder. You can use your fingers to put the new tip on, but will need the plyers again to ti... | Quickly changing the temperature like that causes the metal to contract/expand at different rates and will cause metal fatigue. So it will shorten the life expectancy of the tip and perhaps the treading used to connect it. Using a moist sponge is a much safer option and you should be able to still cool it down fairly q... | https://electronics.stackexchange.com |
341,091 | [
"https://physics.stackexchange.com/questions/341091",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/147093/"
] | I have come across this term "Differential distribution" in particle physics but have not been able to clearly understand it. Does it mean how the decay observables change with change in some input parameter? Any small help would be appreciated!
| A differential distribution in particle physics is usually a measure of the rate of a process as a function of some parameter. For example, you might have a differential distribution for the cross section of e+ e- > $\mu$+ $\mu$- as a function of the collision energy, $\frac{d\sigma}{d\sqrt{s}}$.
The idea is to integ... | One of the main use of that phrasing I know of relates to the Parton Distribution Functions (PDF). Consider a proton of momentum $p$. What is the probability $dP_u(x)$ to find a quark $u$ with a momentum that is a fraction of $p$ between $x$ and $x+dx$? It can be written as $dP_u(x) = f_u(x)dx$. Then
$$f_u(x) = \frac{... | https://physics.stackexchange.com |
80,095 | [
"https://cs.stackexchange.com/questions/80095",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/41391/"
] | It is listed as such in wiki, however my literature never refers to it as such.
My gut tells me to follow papers and monographs on the matter, but my concern is that I will make an easily avoidable mistake, or that I simply misunderstood the text.
| One of my first Google hits is a widely-cited article:
Eberhart, Russell C., and Yuhui Shi. "Comparison between genetic algorithms and particle swarm optimization." In International Conference on Evolutionary Programming, pp. 611-616. Springer, Berlin, Heidelberg, 1998.
The third paragraph of the article says:
<bloc... | There's no authorized body to pronounce the truth on this sort of thing. Personally, I would say it absolutely is a genetic operator. Someone else might disagree. I don't thing either is objectively wrong or makes one look amateurish or silly.
| https://cs.stackexchange.com |
519,890 | [
"https://physics.stackexchange.com/questions/519890",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/195374/"
] | I realised that Newton's first law has significance in that it defines the inertial reference frame which is the condition for Newton's second law to hold. This means, from second law we should be able to find an implicit reference to the first law (i.e. we should be able to reach the first law by manipulating expressi... | Obviously the first law applies to an isolated system. For this the mass cannot change. Take the example of an evaporating mass, as in one of the answers. The vapour carries off mass but also momentum. So either you consider the total system to be the evaporating particle plus vapour, for which the sum of forces over a... | Imagine a body moving with some velocity <span class="math-container">$\vec{v}$</span> which slowly evaporates. There are no external forces. It's mass decreases with time, it's velocity remains constant, it's momentum decrease as well. The equation <span class="math-container">$\vec{F}=\frac{d\vec{p}}{dt} $</span> obv... | https://physics.stackexchange.com |
214,859 | [
"https://security.stackexchange.com/questions/214859",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/52102/"
] | Scenario: User searches for something and gets a link to a sub-page on a site, proceeds to go there, but is asked to sign-up/login - they do so, only then once they are logged in, they are redirected to the main page or the "mobile" version of the page, as opposed to the one that they were originally intending to view
... | This practice of resuming the original request after authentication is sometimes called "bookmarking," because it allows users to bookmark specific parts of a site and "go there" by way of the login process.
When bookmarking isn't available, it's usually a sign that the site wasn't designed or architected to allow it.... | <blockquote>
Is there any sort of security reason for this, or is it just lazy
programming, or something else?
</blockquote>
There is no security reason to it as long as the website doesn't take user input and then redirect them there without any input validation.
The only thing i can see here is just poor design... | https://security.stackexchange.com |
102,056 | [
"https://electronics.stackexchange.com/questions/102056",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/37963/"
] | I'm using 220v -> 48v 830 W transformer directly connected to the 220v grid and used to operate my PCB drill.
The issue with transformer is that it's making irregular (with 0.1-2 min interval) sudden "BZZ" noises, accompanies with current spike in input coil and voltage spike in output coil.
Before using it as drill ... | Sounds like some kind of arcing within the transformer (perhaps turn-to-turn). If the transformer does this with no load and there is no sign of any other disruption on the incoming power, I would suggest discarding the transformer as it could be dangerous.
| It could be a lamination issue - do you have it mounted so that the iron core laminations do not come into contact with something metallic - mounting it on a steel sheet might cause problems. Any conducting material touching/shorting the laminates could have this effect - think mounting brackets, fasteners thru holes i... | https://electronics.stackexchange.com |
41,876 | [
"https://stats.stackexchange.com/questions/41876",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/16524/"
] | This is probably a very simple question! I am trying to compare two groups on a balance evaluation, a normal group and one with a neuro deficit. However, these two groups are significantly different in age. Since age is significantly correlated to the balance score, is there a way to compare my two groups while taking... | The simplest solution is to add age as a covariate in a regression equation. However, if the two groups have no overlap on age, that may not be enough.
| Not without more information. If you have enough overlapping data, you can follow Peter's advice, and use both variables in your regression. If not, you may be able to add in some information from previous studies on the relationship between age and the balance score. If the relationship is very robust, you may be able... | https://stats.stackexchange.com |
221,236 | [
"https://security.stackexchange.com/questions/221236",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/209137/"
] | AWS provides access to EC2 by downloading the private key(<code>.pem</code>) into management host that connects to EC2.
AWS uses <code>openssl</code> tool
Key providers generally provide public key but not private key, because with keypairs, one can encrypt either with public key or private key and decrypt with other... | <blockquote>
1) Why AWS distributes private key instead of public key? for secure communication...
</blockquote>
Without the private key, you cannot prove you are the owner of the public key. Without proving you are the owner, you cannot use SSH Public Key Authentication.
So Amazon generates a key itself, and send ... | In this specific case, the private-public key pair is more for authentication.
You do not have to generate the private-public key pair on the AWS console, you can do this yourself and upload <strong>just</strong> the public key onto AWS.
The important thing is that the public key loaded onto the EC2 instances matche... | https://security.stackexchange.com |
40,742 | [
"https://softwareengineering.stackexchange.com/questions/40742",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/2099/"
] | I am a teenage hobbyist in high school, and I mostly release my software as open source; therefore I have little experience actually selling software, let alone in a specific area -- Android phones.
Now, I have received requests from several users of one of my applications (basically, an advanced calculator) to create... | I think probably if you think you can do it for not too much effort, you should do it just so you can show your potential employers in the (near) future that you released an app for Android while in high school.
That will demonstrate to them that you have what it takes to Get Stuff Done.
| Nothing could possibly matter less than whether you make any money off this. You have decades ahead of you to weigh costs and benefits, to worry about supporting a family, maintaining a house, paying your dental bills, to be devoured by the minutae of life and commerce.
Right now, you have a technical problem to solve... | https://softwareengineering.stackexchange.com |
80,608 | [
"https://physics.stackexchange.com/questions/80608",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/20747/"
] | I'm trying to wrap my mind around the twin paradox, but I can't figure out this one problem from my textbook. It uses the relativistic Doppler effect to demonstrate how the paradox works. The first part of the problem is as follows:
Amelia and Casper are twins. Amelia is going on a trip to a distant planet while Caspe... | There are a couple of solutions based on how we interpret the sentence <code>"Amelia sent a message to him on each of</code><strong>his</strong><code>birthday"</code>.
<strong>1. If Amelia wanted to make sure that Casper receives the messages on <em>HIS 40 birthdays :</em></strong><br> She must do some calculations by... | I just think of the way time dilation works on an earth-bound scale and extrapolate that idea. It's pretty simple but it takes a lot of imagination and may seem a bit silly. Imagine you're standing still on the ground, and a toy car is moving in a circle around you. Time dilation says that your time moves at the sam... | https://physics.stackexchange.com |
171,829 | [
"https://math.stackexchange.com/questions/171829",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/35828/"
] | If $\log 0.318 = x$ and $\log 0.317 = y$, can $\log 0.319$ be expressed in terms of $x$ and $y$ ?
Is there any way or we have to find $\log 0.319$ using log tables only?
I'm not getting any expression in $x, y$ which will represent $\log 0.319$.
Please help.
| The question clearly hints at the following reasoning.
One is given $x=u(t+h)$ and $y=u(t)$, where $t=0.317$, $h=0.001$, and $u$ is the logarithm function, and one is asked an approximation of $z=u(t+2h)$.
Since $h$ is small, $x\approx y+hs$ where $s=u'(t)$ is the slope of the function $u$ at point $t$ (whose value ... | One has $0.318=e^x$, $0.317=e^y$ and $0.319=2\times0.318-0.317$.
Thus, $0.319 = 2e^x-e^y$ and
$$\log 0.319 = \log( 2e^x-e^y)$$
| https://math.stackexchange.com |
58,113 | [
"https://mathoverflow.net/questions/58113",
"https://mathoverflow.net",
"https://mathoverflow.net/users/13566/"
] | There is a famous old theorem by Kronecker that for every positive real $\alpha$ and $\epsilon>0$ there exists a positive integer n such that $\alpha n$ is within $\epsilon$ of an integer.
Recently I found that the same result is true if we replace $\alpha n$ by $\alpha n^2$ or any polinomial p such that $p(0)=0$... | Well, as Gerry has pointed out, this is certainly not true for all $\alpha$. On the other hand, this is true for a.e. $\alpha$. More precisely, the sequence $2^n\alpha$ is equidistributed mod 1 for a.e. $\alpha$.
I believe this result is due to H. Weyl and can be found in Cornfeld, Fomin and Sinai `Ergodic Theory'. (I... | $\alpha2^n$ is clearly not going to work, e.g., for $\alpha=1/3$. One place to look is the Kuipers and Niederreiter book on uniform distribution of sequences, although uniform distribution is a bit of overkill for the question you are asking about.
| https://mathoverflow.net |
78,873 | [
"https://mathoverflow.net/questions/78873",
"https://mathoverflow.net",
"https://mathoverflow.net/users/18719/"
] | All of the models of CH which I know of also satisfy $\diamondsuit$. What is the easiest way to produce a model of CH wherein $\diamondsuit$ is false?
| "The easiest way" to produce a model of CH in which $\diamondsuit$ is false is to start with a model of GCH and then do a countable support iteration of length $\omega_2$ killing off a potential $\diamondsuit$ sequence at each stage.
The forcing for doing this is straightforward: supposing $\langle A_\alpha:\alpha<... | The first one of these was probably Jensen's model for the consistency of Suslin's Hypothesis (SH) with the Continuum Hypothesis (CH). This is by no means an easy model, e.g. it involves an iteration of length $\omega_2$ which is neither finite support nor countable support. Devlin and Johnsbråten wrote a monograph abo... | https://mathoverflow.net |
61,286 | [
"https://mechanics.stackexchange.com/questions/61286",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/43607/"
] | My 2003 dodge ram emergency brake is not working. I push the pedal down and there is zero resistance. I release the pedal and it doesn't spring back to starting position, I have to grab the pedal and pull it up to get the light to go off. I can drive the truck without a problem with the emergency brake on, no sounds/... | This is a quick-n-dirty test done to see if there might be an issue with a burnt exhaust valve. Another similar test uses a dollar bill (or whatever currency you happen to have). What is being checked for is <em>reversion</em>. This is an issue where if the exhaust valve is open when the piston is on the intake stroke,... | Most probably due to the velocity profile of the gases coming out of the pipe and any constrictions upstream of the exit - causing backflow at the specific exhaust gas speed.
| https://mechanics.stackexchange.com |
111,361 | [
"https://chemistry.stackexchange.com/questions/111361",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/76037/"
] | i read in my textbook that the energy diagram of moleculal orbits is different for atoms nitrogen and below them.
my question is which one of the energy diagrams to use in heteronuclear compounds like CO?
| (In heteronuclear diatomic molecules, atomic orbitals only mix when the electronegativity values are similar.)
(While MOs for homonuclear diatomic molecules contain equal contributions from each interacting atomic orbital, MOs for heteronuclear diatomics contain different atomic orbital contributions.)
(Orbital interac... | It depends on how many electrons your compound has. In <span class="math-container">$\ce{O2}$</span> , there are 16 electrons.
Likewise all compounds with more than 16 electrons follow the <span class="math-container">$"\ce{O2}"$</span> diagram and those with less than 16 follow the other diagram.
| https://chemistry.stackexchange.com |
136,470 | [
"https://electronics.stackexchange.com/questions/136470",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/50265/"
] | I was talking to a friend today that I have a transmitter which is capable of sending 128 samples/second. Note that each sample is a voltage level represented by a floating point decimal. He then insisted that I call it a 7 bit system since 128 is 2^7.
What useful information can I possible get from representing the ... | Your transmitter can send 128 samples of data. Each sample contains a x number of bits depending on your transmitter. If your transmitter is a 8 bit transmitter then you can send 128 times a 8 bit sample in one second.
| Your friend confuses two completely different things.
The number of bits per sample tells how many distinct values you can have: be it 8, 10, 12 or whatever number n of bits means you can have 2^n distinct values. On a CD, you have 16 bit samples, and thus 65536 different values.
The number of samples per second is c... | https://electronics.stackexchange.com |
602,360 | [
"https://physics.stackexchange.com/questions/602360",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/213658/"
] | Suppose if we have a varying electric field <span class="math-container">$\vec{E}$</span> and magnetic field <span class="math-container">$\vec{B}$</span> that are independent and acting on particular area such that <span class="math-container">${\rm curl}\,\vec{E}\neq 0$</span>. Then can we write, by Maxwell's equatio... | The equation you cite, which is Faraday's Law, always holds. In fact the four complete Maxwell's equations, which are (in SI units)
<span class="math-container">$$\vec{\nabla}\cdot\vec{E}=\frac{\rho}{\epsilon_{0}}\\
\vec{\nabla}\cdot\vec{E}=0\\
\vec{\nabla}\times\vec{E}=-\frac{\partial\vec{B}}{\partial t}\\
\vec{\nabl... | There is no difference between "electromagnetic field" and "electric field and magnetic field". An electric field and a magnetic field are always just two components of an electromagnetic field, witch can be characterized by two vectors <span class="math-container">$\vec E$</span>, <span class="math... | https://physics.stackexchange.com |
275,732 | [
"https://softwareengineering.stackexchange.com/questions/275732",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/168619/"
] | Regardless of library, language, or spec, asynchronous send/write operations make sense to me, because code later in the execution path may not have any dependency upon the success or failure of the write operation.
However, asynchronous receive/read operations are not motivated for me.
<em>Can anyone suggest specifi... | The main reason async read is handy if you still have other work to do or more often because there is another input source you also need to listen to.
For example a GUI you need to listen to the user input besides getting the image from the disk.
Without asynchronous IO you would need to start a new thread every tim... | Non-blocking reads can be especially useful in contexts where data may or may not arrive, but if data does arrive it should affect the behavior of an action being performed. For example, if an interactive program that communicates via TCP expects to perform some computations in response to a request, but it's possible... | https://softwareengineering.stackexchange.com |
7,906 | [
"https://physics.stackexchange.com/questions/7906",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/2896/"
] | How does an antenna behave when it is cooled so that its black-body radiation is emitting energy at its resonant frequency?
Edit: To clarify, its not how they're related in general, but how might thermal radiation and resonance interact with each other when their spectra are aligned well?
Edit: Also, I'm sure that th... | OK, the simple answer:
When there is a resonance in the antenna you have a coherent phenomenon. All the bands of electrons of the antenna are marching in tune.
The black body radiation is an incoherent phenomenon coming from the individual atoms of the antenna. Even if the peak of the black body radiation were sitt... | They're not related. The black-body radiation as well as the resonance curve may look like "bumps" but they are very different bumps mathematically. The black body radiation gets emitted at all frequencies, and the "uncertainty of the frequency" is maximized, in some sense. On the other hand, resonances are peaked arou... | https://physics.stackexchange.com |
679,451 | [
"https://math.stackexchange.com/questions/679451",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/80524/"
] | If I know for two numbers a and b that
$${a < b }$$
Then is it correct to say that
$$ a \leq b $$
I know that the second statement is true as long as the first one is.
It seems OK as it is true but from the other side it seems kinda weird (to me), to say that, if you know, that a is <strong>strictly</strong>... | It is correct to say that if $a < b$, then $a \le b$. And yes, I completely agree with you: if explicitly being shown $a < b$, there is a certain "loss" of information when we write $a \le b$ instead of being concrete.
"$\le$" is, however, very useful in real-life, practical mathematical problems such as opt... | You have a sound understanding of the matter:
It is true that $a\lt b \implies a\leq b$, since $$a \leq b \iff (a\lt b\;\text{or}\;a = b)$$
If you need to prove $a \leq b$, then it suffices to prove either $a < b$ or $a = b$.
On the other hand, if you need (or want) to prove $a$ is <em>strictly less than</em> $b$... | https://math.stackexchange.com |
167,106 | [
"https://security.stackexchange.com/questions/167106",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/108682/"
] | As title says, I was asked for my online banking password while on the process of getting in touch with a real person. This is something I'd never do and knowing that the call was being recorded (for further improvement of the bot I was <em>talking</em> to) makes it even worse.
For sure, after that, I hung up and I'm ... | Assuming that you called them on a published number, I'd say that this sounds like it was an interactive Voice Response (IVR) system, which is pretty common in the banking world.
The concept is that the system takes your authentication information before passing you on to a contact centre agent. The benefit of this... | Yes you should take action, report it to your bank, in all likelihood this was a phishing attempt.
This shouldn’t happen and isn’t normal practice.
Your bank will never ask you for your pin number or password.
EDIT: After reading your comments and the clarification (posted after my awnser) that the contact was ent... | https://security.stackexchange.com |
61,312 | [
"https://physics.stackexchange.com/questions/61312",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/10549/"
] | When light from an object passes through a convex lense, it gets reconverged at a single real image. From there, the light rays presumabely begin diverging again, exactly as if there was a light-emitting object sitting at the real image point.
Why is it that if we look at the real image point, we don't see a hologram ... | You're operating under a misconception. When a real image is formed, we <em>can</em> see it, provided that our eye is positioned in a location such that rays from the image can enter your eye. Compared to a hologram, the situation is different for a couple of reasons. (1) The possible locations of your eye are more res... | It's not exactly the same as if an actual object was sitting there. An actual object emits light in all directions but the light from the focal point only travels in a straight line from the surface of the mirror, through the focal point. If you place a screen at the focal point you will see an image on the screen. You... | https://physics.stackexchange.com |
732,089 | [
"https://physics.stackexchange.com/questions/732089",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/348245/"
] | If in a straight wire, current flows, and magnetic field is created there are no poles to it. Then why does a compass brought near it gets deflected if there are no poles?
if the direction of the magnetic field affects the pointing of the poles then please explain how it affects ?
| The magnetic field around conductor having a current is having relativistic poles.
Meaning; if you select a point in the field it will be north pole to the point before it and at the same time it is a south pole to another point after it.
| It is neither necessary nor useful to look for poles in the magnetic field of the wire, since its field lines are circles around the wire, so they don't have a dipolar structure.
The needle of the compass, however, is a magnetic dipole. As such, it gets a potentiel energy when it's inside a magnetic field:
<span class=... | https://physics.stackexchange.com |
1,345,863 | [
"https://math.stackexchange.com/questions/1345863",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/79127/"
] | <blockquote>
A total of $n$ bar magnets are placed end to end in a line with random
independent orientations. Adjacent ends with equal polarities repel
each other, and adjacent ends with opposite polarities join to form
blocks. Let $X$ be the number of blocks of joined magnets. Find $\text{E}(X)$
and $\text{V... | <blockquote>
The obvious method to solve this problem is to find the pmf of $X$...
</blockquote>
This problem illustrates perfectly that the "obvious method" might not be the easiest.
Here we can find $\mathbb{E}(X)$ and $\mathbb{V}(X)$ <strong>without</strong> working out the pmf of $X$.
Instead let's represen... | Let $0$ represent one orientation and $1$ represent the other orientation. Each arrangement of the $n$ magnets then corresponds to an $n$-bit binary string. Two adjacent magnets repel if they are in opposite orientations and attract if they are in the same orientation. Thus, a block of magnets corresponds exactly to a ... | https://math.stackexchange.com |
420,765 | [
"https://mathoverflow.net/questions/420765",
"https://mathoverflow.net",
"https://mathoverflow.net/users/12518/"
] | I am embarrassed to be stuck on this seemingly simple question.
Suppose that <span class="math-container">$X,Y$</span> are mean-zero real-valued random variables and <span class="math-container">$\tilde X,\tilde Y$</span> are their "independent copies": <span class="math-container">$\tilde X,\tilde Y$</span> ... | [<strong>EDITED</strong> <em>to ensure that the random variables have expectation 0</em>]
I think the answer is no.
Let <span class="math-container">$Z$</span> be a random variable taking the values <span class="math-container">$\pm 1$</span> with probability <span class="math-container">$p$</span> each and 0 with prob... | Let <span class="math-container">$\varepsilon \to 0$</span> and <span class="math-container">$M= \sqrt{(1-\varepsilon)/\varepsilon}$</span>. Define <span class="math-container">$X$</span> and <span class="math-container">$Y$</span> as follows:
<ul>
<li>w.p. <span class="math-container">$\frac{1-\varepsilon}2$</span>: <... | https://mathoverflow.net |
377,792 | [
"https://physics.stackexchange.com/questions/377792",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/37677/"
] | Let’s take the a simple $H_2^+$ molecule, where there is only electron which is $r_a$ away from the first proton and $r_b$ away from the other one.
Let’s call the separation between the two protons $R$.
As $R\rightarrow \infty$, the electron will stick to one of the two protons, so the wavefunction will be: $$ \phi =... | Think physically about the shape of the odd state. It has a nodal plane at the midpoint of the line connecting the two nuclei; this is the only node to the wave function. As the nuclei become coincident, that nodal plane ends up passing through the combined nucleus; since this is the only node, the resulting wave fun... | Let $\Psi(\vec{r})$ be $1s_a$ wave function and $\Psi(\vec{R}+\vec{r})$ be $1s_b$ wave function. As $\vec{R} \to 0$ we have for ungerade state:
$$
\phi_{-}(\vec{r}) = N_{-}(\Psi(\vec{R}+\vec{r})-\Psi(\vec{r})) \approx N_{-} \nabla\Psi(\vec{r})\vec{R}
$$
Thus the limit (not normalized) is $\nabla\Psi(\vec{r})\vec{n}$, w... | https://physics.stackexchange.com |
203,898 | [
"https://physics.stackexchange.com/questions/203898",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/75693/"
] | So I was watching MinutePhysics' video 'How long can you balance a pencil?' and it turned out to not be for very long.
Would introducing a magnet to the equation make for different results?
We can assume the magnet and non-magnetic surface are both uniform and perfectly flat, and that the needle is uniform to a 'sha... | When you say "laser", I'm guessing that you mean visible or near-visible light.
The problem is, where are they going to build their laser? If they build it on their home planet, then it's going to be so close to their sun, that our telescopes won't be able to distinguish the one from the other. We won't even be able... | The divergence of the signal is closely related to the angular resolution of the "antenna" (lens, reflector, ...) you use. This will in turn be a function of the size of the source in relation to the size of the lens - or in the limit, the ratio of dish diameter and wavelength (the familiar $\frac{1.22\lambda}{d}$ rela... | https://physics.stackexchange.com |
267,838 | [
"https://security.stackexchange.com/questions/267838",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/106741/"
] | I received an email that passed all the email sender checks (spf, dkim, dmarc), and the sent-from domain is a legitimate domain from a legitimate company. However, the email content itself was suspiciously phish-y. I even started to write a reply email advising they were mistaken, until I noticed that the reply-to was ... | Even if you don't share volumes, there are still potential security risks when running a remotely downloaded Docker image. For instance, the image may contain code that runs with elevated privileges inside the container, allowing it to perform actions such as accessing the host's network or consuming host resources.
Ad... | If the image is malicious, it <em>can</em> run malicious code on your system, even if you don't map any volumes for it.
Docker images are not intended to isolate malicious code from your host, it's intended to make easier the transition from development to testing to production. If you use a controlled and vetted image... | https://security.stackexchange.com |
315,440 | [
"https://math.stackexchange.com/questions/315440",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/39754/"
] | $$\lim_{x\to 2}\frac{(3x-6)\sqrt{x^2+1}}{5x-10}$$
I have tried to simplify, multiply by the conjugate of the denominator, but I cannot seem to figure out how to find the limit. Do I need to apply the limit laws in order to find the limit of this function? Could someone explain the steps I should take to approach this ... | We're looking at
$$\lim_{x\to 2}\frac{(3x-6)\sqrt{x^2+1}}{5x-10}$$
Note that the problem arises since $3\cdot 2-6=0$ and since $5\cdot 2-10=0$, but there is no problem with $\sqrt{x^2+1}$.
Also note that $3x-6=3(x-2)$ and that $5x-10=5(x-2)$. Simplify, and then use the product rule for limits, i.e $\lim f(x)g(x)=\l... | This problem displays that without simplification the limit will be indeterminate or in other words $\frac{0}{0}$ when $x$ is subbed in. Currently you have the following equation
$$\begin{align}
\lim_{x\to 2}\frac{(3x-6)\sqrt{x^2+1}}{5x-10}
\end{align}$$
When you sub in $x$, you will get the following and it will redu... | https://math.stackexchange.com |
5,627 | [
"https://mechanics.stackexchange.com/questions/5627",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/2898/"
] | My 2006 Nissan Titan makes an odd sound, like a grinder, mostly when travelling steady at 35, 40 or 45 miles per hour. Even 1-2 mile per hour difference will cause the noise to stop.
The noise will go away if I accelerate slightly or slow down by letting off the gas. I've noticed that it will also make the same sound ... | My truck had the same "grinding thing" at certain speeds like you all said...
The problem is called -VIBRATION- this happens when you're running your car, not when it's in parking, so this "grinding thing is the laminate cover that protects your catalytic converter that vibrates, you just need to get a long screwdrive... | I would imagine that something is just rattling.. I say this because if it was an issue of possibly a pulley or a wheel bearing or anything at all for that matter. Then the noise would be more consistent. It isn't too often where you have a vehicle that <em>only</em> has an issue in that tight of a situation.
<blockqu... | https://mechanics.stackexchange.com |
139,876 | [
"https://softwareengineering.stackexchange.com/questions/139876",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/49848/"
] | This is basically a logging/counting application that is counting the number of packets and counting the type of packet, etc. on a p2p chat network. This equates to about 4-6 million packets in a 5 minute period. And because I only take a "snapshot" of this information, I am only removing packets older than 5 minutes e... | Instead of having one dictionary and searching that dictionary for entries that are too old; have 10 dictionaries. Every 30 seconds or so create a new "current" dictionary and discard the oldest dictionary with no searching at all.
Next, when you're discarding the oldest dictionary, put all the old objects onto a FILO... | The first thought that springs to mind is why you wait 5 minutes. Could you do the snap-shots more often and thus reduce the big overload you see at the 5 minute boundary?
Secondly, LINQ is great for concise code, but in reality LINQ is syntactic sugar on "regular" C# and there is no guarantee that it will generate th... | https://softwareengineering.stackexchange.com |
289,349 | [
"https://stats.stackexchange.com/questions/289349",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/160416/"
] | I am studying time series analysis for the very first time and I cannot seem to get my head around the fact that the entire time series analysis is about studying and modelling the behaviour of the noise component of the time series model. But why is it that we are so concerned about the noise?
From what I could gathe... | A time-series breaks down to the components seasonality, trend and noise.
Let's take oil prices.
Trend component tells you whether oil prices are going up or down and by approximately how much (steep upward slope, plateauing downward slope etc.)
Seasonality says that given it is winter, what effect does that have on... | Why to be concerned about the noise? If you are not concerned about, than you can have troubles with overfitting (you are modelling noise instead of signal). Also good knowledge about noise allows you to filter that noise, so you can get pure signal. Imagine, that you want to study signal y(t)=sin(t)+sin(3t), where sin... | https://stats.stackexchange.com |
384,377 | [
"https://softwareengineering.stackexchange.com/questions/384377",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/280882/"
] | I often encouter the following pattern: A bug is resolved/closed as duplicate of another still open/unfixed bug.
I have trouble understanding the reasoning behind this strategy. From my naive point of view, there is at least one major downside: there is no easy way to verify that this is really a duplicate.
Normally... | In the cases in which it is absolutely clear that the issue/root cause is the same (for example crashes with the same traceback occurring in the same conditions, only reported by different customers) yes, closing it as a duplicate is IMHO perfectly OK - tracking the 2 (or more) issues separately is just a waste of time... | You want to have a clear view of the issues with a software.
If a system has 10 issues, you want 10 open bugs. Not 10.000 open bugs of duplicates.
You also want the minimum amount of administration. So closing a bug as duplicate when someone finds out it is, is the sensible thing to do. Time is expensive and you want... | https://softwareengineering.stackexchange.com |
53,080 | [
"https://security.stackexchange.com/questions/53080",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/971/"
] | Some companies build their own software. Others outsource software development by hiring contractors or other companies to build software they need.
When we need to build new custom software, is there any evidence whether the choice to develop in-house vs outsource software development has an effect on security? All... | I'd say that there is nothing per se which would always lead to outsourced software being less secure than an in-house development but there are some common factors which may in practice lead to this commonly being the case
<ol>
<li>Cost concerns. If a key factor in winning the work is low cost, then the risk of inse... | As a rule of thumb involving a third party complicates security. It does not mean the product (software or otherwise) cannot be secure, but it does add another factor into the equation. Things to consider:
<ol>
<li>Quality of the staff</li>
<li>Experience of the third party in creating software with a focus on securit... | https://security.stackexchange.com |
78,523 | [
"https://softwareengineering.stackexchange.com/questions/78523",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/4553/"
] | We have many windows services and web apps which all rely on a lot of common settings. Currently we use just appSettings as needed and they are referenced by key. Some of the settings are app specific and only know by the app team while others are environment specific and only know by the deployment team. Also we hav... | I suggest to use some sort of wiki or documentation engine in your workplace.
Sure you can upload quite similar files to server, but thats the place where sometimes we forget to look at. Just think how many times you downloaded an app for development purposes and started reading README.txt, or just jumped to that app ... | We had a similar problem at my work. We solved it by not using the config files. We wrote the things that when changed required a code change any way in code, and then moved the rest of the properties to a settings table in a database. We wore able to do this because we control every server we deploy code on. This ch... | https://softwareengineering.stackexchange.com |
265,349 | [
"https://security.stackexchange.com/questions/265349",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/277634/"
] | I need to hash 32 cryptographically random bytes, but later the verification the value with hash must be very fast, so I decided to use SHA256. Is it a security issue if my passwords are 32 cryptographically random bytes? Maybe you know some other fast and secure hashing algorithm?
| If the server response only needs to be "fairly" fast, a relatively strong hash like bcrypt, scrypt, or Argon2i, tuned to match your expected performance, is still the preferred approach. The hash "work factors" should take as much processing time - and latency - as your application and users can to... | Requiring password hashing to be fast makes no sense to me. Usually passwords are hashed to prevent password disclosure in case the attacker gets access to the database. But hashes for this purpose are intentionally constructed to be relatively slow and to require relatively much memory. The examples of such password h... | https://security.stackexchange.com |
197,987 | [
"https://dba.stackexchange.com/questions/197987",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/25140/"
] | I am wondering: assuming everything is set up properly, meaning a column with charset <code>utf8mb4</code> and a client connecting to the database setting the connection encoding to <code>utf8mb4</code>, is there any way in which said client could <code>SELECT</code> data which contains invalid UTF-8 byte sequences? Is... | I don't <em>think</em> so. But I have come close:
<pre><code>mysql> CREATE TABLE invalid (c VARBINARY(22));
Query OK, 0 rows affected (0.02 sec)
mysql> INSERT INTO invalid (c) VALUES (UNHEX('65c3c3c366'));
Query OK, 1 row affected (0.01 sec)
mysql> SELECT HEX(c) FROM invalid;
+------------+
| HEX(c) |
... | Ok, you´re welcome to help me to understand the problem...
In the reffered thread the data comes from a html form field und was inserted in a prepared field.
There was no data conversion after the insert!
First of all, why it happens: A Javascript tries to interpret the string U+1F601 as expression for the unicode char... | https://dba.stackexchange.com |
32,106 | [
"https://electronics.stackexchange.com/questions/32106",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/5694/"
] | It seems to me that the maximum current output of a linear regulator should be determined primarily by power / heat dissipation, not the actual current. For instance, a 3.3V regulator that has an output current of 100mA with a 40V input could not easily dissipate the voltage drop (3.67 W), whereas it would have no prob... | Good question.
Almost always, what damages things is heat, not current. The answer to your question is in the difference between local and global, or between sub-parts and the whole thing. The current may go through several sub-parts, and each one of them may have different power ratings. A bonding wire may have, by i... | General guidelines? They're actual limitations, most of the times deliberately designed for. This is often with maximum power dissipation in mind. That dissipation situation can be different for different implementations. It's up to the design engineer to ensure that the device is used within its specifications. It wou... | https://electronics.stackexchange.com |
23,277 | [
"https://mathoverflow.net/questions/23277",
"https://mathoverflow.net",
"https://mathoverflow.net/users/2024/"
] | While studying some class field theory there was a lot of talk on galois extensions. Of course. When talking about non-galois number fields, usually the text will quickly take the galois closure. At this point it occurred to me that this implies many properties are shared by number fields with the same galois closure.
... | There's a huge amount of literature on this problem starting with
<ul>
<li>F. Gassmann, <em> Über Beziehungen zwischen den Primidealen eines algebraischen Körpers und den Substitutionen seiner Gruppen</em>, Math. Z. 25, 661-675 (1926)</li>
</ul>
Gassmann constructed number fields with the same normal closu... | Fields with the same Galois closure can be completely different. For example take a random irreducible polynomial of huge degree, let $F$ be the field obtained by adjoining one root of the polynomial, and let $K$ be the field obtained by adjoining all of the roots. These fields have the same normal closure, and one is ... | https://mathoverflow.net |
3,698,011 | [
"https://math.stackexchange.com/questions/3698011",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/283383/"
] | When I take some calculation, I find an equation
<span class="math-container">$$
0=1+\frac{x}{3!}+ \frac{x^2}{5!}+ \frac{x^3}{7!}+...
$$</span>
Obviously, when <span class="math-container">$x=0$</span>, the right part is positive. And when <span class="math-container">$x=-1$</span>, the right part is negative, so, the... | Restricting our attention to real <span class="math-container">$x$</span>, note that
<span class="math-container">$$\sum_{n=0}^{\infty} \frac{x^n}{(2n+1)!}
= \begin{cases}
\frac{\sinh\sqrt{x}}{\sqrt{x}},&x>0;\\
1,&x=0;\\
\frac{\sin\sqrt{-x}}{\sqrt{-x}},&x<0.
\end{cases}$$</span>
So it vanishes exact... | As you noticed, the rhs is positive if <span class="math-container">$x >0$</span>. So consider instead
<span class="math-container">$$\sum_{n=0}^{\infty} (-1)^n\frac{x^n}{(2n+1)!}=\frac{\sin \left(\sqrt{x}\right)}{\sqrt{x}}$$</span> So, if you want that to be zero, you need to solve <span class="math-container">$\si... | https://math.stackexchange.com |
44,087 | [
"https://datascience.stackexchange.com/questions/44087",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/62528/"
] | How can I check if a bigger training data set would improve my accuracy of my scikit classifier, is there a method or something?
| In a correlation framework above, the biggest driver of the output is the input which has the greatest <strong>absolute</strong> correlation value.
Correlation lies in the range [-1,1], and:
<ul>
<li>Negative correlation (correlation < 0) implies that the input and output move in opposite directions - i.e. as the ... | You are in right way! Length of each box is actually correlation that you are looking for, positive, if box is on the right side from start (null) position, or negative on the left side. You can see that grey box have length 1, which is obvious because output has perfect correlation with itself. Regarding features: it ... | https://datascience.stackexchange.com |
669,768 | [
"https://physics.stackexchange.com/questions/669768",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/315617/"
] | A pot is partially filled with water, in which a plastic cups is floating. Inside the floating cup is a small block of lead. When the lead block is removed from the cup and placed in the water,
block sinks to bottom. When this happens, does the water level in the pot: fall, rise, or stay the same?
The answer is apparen... | Most sanitizers contain alcohol. The alcohol evaporates faster than water at room temperature. So it absorbs heat at a faster rate than water. You can compare the sensation with those sticky sanitizers used by some stores, that contain no alcohol and leave your hand greasy/sticky. These do not feel cool to me. And by &... | <blockquote>
Which absorb more heat from hand? Sanitizer or Water?
</blockquote>
By "sanitizer" I'm assuming you mean an alcohol based product with minimum 60% alcohol as recommended by the FDA.
Given the same mass of water and alcohol, and allowing both to completely evaporate, the water will absorb more hea... | https://physics.stackexchange.com |
722,354 | [
"https://physics.stackexchange.com/questions/722354",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/281431/"
] | Imagine you have the wave equation:
<span class="math-container">$$
\frac{1}{c^2}\frac{\partial^2 u}{\partial t^2}= \nabla^2
u$$</span>
And you have your solution wave is "confined in a box" (or with the extremes of the ropes attached). Or maybe, your rope is confined to a paraboloid well. How would you add t... | Suppose that the potential energy per unit length associated with a displacement <span class="math-container">$u$</span> at a point <span class="math-container">$x$</span>, is <span class="math-container">$V(u,x)$</span>, then the wave equation will have an extra term of the form <span class="math-container">$-\frac{\p... | You can try to add a source term. First, I'm going to define:
<span class="math-container">$$\partial^2 \equiv \frac{1}{c^2}\frac{\partial^2}{\partial t^2} - \nabla^2$$</span>
and so the wave equation becomes:
<span class="math-container">$$\partial^2 u = 0$$</span>
you can add a source term like the following:
<span c... | https://physics.stackexchange.com |
8,905 | [
"https://bioinformatics.stackexchange.com/questions/8905",
"https://bioinformatics.stackexchange.com",
"https://bioinformatics.stackexchange.com/users/1008/"
] | Could you please suggest me how I can remove range from fasta header:like these number from below sequences which has some colons indicating the range of the genome
:147010-147657
:149201-149845
<pre><code>> FVJT01000004.1:147010-147657 Mycobacterium abscessus subsp. massiliense strain 974
GTGACCAGCAGCGCAGACCGA... | Invest some time in learning unix tools such as <code>grep</code>, <code>sed</code> and <code>awk</code>.
<pre><code>sed -r '/^>/s/:[0-9]+-[0-9]+//' input.fasta > output.fasta
</code></pre>
This line of code matches the pattern <code>:<numbers>-<numbers></code> on lines that start with <code>></c... | Alternatively, Perl provides a slightly simpler regular expression syntax (<code>\d</code> instead of <code>[0-9]</code>) that I prefer over <code>sed</code>.
<pre><code>perl -pe 's/:\d+-\d+//' input.fasta > output.fasta
</code></pre>
| https://bioinformatics.stackexchange.com |
44,880 | [
"https://physics.stackexchange.com/questions/44880",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/16213/"
] | Saw a question about faster than light travel... I still have the same question though none of the answers offered any resolution for me.
It is so summarily assumed by all physicists and commentaries that exceeding the speed of light would turn back the clock. I can't see the relation.
Doubling the amount of any spe... | <blockquote>
Disclaimer.. I hate calculations, stink at them and have no idea what mathematical formulas are used to arrive at the accepted conclusions so I'm not trying to dispute any findings etc by the experts, just trying to align my lay understanding to their conclusions.
</blockquote>
Main thing is, all these ... | If you have a clock at earth and a lot of clocks on a spaceship setting off at faster than the speed of light. When you send each clock back to earth you are saying each of the clocks will show a different time to the one on earth when it arrives back. (And I'm talking about a clock unaffected by the lack of gravity). ... | https://physics.stackexchange.com |
118,785 | [
"https://dba.stackexchange.com/questions/118785",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/78740/"
] | I'm writing an SSIS Package to take data from a production DB to a reports server/data warehouse. The other databases on my reports server are all in Mountain Time but this DB is in UTC should I convert the data as I'm importing it with SSIS, or should I leave the data as is and just keep track of what time zone the se... | Yes, there is a reason to <em>not</em> convert the timezone, and to keep the times in UTC. Times in UTC can be converted on-the-fly to any given timezone (and timezone's do occasionally shift, and Daylight Savings rules shift even more often). UTC is always accurate, whereas timezones and regions that allow for Dayligh... | Srutzky has offered the more resilient solution, which you should consider, but from the description of a single location and single reporting time zone I'd suggest the most easily implemented and maintained solution is to simply convert to MTN during ETL (and/or change that server's settings). Don't build a cruise shi... | https://dba.stackexchange.com |
1,891,758 | [
"https://math.stackexchange.com/questions/1891758",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/277062/"
] | I am trying to show that $n! = \binom{n}{0}D_n + \binom{n}{1}D_{n-1}+...+\binom{n}{n-1}D_1 + \binom{n}{n}D_0$ where $D_k$ is the number of derangements of $k$ objects.
However the last step of my proof requires me to show that
$\sum\limits_{r=0}^n({1 \over r!} \sum\limits_{s=0}^{n-r}{(-1)^s \over s!}) = 1$.
I am co... | Your original problem $n! = \binom{n}{0}D_n + \binom{n}{1}D_{n-1}+...+\binom{n}{n-1}D_1 + \binom{n}{n}D_0$ can be solved by noticing that the number of permutations with exactly k fixed points is $\binom{n}{k}D_{n-k}$.
However, your identity $\sum_\limits{r=0}^n({1 \over r!} \sum\limits_{s=0}^{n-r}{(-1)^s \over s!}) =... | As regards your identity,
$$\sum_{r=0}^n({1 \over r!} \sum_{s=0}^{n-r}{(-1)^s \over s!}) = \sum_{r=0}^n\sum_{s=0}^{n-r}{(-1)^{s} \binom{r+s}{r}\over (r+s)!}\\=
\sum_{r=0}^n\sum_{m=0}^{n}{(-1)^{m-r} \binom{m}{r}\over m!}=\sum_{m=0}^{n}{(-1)^m\over m!}\sum_{r=0}^m(-1)^{r} \binom{m}{r}=
1+\sum_{m=1}^{n}{(-1)^m\over m!}(1-... | https://math.stackexchange.com |
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