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4,519,166 | [
"https://math.stackexchange.com/questions/4519166",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/704937/"
] | OK so I have this function <span class="math-container">$f(x)=\begin{cases} x,& x<0
\\x+1, & x\geq0
\end{cases}$</span>.
It's clearly not continuous at all points. But I somehow find that its left-hand and right-hand derivatives at <span class="math-container">$0$</span> are <span class="math-container">$f... | <span class="math-container">$$f_-'(0)=\lim_{h\to 0^-}\frac{f(0+h)-f(0)}{h}=\lim_{h\to 0^-}\frac{h-1}{h}\ne 1$$</span>
| The result that if the one sided limits of <span class="math-container">$f'$</span> at <span class="math-container">$0$</span> exist, then <span class="math-container">$f'(0)$</span> exists, only holds for continuous <span class="math-container">$f$</span>, because it needs the mean value Theorem.
| https://math.stackexchange.com |
49,795 | [
"https://security.stackexchange.com/questions/49795",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/38369/"
] | If I download a file through Tor, will it be any different than if I download a file through a normal internet browser? I've downloaded stuff through Tor but it's always really slow. I was wondering if I browse through Tor, but then download a file through a normal browser, will it practically be the same thing or comp... | There are a couple different answers to this depending on the circumstances...
<h2>Peer to peer downloads (like torrents or filesharing networks)</h2>
If you are talking about downloading data illegally (e.g. copyrighted data in many countries), you shouldn't be doing that in the first place. Be a man and download th... | Tor works just like any other network interface. The site serving the data is unaware that a Tor node is requesting it. If you're getting the same file from the same location, they'll both have identical contents.
But you will lose all the anonymity Tor offers. You can give yourself away just by browsing to the same... | https://security.stackexchange.com |
3,388,293 | [
"https://math.stackexchange.com/questions/3388293",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/582290/"
] | The particular solution given in the question is <span class="math-container">$u(t) = t$</span>. I know that this is a Ricatti differential equation, therefore it's solution is <span class="math-container">$y=u(t)+\frac{1}{v(t)}$</span>. So first I've write the ODE in the Ricatti form:
<span class="math-container">$$\... | I found out where that was my mistake: that's when I expanded <span class="math-container">$t^3(y-t)^2$</span> at the beggining.
<span class="math-container">$$\frac{dy}{dt}=t^3(y−t)^2+yt$$</span>
<span class="math-container">$$\frac{dy}{dt}=t^3(y^2−2yt+t^2)+yt$$</span>
<span class="math-container">$$\frac{dy}{dt}=t^3... | After change <span class="math-container">$y=v(t)+t$</span> we get Bernoulli equation
<span class="math-container">$$\frac{d}{d t} \operatorname{v}(t)-\frac{\operatorname{v}(t)}{t}={{t}^{3}}\, {{\operatorname{v}(t)}^{2}}$$</span>
You can continue.
| https://math.stackexchange.com |
162,475 | [
"https://security.stackexchange.com/questions/162475",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/123462/"
] | I was wondering if having an antivirus installed is still a necessary thing in 2017 for me, I will explain myself:
I never use my antivirus to scan file that I'm not really sure about (I use a service like VirusTotal for example), the only case is when a file is large.
I have add-on on my web browser to block script ... | I spoke to the CTO of Symantec at a conference once who said something that I'll never forget:
<blockquote>
If every user does everything they are supposed to do all the time,
then you do not need anti-virus.
</blockquote>
The question becomes, can you ensure all those conditions? Do you live with others? Are you... | Since you've said it's just your machine, this is about your risk comfort level with your own web habits. If you don't think you need one, and if you aren't worried about what you might lose if your computer becomes infected, don't run one.
However, once you start talking about sharing data with others, such as hooki... | https://security.stackexchange.com |
167,592 | [
"https://chemistry.stackexchange.com/questions/167592",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/126956/"
] | The metal is dipped into a beaker that has the two acids, sulphuric (VI) acid and nitric (V) acid in equal concentrations.
In my preliminary research, I found that there might be several possibilities. I disregarded the case of the nitrating mixture since in this case the acids are dilute. There might be formation of a... | <blockquote>
For points 1 and 3, if they're both agreed to be true then <span class="math-container">$\Delta S_{\text{total}} = 0=R\ln K \implies K = 1$</span>, no matter the reaction.
</blockquote>
The entropy change of a reaction is concentration-dependent:
<span class="math-container">$$\Delta S = \Delta S^\circ - R... | (1) The entropy reaches a maximum at equilibrium, thus any change will depart from equilibrium, but this cannot happen if we are at equilibrium unless some other parameter changes such a temperature or pressure thus any change in entropy is zero.
If the reaction is <span class="math-container">$A+B\rightleftharpoons C$... | https://chemistry.stackexchange.com |
23,407 | [
"https://engineering.stackexchange.com/questions/23407",
"https://engineering.stackexchange.com",
"https://engineering.stackexchange.com/users/17257/"
] | Reading an article about multistage rockets (not educated at all on the topic), and from the get go it seems to assume they are used/need to be used.
| As the rocket is propelled upwards, it expends fuel. So there is no need to carry half empty fuel tanks. By splitting it up into separate stages, you can simply drop off unneeded mass.
| Another reason is that each motor stage can be designed for different purposes to match the flights characteristics.
So, a relatively short high thrust burn to escape gravity
And a longer or continuous burn to accelerate over a course / trajectory
Can save weight and complexity so adjustable nozzles are not always n... | https://engineering.stackexchange.com |
648,656 | [
"https://physics.stackexchange.com/questions/648656",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/252367/"
] | I'm exploring this question out of personal curiosity. If I take a cloud of atoms of a given element and release it in space at a distance of Earth's orbit from the sun (but not so close to Earth as to be affected by Earth's atmosphere/gravity/mag-field/radiation belts, etc.), will the neutral cloud turn into an ionize... | <blockquote>
GR describes curvature as being caused by stress-energy.
</blockquote>
This statement is slightly wrong and is the cause of your confusion here.
Technically, in GR the stress energy tensor is the source of curvature. That is not quite the same as being the cause.
An easy analogy is with Maxwell’s equations... | <blockquote>
if there is no matter, no mass, nothing with stress-energy inside the black hole, and the singularity is not part of our spacetime, then neither can cause curvature
</blockquote>
On top of the great Dale answer - the Einstein field equations (EFEs) are local. Imagine a lone planet in the universe. Then eve... | https://physics.stackexchange.com |
407,563 | [
"https://math.stackexchange.com/questions/407563",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/42912/"
] | I'm having troubles to understand the concept of coordinates in Linear Algebra.
Let me give an example:
Consider the following basis of $\mathbb R^2$:
$S_1=\{u_1=(1,-2),u_2=(3,-4)\}$ and $S_2=\{v_1=(1,3),v_2=(3,8)\}$
Let $w=(2,3)$ be a vector with coordinates in $S_1$, then $w=2u_1+3u_2=2(1,-2)+3(3,-4)=(11,-16)$.
... | The vector $(11,-16)$ is written in the standard basis - it means $11\times(1,0)+(-16)\times(0,1)$. This is exactly the computation that you do when you write:
$$2u_1+3u_2=2(1,-2)+3(3,-4)=(11,-16)$$
If we write $e_1=(1,0)$ and $e_2=(0,1)$, then what you're writing is:
$$2u_1+3u_2=2(e_1-2e_2)+3(3e_1-4e_2)=11e_1-16e_2... | When starting out, it's often easy to confuse the <strong>basis coordinates</strong> and the <strong>elements of $\Bbb R^2$</strong> because they are all written as ordered pairs with parentheses.
Let's try to keep them separate by using $\langle a ,b\rangle$ for coordinates in $S_1$, $[a,b]$ for coordinates in $S_2$... | https://math.stackexchange.com |
275,686 | [
"https://dba.stackexchange.com/questions/275686",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/164796/"
] | I've inherited a somewhat strange table whose records must somehow be linked to each other. In the real world, these two "structures" are combined into one larger "structure", so there should be a way of linking them.
Should I use an "association table" to link the two IDs, <strong>or is t... | <code>AUTO_INCREMENT</code> cannot be trusted to avoid gaps in the numbers. It appears that you <em>must</em> avoid them.
Plan A: Switch to MariaDB and use a <code>SEQUENCE</code> table.
Plan B: Simulate such.
Have a table with 2 columns in one row. (A second sequence could use a second row, etc.) Something like:
... | All(?) <code>INSERTs</code>, including IODKU and <code>REPLACE</code> can <em>burn</em> ids, leading to gaps. This happens with the statement is rolled back. And for <code>INSERT IGNORE</code> when some <code>UNIQUE</code> triggers "ignore".
Even without multi-master, you can get <code>AUTO_INCREMENT</code>... | https://dba.stackexchange.com |
24,845 | [
"https://chemistry.stackexchange.com/questions/24845",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/7612/"
] | I am having a difficult time understanding what makes a buffer a buffer.
Buffers in my textbook are defined as a solution of a weak acid or base and their conjugate acid/base. So if I were to just dissolve acetic acid (a weak acid) in water why would this not be defined as a buffer. Acetic acid incompletely dissociate... | The term "buffer" implies a purpose of resisting changes in pH when small amounts of acid or base are added. The buffer will be most effective when the pH equals the pKa of the acid. This is when there are equal amounts of acid and conjugate base. For each pH unit the system deviates from pH=pKa, the buffer is about ... | I totally agree with @DavePhD and I want to show you how the pH dependence of the degree of acid-base ionisation provides the background of the buffering mechanism. Titration curves effectively illustrate the influence of the pH on the ionisation of weak acids and bases. The Figure below shows the change in the pH of a... | https://chemistry.stackexchange.com |
252,433 | [
"https://electronics.stackexchange.com/questions/252433",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/120771/"
] | I am putting DC current through a wire to heat it. I would think the wire would heat up evenly but I have found that it is hotter the closer I get to the middle, or, respectively, colder the nearer to the clamps. Can anyone explain this?
| There are two effects going on. The heat sinking effect of the connections and the temperature coefficient on the wire.
Initially the wire is all at the same temperature.
You turn the power on and it starts to heat up.
The heating is determined by the electrical power dissipation in the wire, for any given section ... | Heat and temperature are two very different things. The equilibrium temperature occurs when the heat flow into a region equals the heat flow out.
In your case, the heat flow in per unit length of wire (the resistive heating) is essentially constant, as you surmise. However, the heat flow out — both along the wir... | https://electronics.stackexchange.com |
51,486 | [
"https://security.stackexchange.com/questions/51486",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/15194/"
] | I'm playing around with certificates and encryption on a Windows system, but either it isn't working or I don't know how it should works.
When I encrypt a file its name changes to green. But then, I can open it without any prompt or something. I thought "well... I'm logged with the same user that encrypted the file, l... | Windows built-in encryption is done transparently at the filesystem level. The encryption key is stored in your account profile encrypted with your login password (this is why there's the warning when you change your password). So logging in unlocks the key, which then makes your encrypted files available.
Since the e... | The encryption key is tied to your account and unlocked when you enter your account's password. As far as I know — but I'm not a Windows expert — what “unlocking” means depends on the version of Windows and whether a TPM is available, but the idea is that the encryption key cannot be retrieved without guessing your pas... | https://security.stackexchange.com |
1,562,503 | [
"https://math.stackexchange.com/questions/1562503",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/204859/"
] | Can anyone help me here?
Question: "X is a normed space and A is a subset dense in the dual of X.
x belongs to X and the sequence (x_n) of X is bounded of E such that f(x_n) converges to f(x) for all f in A. Show that x_n converges to x weakly"
My try: I think that if I show that A=cl(A) so I prove what is required.... | We want to prove
$g(x_n) \to g(x)$, for every $g \in X'$,
where $X'$ is the dual space of $X$. Equivalently, we can prove
$|g(x_n) - g(x)| \to 0$, as $n \to \infty$.
Let $\{f_k\}$ be a sequence in $A$ such that
$\|f_k - g\|_{X'} = \sup_{\|x\| \leq 1} |f_k(x) - g(x)| \to 0$, as $k \to \infty$.
The above implies... | You're on the wrong track because $A$ is never closed except in the trivial case where $A=X^*.$
You need to prove that for arbitrary $f\in X^*$
$$\lim_{n\to\infty}f(x_n)=f(x).$$
You can do this using the $\epsilon$-definition of the limit of a sequence, first choosing a $g\in A$ sufficiently close to $f$ in the sens... | https://math.stackexchange.com |
114,227 | [
"https://mathoverflow.net/questions/114227",
"https://mathoverflow.net",
"https://mathoverflow.net/users/20491/"
] | Let K and L are fields,L is a sub field of K,and L is isomorphic to K,whether can we get K=L?If true,how to prove? Thanks.
| No. ${\mathbb C}(X^2,Y)=L$ is a subfield of $K={\mathbb C}(X,Y)$ where $X,Y$ are algebraically independent variables over $\mathbb C$. Hence $L$ is isomorphic to $K$ but not equal.
| If K and L are F-field extensions, K/F and L/F are both finite dimensional, and the isomorphism from K to L is an F-homomorphism, then the proof is easy, but the general case seems difficult.
| https://mathoverflow.net |
255,717 | [
"https://security.stackexchange.com/questions/255717",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/34482/"
] | Let's assume we have an example machine connected to the internet. This machine is typically a client one, and it has no services like ssh running on. Does this kind of machines need any firewall to restrict incoming connections? On the one hand, there's no services that would accept the network packets, so there's no ... | This is close to ask whether a shutdown computer needs updates. The answer is not if and only if you are sure that it will always stay off. Your question should receive a similar answer: if you are sure that no listening services are active and will never be you do not need to block incoming connections.
But in real wo... | Does a machine with no listening services strictly need a firewall? Not really.
Does a machine with no listening services exist in practice? Not really, if we're talking about the more common desktop & server operating systems.
If you somehow identify and disable every single service that listens on a TCP or UDP po... | https://security.stackexchange.com |
19,933 | [
"https://math.stackexchange.com/questions/19933",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/6497/"
] | <strong>Second Update:</strong>
I see that some answers that reference my image are more closely answering my question.
Here is a second image to clarify my point.
Take this image representing a checkerboard like configuration. If I asked you to move the X negative 1 space it would be impossible. You would have to m... | Where the philosophy of negative numbers "existing" is concerned, I think Nate Eldredge answered the question very nicely. However, let me post a separate answer (really an extended comment) to address the diagram.
I think there's some confusion regarding the distinction between <em>position</em> and <em>distance</em... | Asking whether a negative number "really exists" is not really a meaningful question, to my mind. A number (of any kind) is an abstract mathematical idea, and it's not really clear how to talk about whether ideas "exist". Does justice really exist?
However, what we can say is that there are situations in the real wo... | https://math.stackexchange.com |
109,636 | [
"https://physics.stackexchange.com/questions/109636",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/45085/"
] | Had physics for 2 years now on highschool, but there is a thing I am wondering about.
You know the in the height above the earth surface around where the satellites are (Or the ISS), I've calculated that there actually is a big amount of gravity-forces, even up there. (9.1 - 9.2 <code>m/s^2</code>) - How come that thi... | These are two different effects. Satellites don't fall down because they are moving on a circular orbit. Actually, they are falling down all the time, since circular motion is accelerated (though the velocity doesn't change absolute value, it changes direction!), so it is kind of "falling around the earth".
The secon... | Paraphrasing Douglas Adams, "Flying is learning how to throw yourself at the ground and miss". This is pretty much how orbits work - you're actually falling all the time, but missing the body you're orbitting.
The reason you're experiencing apparent weight-lessness is because every piece of your body is accelerated th... | https://physics.stackexchange.com |
2,722 | [
"https://security.stackexchange.com/questions/2722",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/1590/"
] | Can <em>xp_cmdshell</em> ever be used safely within a stored proc and are there any situations for which there really is no other option? In other words, should its usage within a stored proc always be flagged as a security issue (as is advised by a well-known source code analyzer)?
Put differently, would you agree wi... | It is always a <em>risk</em>. It should always be <em>reviewed</em>. It can be properly <em>mitigated</em>.
There are legitimate uses, sometimes necessities, but watch your input closely!
| Turning off xp_CmdShell is a bit like putting a veil over rotting meat. It brings a false sense of security to the table and the flies can still get at the meat. Allow me to explain.
Who can use xp_CmdShell? That's right. Only people/app logins with "SA" privs or people that you made the horrible mistake of granting a... | https://security.stackexchange.com |
124,900 | [
"https://softwareengineering.stackexchange.com/questions/124900",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/42827/"
] | I have been writing in C# 4.0 a lot lately and trying to write as lean as possible. As such, I have not been using the classic <code>try/catch</code> blocks and <code>using</code> statements as often.
I understand the general function of .Net's garbage collection and exception handling - <em>I want to bulletproof my c... | <code>using</code> is not a "bloat", it's necessary to free resources wrapped into <code>IDisposable</code> objects. <code>using</code> is compiled into <code>try...finally</code>, with a call to the <code>Dispose</code> method in the <code>finally</code> section. For example, when StreamReader and StreamWrit... | <code>try/finally</code> and <code>using</code> are almost totally orthogonal concerns to memory leaks. You should use <code>using</code> or <code>try/finally</code> when you have non-memory resources, like a lock, a handle, or a connection, and you need to make sure those are cleaned up.
There are very few or no <em... | https://softwareengineering.stackexchange.com |
15,642 | [
"https://physics.stackexchange.com/questions/15642",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/5648/"
] | I'm guessing that, this would be someone in a rocket or something... When they hit their top speed, at what fraction of $c$ are they traveling?
| Maximum velocity attained by the Apollo spacecraft was 39,897 km/h which is $3.6\times 10^{-5}$ times the speed of light...
| When swinging my comfy hammock, I travel all day even up to 0.99 $c$, some days even more, depending on what particles are passing me by and measuring my exorbitant speeds with their atomic clocks and photons..!
| https://physics.stackexchange.com |
2,802,264 | [
"https://math.stackexchange.com/questions/2802264",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/296130/"
] | Considering the plane autonomous system below:
<blockquote>
$$\frac{dx}{dt} = ye^{xy}$$
$$ \frac{dy}{dt} = 1-x^{4} - y^{2}$$
</blockquote>
I have been asked to show that the point $(1,0)$ is a critical point (it is a centre), and then to use $x = 1 + u$ and $y= v$ for $u,v$ sufficiently small to deduce that appro... | You get
$$
ve^{u+uv}=v(1+u+u^2/2+...)(1+uv+u^2v^2/2+...)
$$
and
$$
1-(1+u)^4-v^2=1-(1+4u+6u^2+4u^3+u^4)-v^2.
$$
Removing terms of degree 2 and higher leaves you with the linearization.
| The first-order approximation of $x$ is $x=x_0+\Delta x= x_0+x'\Delta t$
Here, $x_0 = 1$, so $x=1+x'\Delta t$, hence $u=x'\Delta t$. Similarly, $v = y'\Delta t$.
Then $\frac{du}{dt}=x'\Delta t=(ye^{xy})'\Delta t =[ y'e^{xy}+(x'y+xy')ye^{xy})]\Delta t$
Substituting in $x=1,y=0$, everything disappears except $y'\Del... | https://math.stackexchange.com |
306,837 | [
"https://softwareengineering.stackexchange.com/questions/306837",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/142254/"
] | I have a script that throws exceptions when something goes wrong. However, for the purposes of testing I also want to capture specific points although I'm not sure whether they would be deemed errors or not.
Below is an example:
<pre><code>if ($model->fetchCurrentlyProcessing() > 0) {
throw new App_Exceptio... | An exception signifies an "exceptional situation" due to which a piece of code stops working. This may be an error, but it may also be any other situation which is exceptional and it prevents the code from doing its job.
So, both of your examples appear very good cases of exceptions to me: your code is supposed to do... | While I agree with most of what Mike said, I would say your way is never the right way to use exceptions. You are basically replacing something like "if queue is empty, return" with an exception. That is, you are using exceptions as a way of flow control to implement your basic algorithm and that is not good. Exception... | https://softwareengineering.stackexchange.com |
292,051 | [
"https://softwareengineering.stackexchange.com/questions/292051",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/95855/"
] | All the documentation, examples and tutorials I've found on <code>$resource</code> were using it directly in the controller. So far I'm just using <code>$http</code> from services and those services are further used in the controllers. I thought <code>$resource</code> should replace <code>$http</code> and not my whole ... | This depends on the complexity and the long term goals of your app, but in my experience, your best bet is to wrap your $resource calls in a service since at some point in the future you'll find yourself pre/post processing the intput/output of your $resource calls and it's better to simply attach a new function (or pa... | <blockquote>
When you want to create a command line interface for your application and you need to copy code out of the controller, than the controller has too much logic and it should be in a service.
</blockquote>
The reason this works is because your UI's are supposed to be independent of the service. A command ... | https://softwareengineering.stackexchange.com |
457 | [
"https://mechanics.stackexchange.com/questions/457",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/282/"
] | I have a 2004 Subaru Impreza WRX, but I suspect my question can be generalized to apply to many smaller cars.
Not being a fan of <em>donut</em> spare tires, when I replaced my tires about a year ago I kept the one that was in the best shape, mounted on a used steel rim that the shop sold me for cheap.
Much to my chag... | I only see three options.
<ol>
<li>Modify the hole where the spare tire goes, read cut and weld in a new one <strong>if</strong> there is room</li>
<li>Find a way to mount it outside the trunk, like under the car, doubtful that this could be done.</li>
<li>See if you can get the tire un-inflated in the same space, my ... | Recently I've helped a friend with the same problem. We created a double bottom with, and attached the top with hinges so you can open it. The cover has 4 bolts on it (the car uses 4x100 rims) and attached the spare to the bolts. When you open the cover you can use the spare and the well is available to store stuff lik... | https://mechanics.stackexchange.com |
117,436 | [
"https://cs.stackexchange.com/questions/117436",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/110055/"
] | I want to find two undecidable languages <span class="math-container">$A$</span> and <span class="math-container">$B$</span> that <span class="math-container">$A$</span> cannot mapping (i.e. many-one) reduce to <span class="math-container">$B$</span>, <span class="math-container">$B$</span> cannot reduce to <span class... | There is no known <em>natural</em> example of such a pair, and indeed there are various results in computability theory suggesting that such a pair does not exist. So to whip up an example one has to do some work.
Below, <span class="math-container">$\Phi_e$</span> is the <span class="math-container">$e$</span>th oracl... | A really simple example is <span class="math-container">$A_\mathrm{TM}$</span> (i.e. the language of <span class="math-container">$\langle M, w \rangle$</span> where TM <span class="math-container">$M$</span> accepts <span class="math-container">$w$</span>) and its complement <span class="math-container">$\overline{A_\... | https://cs.stackexchange.com |
312,296 | [
"https://softwareengineering.stackexchange.com/questions/312296",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/-1/"
] | I am very new to programming and a bit confused from reading\hearing different conventions from different sources:
Does Object-oriented programming have 4 or 5 concepts?
As a newcomer, I understand these are the 5 concepts:
<ul>
<li>Abstraction</li>
<li>Inheritance</li>
<li>Encapsulation</li>
<li>Polymorphism</li>
<... | The reason you find different explanations of what object-oriented programming means is because there is no single person or organization with the authority to formulate a strict universally-applicable definition.
Object-oriented programming is not an ISO standard or a scientific law. It is a philosophy. And as with a... | <blockquote>
Does Object-oriented programming has 5 or 4 components?
</blockquote>
As others have mentioned, "OO" doesn't really have any <em>components</em>, because it's a way of thinking about modelling solutions to problems and not a toolkit nor a set of clearly defined processes.
<blockquote>
As a newcom... | https://softwareengineering.stackexchange.com |
58,758 | [
"https://biology.stackexchange.com/questions/58758",
"https://biology.stackexchange.com",
"https://biology.stackexchange.com/users/28324/"
] | Why do fruits have such a low protein content (with a few exceptions) ? Don't seeds need protein while growing up? In comparison, the egg of a hen contains lots of protein, used to make a chick.
| I don't have time to document this enough to make it a good answer but it's too long for a comment so I'll post it anyway.
A big difference between plants and animals is how they get their food, that implies they have different elements to work with. Proteins are made of amino acids, which contain nitrogen, meaning to... | In brief the lower protein content of a seed compared to an egg is caused by two reasons.
(1) The structural components of a plant versus that of an animal.
The structural components of plant is made by cellulose which is made of glucose subunits. Animal in contrast are have large amount of muscle tissue to allow move... | https://biology.stackexchange.com |
37,236 | [
"https://datascience.stackexchange.com/questions/37236",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/41631/"
] | A simple question about convolution over volume .
Say we have an image with dimensions $(n, n, 3)$ and we apply a filter of dimension $(k, k, 3)$ this outputs an matrix of dimension $(n-k+1, n-k+1)$.
Why do we sum across channels in this case. Don't we lose information by mixing different channels. In case of images... | CNN filters are used for edge detection only. These edges are basically detected by a mathematical functions and as a result get more and more complex in deeper layers (cascading functions) enabling it to detect complex features.
In your question 2 points need to be noted:
<ul>
<li>Traffic lights are not pure red, gr... | There are multiple points that I try to explain them.
First, each filter for convolutional networks for images is a 3d volume. Consequently, whenever it is said we have $n$ filters, means we have $n$ volumes of those 3d filters.
Second, you can consider each convolutional layer as an MLP which is applied to small r... | https://datascience.stackexchange.com |
27,693 | [
"https://mathoverflow.net/questions/27693",
"https://mathoverflow.net",
"https://mathoverflow.net/users/828/"
] | I hope this question is suitable; this problem always bugs me. It is an issue of mathematical orthography.
It is good praxis, recommended in various essays on mathematical writing, to capitalize theorem names when recalling them: for instance one may write "thanks to Theorem 2.4" or "using ii) from Lemma 1.2.1" and so... | In English, proper nouns are capitalized. The numbered instances you mention are all usages as proper nouns, but merely refering to a lemma or corollary not by its name is not using a proper noun, and so is uncapitalized.
Thus, for example, one should write about the lemma before Theorem 1.2 having a proof similar to ... | No. "Theorem 2.4" is a title, hence capitalized. But the word "lemma" in "the previous lemma" is simply a non-proper noun, hence uncapitalized in English. In both your examples I would use the second version.
Although I wrote the above paragraph as though it were definitive, it is of course only my own opinion. Th... | https://mathoverflow.net |
378,560 | [
"https://softwareengineering.stackexchange.com/questions/378560",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/315480/"
] | <strong>GOAL:</strong> naming a class which has the responsibility to save data in Android database.
The class structure is like this
<pre><code>ClassName {
public void saveSync(...){
/* implementation*/
}
public void auxiliaryMethod(...){
/* implementation */
}
}
</code></pre>
<strong>QUESTION:</stro... | If a class is properly encapsulated it's hard to tell if it even has fields from outside.
Some classes are immutable. You can't change their fields once their objects are constructed. Your class is also immutable. But not only can't its object change, you only have one way to build it.
That doesn't mean you have to ... | When I find myself in this situation, I take a step back and think about how I am going to call the methods. Forget nouns vs. verbs for a minute. Then I play fill-in-the-blank and go with what sounds most natural.
<ul>
<li><code>___________.saveSync()</code></li>
<li><code>dataManager.saveSync()</code>?</li>
<li><code... | https://softwareengineering.stackexchange.com |
373,128 | [
"https://softwareengineering.stackexchange.com/questions/373128",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/277373/"
] | Say I have class with a <code>boolean</code> member variable <code>fancy</code>:
<pre><code>public class MyClass {
private boolean fancy;
}
</code></pre>
<strong>Case 1.</strong> I could the define the <code>setter</code> and <code>getter</code> as follows:
<pre><code>// getter
public boolean isFancy() {
ret... | Using <code>isFancy</code> for a setter is certainly not a standard convention and may therefore confuse the code's readers. However, the idea of using a fluent interface is a good one. If I had to choose a name for use in your second example, I would simply use <code>fancy()</code>. This is a name which you can use fo... | I wouldn’t say anything is inherently wrong with it. But it’s not the kind of method name I would be looking for if I was looking for a setter. With that name I might expect to be able to retrieve whether the object is fancy, with some kind of boolean switch (which I would not suggest either, but that’s off-topic)
I s... | https://softwareengineering.stackexchange.com |
3,217 | [
"https://electronics.stackexchange.com/questions/3217",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/914/"
] | I can connect many resistor in parallel or series to get a final resistance I want in case when I have no necessary nominal out of the box.
I wonder is there such trick that will allow me to easily change varistor's nominal. Say, originally I have a varistor that changes its resistance from 0 to 200 kilohm. Can I cons... | An easy trick is to place a fixed resistor, say 10 kOhm, in parallel with the varistor. The total resistance will now vary between 0 and 9.5 kOhm. But the problem here is that the total resistance does not vary linearly with the varistor's resistance, f.i. when the varistor is at one quarter, at 50 kOhm, my circuit wil... | Wim is correct with his ideas but I wanted to say that in general placing nonlinear circuit elements in parallel or series will produce undefined behavior. Unless the two nonlinear elements are precisely matched (ie, their trip points, V-I characteristics, etc are within <1% of each other) then there's no telling w... | https://electronics.stackexchange.com |
201,851 | [
"https://softwareengineering.stackexchange.com/questions/201851",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/56275/"
] | Imagine this scenario:
You've created a multiplayer game app for a mobile platform such as Android. You successfully set it up with one server, you figured out how to do that. It works quite well.
To your surprise, the game TAKES OFF. Server demands explode. You need to scale, but you have hardly any experience with ... | <strong>1. On app level</strong>
Running an app on multiple servers isn't terribly hard. There are techniques which are difficult to understand and implement, but there are also some basic techniques any one can use.
Learn those basic techniques. If it's not enough, then yes, you'll have to find a freelancer or to hi... | The first three things you want to do are profile, profile, profile.
To be able to effectively scale, you need to know exactly what it is that needs to be scaled up. Whats the bottleneck? Is it concurrent connections? database access? Heavy processor usage due to your code? Once you find out the problem, then y... | https://softwareengineering.stackexchange.com |
93,391 | [
"https://cs.stackexchange.com/questions/93391",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/89421/"
] | Basically what the title says. Why can you "ignore" the "xy" part if you want to prove whether a language is regular?
| Both pumping lemmas have an intuitive explanation in terms of an automaton that can recognize a language.
A regular language can be recognized by a finite automaton. All words are recognized through:
<ul>
<li>either a finite path through the automaton: words that are shorter than the pumping length;</li>
<li>or a pat... | That is because of the "structure" of the languages that is observed by the respective pumping lemma's. Have a look at the proofs of the respective pumping results.
For regular languages the structure is linear, and for every long word there is a state that is repeated twice in the accepting computation of a finite st... | https://cs.stackexchange.com |
62,607 | [
"https://cs.stackexchange.com/questions/62607",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/56696/"
] | We know that any "effectively computable" process is computable by a Turing machine (Turing-Church thesis).
Although it seems that "effectively computable" is still open to discussion, the intuitive interpretation is that any process that is "mechanical enough" can be computed by a Turing machine.
Turing's initial ob... | There are only three high level abstractions in the network stack.
<strong>Link Layer</strong>: Knows how to transfer data (physically*) across <strong>one</strong> hop. This layer with deals encoding and framing bits, controlling channel access, detecting and recovering from collisions in the transfer medium. Eg: Eth... | Network layer is required for routing purposes. The network layer at router takes the destination IP address of the packet and compares it with its routing table. <strong>Next hop of the packet</strong> is decided by seeing the routing table.<br>
<blockquote>
The table helps to decide the router whether the packet b... | https://cs.stackexchange.com |
1,848,644 | [
"https://math.stackexchange.com/questions/1848644",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/271257/"
] | Let $A=\{a_n\}$ is a strictly-increasing sequence of positive integer. The natural density of this sequence is defined by $\delta(A)=\lim_{n\rightarrow \infty} \frac{A(n)}{n}$ whenever the limit exists and where $A(n)$ is the number of elements of $A$ not exceeding $n$. Is there a strictly-increasing sequence of positi... | Yes. Start with $1$. Then omit enough integers to reduce the ratio below $\frac{1}2$. Then include enough consecutive integers to increase the ratio above $1-\frac{1}3$. Then omit enough to reduce it below $\frac{1}4$. Then include enough to increase it above $1-\frac{1}5$. Keep going.
| Consider the sequence of integers with an odd number of decimal digits.
| https://math.stackexchange.com |
1,727,473 | [
"https://math.stackexchange.com/questions/1727473",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/312139/"
] | I've been trying to calculate the limit of the following, as $x$ tends to $0$:
$$f(x) = \left(\frac{e^x-1}x\right)^{1/x}$$
I've tried writing it as $e$ raised to the power of its log, but I am unable to solve it. Any tips on how to proceed will be appreciated!
| $\lim_{x \to 0}\left( \dfrac{e^x-1}x \right)^\frac1x=\lim_{x \to 0}\left( \dfrac{(1+x+\frac{x^2}2+o(x^2))-1}x \right)^\frac1x=\lim_{x \to 0}\left( {1+\frac{x}2+o(x)} \right)^\frac1x=\lim_{x \to 0}e^{\frac1x \ln\left( {1+\frac{x}2+o(x)} \right)}=\lim_{x \to 0}e^{\frac1x \left( {\frac{x}2+o(x)} \right)}=\sqrt e$
| A good trick for $1^\infty$ indeterminate forms that may be worth rembering is, if $\lim f(x) = 1$ and $\lim g(x) = \infty$ then
$$\lim f(x)^{g(x)} = \exp[\lim g(x)(f(x)-1)].$$
In this case
$$\lim_{x \to 0}\left(\frac{e^x-1}x\right)^{1/x}=\exp\left(\lim_{x \to 0}\frac{e^x-1-x}{x^2}\right).$$
Using L'Hospitals rule
... | https://math.stackexchange.com |
191,809 | [
"https://electronics.stackexchange.com/questions/191809",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/29617/"
] | SE fellas
Today I was thinking about electrolytic capacitors. it's a question to me <strong>Why some electrolytic capacitors have(or manufacturers make) a bigger distance between their two legs?</strong> I'm talking about this:
<img src="https://i.stack.imgur.com/ZYWWd.jpg" alt="Figure1">
While other is like this:
... | Every year they seem to figure out ways of making e-caps smaller and smaller so at some point forming the leads is required to fit the pattern of the old capacitors.
Changing hole layout of high volume inexpensive boards can involve changing hard tooling (punch positions in the die) rather than the soft changes that ... | Those indents, or bends make it so the part is lifted above the PCB, (through hole), make it so the component has some room for air flow. Or that was the explanation I received when I asked the same question about 10 years ago.
Straight through leads you could optionally bend to put in the gap for air flow.
Other m... | https://electronics.stackexchange.com |
64,892 | [
"https://dba.stackexchange.com/questions/64892",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/38195/"
] | I have a massive flat text file with ~4M records that I would like to read into MySQL using Ruby/Rails, Python, or any other method.
The flat file is roughly this format:
business_name,address,city,state,zip,employee_name
If there are multiple employees at the business, there will be one row for each employee with th... | Try using LOAD DATA INFILE into an ISAM table with few or no indexes. ISAM should reduce memory consumption and get all data in there the fastest possible way. I have never run out of memory when doing this with larger data sets than yours, on pretty old servers with alot less memory than is used today.
Once the data... | I would make two passes through the input the file in a script, and generate two output .sql files, one for business and one (or more) for employees.
On the first pass, just build inserts for the businesses table. During the processing keep track of names of businesses you've already inserted and skip them when they s... | https://dba.stackexchange.com |
316,485 | [
"https://stats.stackexchange.com/questions/316485",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/89188/"
] | Why cant we use negative binomial to calculate the probability of getting a 7th game in best of 7 game series?
P(X=7, r=4) = {7-1 C 4 -1} * (.5)^(7-4) * (.5)^4 = .15 , so there is 15 % chance of 7 trails to get 4 success or 15% chance its a 7 game series.
Another way to look at this would be : Binomial , you need exa... | Summary: the negative-binomial based approach in the question ignores that either team can win Game 7. After correcting for this the results agree.
<h3>Assumption</h3>
Not explicitly stated in the question, but it seems we are assuming the games are iid. with probability 0.5 for either team to win (a sequence of fai... | The negative binomial would be appropriate if you wanted to know how many games it would take before team A won 4 games. However, that might be, say, 104 games, in which case team B would have won 100 games. Obviously that's not the way an actual seven game series works!
Your calculation - $P(X=7 | r=4)$ - using the... | https://stats.stackexchange.com |
487,852 | [
"https://physics.stackexchange.com/questions/487852",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/205615/"
] | I'm slowly reading my way through Peskin and Schroeder. Near the end of section 7.1 they compare the mass shift of the electron from QFT to the classical value, both of which are divergent but in different ways.
The calculation from QFT gives:
<span class="math-container">$$\delta m = \frac{3\alpha}{4\pi}m_0\log\lef... | From a dimensional analysis point of view: Assume you need linearity in <span class="math-container">$m_0$</span> (aka. <span class="math-container">$\delta m \propto m_0$</span>) by the argument given by MadMax above. This means you have <span class="math-container">$ \delta m \sim m_0 f(\Lambda,m_0)$</span> for some ... | The logarithms in amplitudes are the major consequence of QFT and appear from integrals over 4-momentum. For the hard cutoff regularization integrals like:
<span class="math-container">$$
\int_0^\Lambda \frac{d^4p}{(2 \pi)^4} \frac{1}{\left( p^2 + \Delta^2 \right)^n}
$$</span>
become
<span class="math-container">$$
2 \... | https://physics.stackexchange.com |
323,959 | [
"https://stats.stackexchange.com/questions/323959",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/12100/"
] | Consider a set of features $x_1,x_2,...x_N$ that may be (pearson-) correlated.
I want to group them whenever they have a high correlation between themselves so that I can interpret the coefficients of a linear regression on them.
To this, I constructed an undirected (because correlation is commutative) graph/network ... | No, correlation is not transitive.
Consider:
X1 = [0,1,2,0]
X2 = [0,1,1,0]
X3 = [0,2,1,0]
Then:
corr(X1,X2) = 0.90
corr(X2,X3) = 0.90
corr(X1,X3) = 0.64
Therefore choosing a cutoff of x=0.8 will add an edge from X1 to X2 and from X2 to X3, but not from X1 to X3.
To make your feature-grouping approach consist... | Pearson correlation is NOT transitive.
For example, let $corr(x_1,x_2) = corr(x_1,x_3) = 0.5$ . Then $corr(x_2,x_3)$ can be any value in the range [-0.5,1].
This can be seen by arranging the known correlations into a 3 by 3 matrix, with $corr(x_2,x_3)$ as an unknown (variable) in the matrix. The unknown (occupying 2... | https://stats.stackexchange.com |
52,469 | [
"https://mechanics.stackexchange.com/questions/52469",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/34435/"
] | I just did a head gasket replacement on my 1995 Acura Legend.
Before the replacement, compression was good on all cylinders except #3, which had blown. While the engine was open:
<ul>
<li>I cleaned off the pistons and cylinder walls with WD-40 until they were shiny. </li>
<li>Heads were within flatness spec, and the ... | The underlying equation that links vehicle speed with wheel revolutions
is:
<pre><code>Linear Speed = Radius x Angular Speed
</code></pre>
The speedometer is calibrated against angular speed (e.g. RPM, revs/unit time). So any change in wheel radius will translate to directly proportional change in the reading on the... | If you want to change tyres to improve the speedo accuracy then do so.
The circumference, when you say "worse" do you mean larger or smaller. If you go larger then the distance travelled for 1 revolution of the wheel is greater and a greater inaccuracy on the speedo. If you go smaller then the distance per rev is smal... | https://mechanics.stackexchange.com |
249,532 | [
"https://stats.stackexchange.com/questions/249532",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/59500/"
] | <strong>Question</strong>
I've recently discovered that a brute-force algorithm is much faster at solving an optimal solution than the optimization routine for a particular problem I've been working on. This is mainly because the number of possible solutions is rather small ($2^{12}=4096$), but it has made me questio... | As @usεr11852 has pointed out, the difference in speed can be the determination in the decision. In the case of my modeling strategy, a nonlinear model was extremely difficult to calculate the maximum value due to discontinuity in the function form. As a result, I simply calculated all possible values through a brute f... | Brute force only works on certain special cases and optimization is a general technique to solve "more realistic" problems.
Here is a small demo: if we add one small change in your problem, the brute force will "fail". Suppose the tuning parameter $x$ is a vector that has $10$ dimensions. Then, the brute force will ha... | https://stats.stackexchange.com |
498,023 | [
"https://electronics.stackexchange.com/questions/498023",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/241587/"
] | I need to understand the nomenclature used for coaxial cable.
For example: UT-085-SS-SS, or UT-085-SS, or UT-085-NbTi
As far as I understood, the number 085 here is the total diameter of the cable in inches (thus 0.085 inches of diameter). The SS-SS means that the material is Stainless-Steel.
But how can I read it i... | UT stands for Uniform Tubes, the company that originally manufactured semi-rigid cable, it doesn't have any other particular meaning.
There is no rule for part numbers. Often they have a pattern, sometimes they are completely anarchic. Only the datasheet can tell you what all the characters mean.
| <blockquote>
For example what is the difference between UT-085-SS-SS and UT-085-SS: both are in Stainless Steel apparently, but why one has it written twice?
</blockquote>
From some googling it looks like the SS-SS variant has stainless steel for both the outer and inner conductors, while the SS variant has a stainl... | https://electronics.stackexchange.com |
614,425 | [
"https://math.stackexchange.com/questions/614425",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/17976/"
] | This problem's taking me a lot longer than I like (probably doing things the hard way...). I have 9 different terms to integrate, some of which are messier than others. This one, though, is messier than most.
$$-{\Large\int}_{\!0}^{a}
\!\raise 0.8ex {at^{3}\,{\rm d}t
\over
\,\sqrt{\,\left(a^{2} -t^{2}\right)\left(b^{... | Using the substitution $u=a^2-t^2$ as suggested by André Nicolas, we get:
$\dfrac{\mathrm du}{\mathrm dt}=-2t$. So,
$\begin{align}&\int-\frac{at^3}{\sqrt{(a^2-t^2)(b^2t^2+a^4-a^2t^2)}}\mathrm dt\\
&=\frac{a}2\int\frac{t^2(-2t)}{\sqrt{(a^2-t^2)((b^2-a^2)t^2+a^4)}}\mathrm dt\\
&=\frac{a}2\int\frac{(a^2-u)}... | $\newcommand{\+}{^{\dagger}}%
\newcommand{\angles}[1]{\left\langle #1 \right\rangle}%
\newcommand{\braces}[1]{\left\lbrace #1 \right\rbrace}%
\newcommand{\bracks}[1]{\left\lbrack #1 \right\rbrack}%
\newcommand{\ceil}[1]{\,\left\lceil #1 \right\rceil\,}%
\newcommand{\dd}{{\rm d}}%
\newcommand{\ds}[1]{\displaystyle... | https://math.stackexchange.com |
64,281 | [
"https://dba.stackexchange.com/questions/64281",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/-1/"
] | I have setup a transactional replication on a sql 2008 database.
Then I backup the database.I got following error when I try to restore the database.
'Exclusive access could not be obtained because the database is in use'
Do I have to remove the replication before restore database?
| If it's a full backup, and there's no real-time application being in use in the database. I would suggest stopping and deleting the database for a complete clean up. After that the restore can be done and the replication could be setup automatically from the restore.
| It means a connection is open to the database while you're attempting to restore. You need to set the database to single-user mode first.
PS. If I remember correctly, you'll lose all the system procedures created by the replication process. You'll have to re-initialize the subscription.
| https://dba.stackexchange.com |
93,554 | [
"https://physics.stackexchange.com/questions/93554",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/29258/"
] | I am thinking about the mathematical process of "differentiating underneath the integral", i.e. applying the theorem $$\partial_s \int_{-\infty}^\infty f(x,s)\,dx=\int_{-\infty}^\infty \partial_s f(x,s)\,dx$$ given some regularity assumptions. I was trying to think of some relevant physical interpretations of this.
O... | For example, consider some water flow in the space, in which the density $\rho(x,t)$ fluctuates in space and in time. You might be interested in how the mass inside some fixed volume $V$ changes over time. The mass is equal to
$$
M(t)=\int_V{\rho(x,t)\mathrm{d}x},
$$
therefore the "mass flow rate", using the rule you m... | I guess a simple example would be the derivation of the continuity equation.
It will first be useful to define the phase space probability density, denoted by $f=f(q_1,\dots,q_{N},p_1,\dots,p_{N},t) \equiv f(\mathbf{x},t)$, that tells us the probability of finding a system near $(q_1,\dots,q_{N},p_1,\dots,p_{N})$ at a... | https://physics.stackexchange.com |
128,736 | [
"https://mathoverflow.net/questions/128736",
"https://mathoverflow.net",
"https://mathoverflow.net/users/10330/"
] | <blockquote>
Is it possible to construct the midpoint of a segment in the hyperbolic plane
using the <em>set square</em> only?
</blockquote>
With the <em>set square</em> one can
<ul>
<li>draw the line through the given two points and </li>
<li>drop the perpendicular from the given point to the given line.</li>
... | Finally, I realized that the answer is NO.
Let us think of hyperbolic plane as about subset of projective plane for $\mathbb R^{2,1}$.
Note that in this model, the set square tool is the same as hyperbolic-cross-product tool;
i.e. for any two vectors $u$ and $v$
you can construct the vector
$$w=J(u\times v),$$
wher... | I can do it if I am allowed to cheat - in axiom one for the set square (which is the axiom for the straight-edge) I'll allow one of the points to be ideal: that is, on the Gromov boundary of $H^2$. Also, I assume that the set square produces infinite geodesic rays. In particular, this means, in the second axiom, that... | https://mathoverflow.net |
1,240,588 | [
"https://math.stackexchange.com/questions/1240588",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/23666/"
] | I'm encountering questions where I'm required to find a transitive closure (and the questions seem to suggest that there is only one), but I probably don't understand something in the definition, because I don't see why is it required that there be only one minimal transitive relation which is a superset of the one I'm... | The transitive closure of a binary relation $R$ is the intersection of all <b>transitive</b> binary relations that <b>extend</b> $R$.
To say that $S$ <b>extends</b> $R$ means that for all $x,y$ in the domain, if $aRb$ and $aSb$.
The intersection $T$ of a set of binary relations is defined by saying that for all $x,y$... | Your relation $R$ is already transitive, so it is its own transitive closure. It appears that what you’re misunderstanding is the notion of transitivity. A relation $R$ on a set $A$ is transitive if it satisfies the following condition:
<blockquote>
<strong>if</strong> $\langle a,b\rangle\in R$ and $\langle b,c\rang... | https://math.stackexchange.com |
129,132 | [
"https://electronics.stackexchange.com/questions/129132",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/45816/"
] | From what I understand, the voltage doubles since the reflected and incident pulses add up. But then why don't all open terminations have double the source voltage since we can consider the voltage there to be a pulse of infinite duration?
| Yes, the voltage doubles when the pulse hits the open end of the cable, but this change then propagates back toward the source end of the cable. What happens there depends on how well the impedance of the source is matched to the impedance of the cable.
If the source is matched to the cable, the initial pulse was only... | That's because a "pulse of infinite duration" is not at all similar to "a pulse of finite duration".
For the reflection, and all what comes from that, to be significant you have to be working with a time base that is somewhat similar to the propagation delay of the cable. Since the latter is generally in the 10ns ball... | https://electronics.stackexchange.com |
303,887 | [
"https://physics.stackexchange.com/questions/303887",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/141464/"
] | What happens when external force on a body equals force of limiting friction?
I wanted to know if it stays at rest or starts motion.
| It will be on the verge of moving or slipping
| The net forces on a (rigid) object determine its acceleration. If the total force is zero, it will remain at rest.
| https://physics.stackexchange.com |
184,276 | [
"https://dba.stackexchange.com/questions/184276",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/133291/"
] | This is for SaaS app (currently in MySQL-InnoDB). Each app contains about 150 tables. The database size is not big, but the number is high.
For each database, InnoDB create directory in <code>/var/lib/mysql</code>. Because too many database, I choose to have individual database per app to prevent file system bottlenec... | Using a simple subquery should help with this:
<pre><code>SELECT e.eid, CONCAT(efirstname, ' ', e.elastname) name
FROM employees e
WHERE e.salary > (SELECT AVG(salary) FROM employees) * 1.10
</code></pre>
| Can u try this:
<pre><code> SELECT e.eid, concat(e.eFirstName, ' ', e.LastName) as name
FROM employees e
WHERE e.salary > AVG(e.salary) * 0.1;
</code></pre>
| https://dba.stackexchange.com |
228,472 | [
"https://softwareengineering.stackexchange.com/questions/228472",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/119362/"
] | I'd like to include some data from a GPLv2 licensed project in my MIT licensed project.
More specifically, I want to use the data from the other project as the training data for my machine learning algorithm and I'd also like to include the trained model in my project.
I don't want to include the whole project sourc... | As I understand it, the GPL files serve as input to your software. In that case, your software can not be considered to be derived from the GPL files and is thus not affected by the copyleft nature of the GPL license.
The output of your program (when taking those GPL files as input) <em>is</em> derived from the GPL fi... | As I understand you are not going to <strong>redestribute</strong> that part that you borrowed from GPLv2 project. You just need carefully maintain dependency on that project - it's perfectly fine to use some GPL data during development as soon as your final product that you will release doesn't contain it.
From GPLv2... | https://softwareengineering.stackexchange.com |
368,131 | [
"https://electronics.stackexchange.com/questions/368131",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/72155/"
] | Between a mains step down transformer and a SMPS, which would be safer for a bench power supply? I cannot look inside the transformer in either case to judge the insulation quality. But, if the insulation does fail there would be 230VAC at the secondary for the mains transformer. And, maybe 325VDC at the output of the ... | <blockquote>
Battery A still had potential of 12V, but the battery B showed 0V.
</blockquote>
When you connect batteries in series, you have to make sure they are of equal capacity (Ampere hours), equal Voltage and equal wear and age.<br>
Basically you may only create a series pack with two brand new batteries.
T... | Voltage is not a reliable indicator of charge state.
Both batteries can have a terminal voltage of 12V, but one be half charged and the other nearly dead. Running them in series will work until the weakest one is completely discharged.
At that point, the battery that still has charge begins pushing current throug... | https://electronics.stackexchange.com |
1,987,576 | [
"https://math.stackexchange.com/questions/1987576",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/254626/"
] | I have the following proposition:
"Let $C$ be the set of continuous functions $f:[a,b] \rightarrow [a,b]$ with the sup metric. The subset of $C$ made by taking the continuous functions that are not surjective is open."
I've managed to prove this, but by using results from complete metric spaces:
<ol>
<li>proved tha... | Suppose $f$ is not surjective, then there is some non empty open $U$ such that
$f([a,b]) \cap U = \emptyset$. This exists because $f([a,b])$ is compact
and hence closed.
Pick some $y \in U$, then there is some $\epsilon>0$ such that
$B(y,2\epsilon) \subset U$.
Now suppose $\|f-g\|_\infty < \epsilon$. Then $g(x... | Let $f$ be non-surjective; without loss of generality, $f$ maps into $[a,b']$ with $b' < b$. Let $\epsilon = \frac{b - b'}2$. Then the open ball of radius $\epsilon$ around $f$ is contained in the non-surjective functions, since no such function can map onto $b$.
This implicitly uses the fact that the continuous im... | https://math.stackexchange.com |
4,034,302 | [
"https://math.stackexchange.com/questions/4034302",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/369816/"
] | The usual proof of the upward Löwenheim-Skolem theorem rests on the use of the equality symbol as a primitive symbol, interpreted in every relevant structure as true equality. My question is on how the upward Löwenheim-Skolem theorem has to be modified if equality is not a primitive symbol anymore and, say, just introd... | <strong>EDIT: There's a major error here which I will fix later today. (I can't delete this post since it has been accepted; if it is unaccepted, I will delete it until I have time to repair it.)</strong>
Per the comments, your main interest is in the behavior of Hanf numbers when we drop equality from a logic. (Below ... | Concerning your first question, the upward Löwenheim-Skolem theorem in FOL without equality is much simpler (and less interesting). Let <span class="math-container">$M$</span> be an infinite model, and pick an element <span class="math-container">$m\in M$</span>. Then you can always adjoin an arbitrary collection <span... | https://math.stackexchange.com |
34,208 | [
"https://softwareengineering.stackexchange.com/questions/34208",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/10282/"
] | i work for a web development company and during the one year i have spent here, there were no improvements in the technologies we used to built our websites. I introduced jquery to them (buying the Novice to Ninja by Sitepoint) and now, i want to get rid of all these crappy PHP from scratch and use a PHP framework inst... | <h2>Find another Job</h2>
The real problem here is culture. If you had to introduce the other developers to jQuery they don't know web development 101 and probably just don't give a damn about best practices. Maybe they just turn up work 8 hours and collect their pay, devoid of any passion for programming. When you tr... | <h1>It saves money</h1>
<blockquote>
So what reasoning i can use to convince my boss to switch, and how to convice the other developers too?
</blockquote>
Using a framework is like prefabricating pieces of a site. As long as you can work within that prefabrication it <em>will</em> speed development, reduce bugs an... | https://softwareengineering.stackexchange.com |
142,670 | [
"https://physics.stackexchange.com/questions/142670",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/24839/"
] | I have the following homework problem:
<blockquote>
Consider two carts of equal mass <em>m</em> on a horizontal, frictionless track. The carts are connected by a single spring of force constant <em>k</em>, but are otherwise free to move freely along the track.
<strong>(a)</strong> Write down the Lagrangian and fin... | There's a sign error in your equations of motion. The Lagrangian of the system will be
$$L=T-U= \frac{m}{2} \left( \dot{x_1}^2 + \dot{x_2}^2 \right)-\frac{k}{2} \left( L + x_1 - x_2 \right)^2$$
So the equation of motion for $x_1$ is:
$$\frac{d}{dt}\frac{\partial L}{\partial \dot{x}_1}-\frac{\partial L}{\partial x_1}=0 ... | I think this problem is trying to get you to use generalized coordinates.
If you make your coordinates the center of mass and the seperation, you will get decoupled equations for the free particle and the harmonic oscillator respectively. Being agnostic about the coordinates is the secret superpower of the Lagrangean a... | https://physics.stackexchange.com |
101,353 | [
"https://physics.stackexchange.com/questions/101353",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/26597/"
] | Suppose a light wave with wavelength 3m. What happens if one tries to contain that wave within a 1m container? If I'm going about this entirely the wrong way or have wrong conceptions about light (which might be the case because I'm not a professional physicist), please tell me that instead.
| "Soft and squishy" is a bulk property. That's not due to either the polyurethane or the air fraction individually, but of the combination.
However, at the scale of a knife's edge, that foam is in fact locally either air or polyurethane. Cutting the air bubbles is trivially easy, but that small fraction of polyurethane... | My guess is that the foam has some type of sillica in it which may stick to the knife having a detremental effect on its cutting ability possibly cleaning the blades regularly with alcohol could help this but i'm no chemist, hopefully this is helpful
| https://physics.stackexchange.com |
8,361 | [
"https://dba.stackexchange.com/questions/8361",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/4084/"
] | I know how to check the DB is in a lock state or not, but my question is how to find out which query caused the lock on table /db.
IS there any log file getting created if something unexpected happens to the database?
| List the current database activities:
<pre><code>db2top -d [dbname]
</code></pre>
This command will show you database locks:
<pre><code>db2pd -d [dbname] -lock wait
</code></pre>
| db2pd as explained in this same post is a very good option.
Add to that, you can use the procedure:
<pre><code>db2 "call monreport.lockwait()"
</code></pre>
This procedure is going to show you in a very organized way the locks currently existing in your database.
After check the application id that holds the locks,... | https://dba.stackexchange.com |
41,170 | [
"https://security.stackexchange.com/questions/41170",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/28350/"
] | I would like to discuss this scenario:
There is a HTTPS service which responds with signed keys to an authenticated client. These signed keys can be used for another service to return very sensible data. Let's assume that because of some internal security hole (unexperienced internship, exposed ssh key on git server,... | Well, there are many options other than SSL to prevent a man in the middle attack, but most all of them have a similar cryptographic basis. Fundamentally, to ensure that a communication can't be attacked by a man in the middle you must be able to prove that a) both parties can validate the other and b) that no other p... | It sounds like what you're asking is: <strong>how do I prevent a MITM if my private key may be compromised?</strong> And to that, the answer is <strong>get a new private key</strong> (and therefore new certificate).
If your security policy is so disastrous that an intern is able to upload your SSL private key to gith... | https://security.stackexchange.com |
375,257 | [
"https://softwareengineering.stackexchange.com/questions/375257",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/310606/"
] | So, we have a dashboard page where a lot of different values are aggregated and presented. These values are calculated based on roughly 500k data points and presents different index values based on surveys. To calculate a certain index we need to aggregate all data points (these are divided by question number and user ... | Since the weights are based on date, you can structure an aggregate data set of the date, total for that date, and the count for that date. This means that you will not need to calculate the WMA on a per question basis, you can weight the total for a day, add all the weighted day totals and then divide by the weighted... | As an aside, I think you're making your formula harder to read than needs be.
<pre><code>((2+3+3+1)/(4*3))*10
</code></pre>
would make more sense when written as
<pre><code>((2+3+3+1)/4)*(10/3)
</code></pre>
<ul>
<li><code>((2+3+3+1)/4)</code> expresses calculating the average</li>
<li><code>(10/3)</code> expres... | https://softwareengineering.stackexchange.com |
67,865 | [
"https://electronics.stackexchange.com/questions/67865",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/19331/"
] | I am using my Arduino Uno as a programmer to program my ATtiny85. I uploaded the ArduinoISP sketch to my Uno. I double checked my wiring, did some googling, but I still can't figure out why I get this error:
<code>avrdude: stk500_getsync(): not in sync: resp=0xff</code>
when I run this command:
<code>sudo avrdude -p... | In the semiconductor industry these are called DUT (for Device Under Test) or Load boards and they map from the ide pins to the tester standard connection format. They can be fairly expensive because of size and thickness (to ensure stiffness).
However, you don't need most of that. at the center of a DUT board are c... | For very small boards, you can test before depanelizing. Bring the test points out into the panel, through an inner layer so the depanelization process doesn't tear up the rest of the traces.
Or just use smaller probes. Semiconductors are tested before sawing up the wafer, using machines that are maybe 60 dB more ex... | https://electronics.stackexchange.com |
252,979 | [
"https://physics.stackexchange.com/questions/252979",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/98517/"
] | I know it sounds very stupid question.
I am working on the AMO physics (Quantum Optics). I recently saw articles about the Spacex, and I thought it is really amazing, and thinking about the possible AMO application to space project.
AMO is applicable for the accurate sensor/detector and future computation and commun... | One possible application of AMO physics would be inertial guidance systems based on atomic interferometers--similar systems are currently being investigated for missile guidance and have shown much higher accuracy than other methods. Inertial navigation systems don't rely on a network of GPS satellites, which obviousl... | <ul>
<li>As @EntropicallyDriven mentions, matter-wave interferometry can be used for inertial navigation.</li>
<li>Better clocks (in terms of both performance and size / weight / power) would help with pulsar navigation.</li>
</ul>
| https://physics.stackexchange.com |
327,018 | [
"https://physics.stackexchange.com/questions/327018",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/153207/"
] | Consider any other variables unchanged (the surface of the road, how the car is driven, wind, etc.), only consider a weaker gravity. How will that affect fuel consumption?
| Based on what I have read in the past, and based on gathering actual data, a modern automobile with a reasonable drag coefficient (relatively streamlined) will experience approximately 50% rolling resistance and 50% air drag at normal highway speeds (approximately 60 mph or 95 kph). Since a major contributor to rollin... | The car would spend less fuel.
This is because , as you know, dynamic friction is proportional to the normal force the ground exerts on the car.
This normal force needs to mantain the car in the plane of the road, equilibrating its weight.
Because of this, normal force, and by extention fiction, are directly proport... | https://physics.stackexchange.com |
10,455 | [
"https://astronomy.stackexchange.com/questions/10455",
"https://astronomy.stackexchange.com",
"https://astronomy.stackexchange.com/users/6661/"
] | I was watching Venus with the naked eye yesterday at about 7 pm and I noticed that it was flickering, almost like a star.
I have always learned that planets don't flicker to the naked eye, only stars (indeed, every other time I've seen a planet it wasn't flickering), so I was rather confused. I even checked Stellarium... | Because planets actually do twinkle. Most people were told that the major difference between stars and planets is that only the former twinkle - but that's an oversimplification. Given the right conditions, planets will twinkle too, it just happens more rarely.
Several factors that contribute to it:
<ul>
<li>lots of ... | It's true that planets usually don't flicker, or twinkle (or scincillate, as astronomers like to call it). The reason is that they are close enough that they are actually seen as a disk with a larger diameter than the atmosphere can "wash out" (the <em>seeing disk</em>). Stars, on the other hand, are point sources, and... | https://astronomy.stackexchange.com |
593,734 | [
"https://physics.stackexchange.com/questions/593734",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/277926/"
] | In the derivation of Bernoulli's equation
<span class="math-container">$$W_{nc} = (P_1 - P_2)V $$</span>
<span class="math-container">$$W_{nc} = \Delta KE + \Delta PE $$</span>
<span class="math-container">$$(P_1 - P_2)V = 1/2mv_2^2 - 1/2mv_1^2 + mgh_2 - mgh_1 $$</span>
<span class="math-container">$$P_1V + 1/2mv_1^2 +... | It depends on the energies you are considering.
You're right in the "introductory mechanics" sense, energy is conserved when <span class="math-container">$\Delta E=\Delta K+\Delta U=0$</span> for a system.
However, in this case the work is being done by the force(s) associated with the pressure. So one can in... | I am going to explain the factors only intuitively withot any kind of mathematics.
According to Bernoulli's theorem the sum of total of these three kinds of energies <strong>kinetic energy</strong>, <strong>potential energy</strong> and <strong>pressure energy</strong> remains constant along a streamline in a steady fl... | https://physics.stackexchange.com |
1,715,246 | [
"https://math.stackexchange.com/questions/1715246",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/290994/"
] | Eight digit number is formed using all the digits: $1,1,2,2,3,3,4,5$. Find
<ol>
<li>Total numbers in which no two identical digits appear together.</li>
<li>Exactly two pair of identical digits occur together.</li>
</ol>
For first part, I thought of finding number of ways such that all three numbers are always togeth... | One way is to use Inclusion/Exclusion. You attempted a form of it, but something more elaborate is needed.
You know that there are $\frac{8!}{2!2!2!}$ possible sequences if we have no restriction.
Now let us count the number that have the two $1$'s together. Tie them together, and think of them as a superletter. Now ... | There are $3$ pairs and $2$ singles.
For the first part, the simplest way is to use inclusion-exclusion, viz.
<em>All ways - ways with at least $1$ pair together + at least $2$ pairs together - all $3$ pairs together</em>
= $\dfrac{8!}{2!2!2!} - \dbinom31\dfrac{7!}{2!2!} + \dbinom32\dfrac{6!}{2!} - 5! = 2220$
<hr>
... | https://math.stackexchange.com |
15,373 | [
"https://mathoverflow.net/questions/15373",
"https://mathoverflow.net",
"https://mathoverflow.net/users/3513/"
] | Suppose $K$ is a (not necessarily algebraically closed) field, and $G_1$ and $G_2$ are <em>split</em> semisimple algebraic groups over $K$ which become isomorphic over $\bar{K}$, the algebraic closure of $K$. Are $G_1$ and $G_2$ isomorphic over $K$? What about if the $G$s are reductive?
It seems like this should follo... | The answer is yes, for arbitrary split connected reductive groups over any field. The main point is that the Existence, Isomorphism, and Isogeny Theorems (relating split connected reductive groups and root data) are valid over any field. One reference is SGA3 near the end (which works over any base scheme), but in Ap... | Here is another reference which seems very readable: A. Borel, J. Tits, Groupes réductifs Publ. Math. IHES , 27 (1965) pp. 55–150, Theorem 2.13: Two reductive $K$-split groups $G$ and $G'$ which are isomorphic over $\bar{K}$ are already isomorphic over $K$.
| https://mathoverflow.net |
294,460 | [
"https://math.stackexchange.com/questions/294460",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/7289/"
] | How can I prove that for any sets $A,B,C,D$<br>
if
$$A \Delta B = C $$
and<br>
$$A \Delta D = C $$
then $B=D$.<br>
Where $\Delta$ denotes symmetric difference.
| First show that $\Delta$ is associative and has inverses; namely, every set is its own inverse. Then apply $A$ from the left to both equations and use what you proved in the first step. In doing this, it may help to think of the symmetric difference as XOR.
| The probabilist in me cannot resist mentioning that symmetric differences can (and maybe they should) be approached through <em>indicator functions modulo 2</em>.
Recall that, for every $A\subseteq E$, the indicator function $\mathbf 1_A$ is defined on $E$ by $\mathbf 1_A(x)=1$ if $x$ is in $A$ and $\mathbf 1_A(x)=0$... | https://math.stackexchange.com |
62,404 | [
"https://physics.stackexchange.com/questions/62404",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/23651/"
] | If you consider the damping force is friction like in:
<img src="https://i.stack.imgur.com/4JAFJ.jpg" alt="enter image description here">
then the force should be $$F=\mu N$$ where $\mu$ is the coefficient of kinetic friction. Why then is the damping force assumed to be linearly dependent on velocity?
| This question is actually one of the lab exercises I teach. For a spring-mass system, if the damping force is friction, then it is independent of velocity (verified experimentally). However, as mentioned in the comments, the damping force may not always be friction. For example, if the mass is a material like aluminium... | Actually, this is just a simple <em>model</em> for resistive forces (usually duo to viscosity) , and in many situations you can not assume this force to be linearly dependent on velocity.(for example ,usually this model is correct only for <em>small enough</em> objects)
In many <em>everyday</em> examples, this force i... | https://physics.stackexchange.com |
367,972 | [
"https://stats.stackexchange.com/questions/367972",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/178146/"
] | <em>This is meant to be a general question, aiming to clarify the topic for a beginner in TSA, as I haven't found any clear introductory explaination yet.</em>
Suppose I am working with some data which includes some time variable.
Then I want to know if this variable is significative and if it is worth to structure m... | As a preliminary matter, it is worth noting that "independence" is a very vague condition unless it comes with a clear specification of <em>what</em> is independent from <em>what</em>. Conceptually, independence is a much broader concept, whereas stationarity of a time-series is a particular condition on the series th... | To the best of my understanding, the concepts mean the following:
Testing for independence, would be useful to see whether there is any relationship between time and your variable of choice at all.
Looking a the definition of stationarity, it does not say anything about independence. The most important takeaway is t... | https://stats.stackexchange.com |
806,172 | [
"https://math.stackexchange.com/questions/806172",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/152825/"
] | I got this question in a exam.There were two more statements in the examination(but they were quite clearly wrong).However I got stuck between these two statements.The contrapositive of the the statement "I go to school if it does not rain" is
$(1)\qquad$If it rains, I do not go to school
$(2)\qquad$If I do not go to... | Note that (ii) and (iii) follow from $10<a<b$.
(i) and (iv) together give the inequality you listed as $r^2-r-1<0$.
We can easily solve this using the quadratic formula to give $\frac{1-\sqrt{5}}{2}<r<\frac{1+\sqrt{5}}{2}$.
Using the upper value, we have $b<10 \times (\frac{1+\sqrt{5}}{2})^2\approx... | The first of the inequalities at the end is the one you want. That gives a limit on $r$. Now try all the $a$ that satisfy $1 \lt r \lt $ this limit. There are not many possibilities-check if $b$ is a natural and you are done.
| https://math.stackexchange.com |
483,542 | [
"https://stats.stackexchange.com/questions/483542",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/290573/"
] | My answer is completely off. Can you please tell me where did my logic go wrong.
Donald Trump and Tori Black are to meet at a specific time and both will be late by <span class="math-container">$ \sim Exponential(\lambda), i.i.d. $</span>. What is the cdf of the arrival time difference.
Let <span class="math-container... | Your integral limits are not correct. If you draw the region of integration, it'll be in the first quadrant and to the right of the line <span class="math-container">$X-Y=z$</span>. It'll be easier to integrate if the order of integration is <span class="math-container">$dy dx$</span>. Otherwise, you'd need to calculat... | I will not answer the OP's question as to where his analysis for the case <span class="math-container">$z<0$</span> went wrong but instead point out an easier way of getting to the correct answer once the value of <span class="math-container">$F_Z(z)$</span> has been determined to be <span class="math-container">$1-... | https://stats.stackexchange.com |
60,257 | [
"https://mathoverflow.net/questions/60257",
"https://mathoverflow.net",
"https://mathoverflow.net/users/14068/"
] | Hey all,
I read the meta, and I realize this question might be pretty elementary for this site, but I'm having trouble computing this, and I know it won't take too much insight for someone to give me an approximation.
Say I have a 5x5x5 tic-tac-toe board (noughts and crosses), where each of the 125 spaces on the cube... | The set of boards which have any symmetry is microscopic compared with the total number of boards, so the number of boards up to symmetry is roughly $3^{125}/48 \approx 9.1\times 10^{57}$.
The probability that a random board has a particular $5$ in a row is $2/(3^5)$, and if I count correctly there are $94$ places to ... | There are a couple of approaches you can take.
One is to reduce things by a dimension: There are 233 possible tuples for a row of 5, instead of 243, if I read your constraints correctly. You can now loop through 3 iterations to determine an approximate number of 5x5 boards: for every 3 tuple of possible rows, comput... | https://mathoverflow.net |
97,039 | [
"https://electronics.stackexchange.com/questions/97039",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/7523/"
] | I'm trying to size wire for UL 508a panels. I have UL's wire gauge requirements, but those requirements are for continuous use. The device I'm designing will only run for two seconds, with minutes or hours between runs. Since currents of interest are 25, 50, 100, and 200 amps, there's a lot to be saved by not using wir... | If this question were in a physics exam, I would answer it as follows; whether this is a sensible idea in practice is entirely another matter. One would have to be pretty certain that no fault state could leave current flowing for more than two seconds.
We know from the specification of the wire the resistance per met... | The resistance of the wire has two primary effects. The first is that it causes a voltage drop at the load, and this is independent of the duty cycle. The second is that it causes the wire to heat up, which can cause it to fail.
In general, wiring should be conservatively rated in all applications, because you really ... | https://electronics.stackexchange.com |
26,548 | [
"https://softwareengineering.stackexchange.com/questions/26548",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/7974/"
] | If I release some code and binaries, but I don't include any license at all with it, what are the legal terms that apply by default (in the US, where I am). I know that I automatically have copyright without doing anything, but what restrictions are there on it?
If I upload my code to github and announce it as a free ... | Without a license, companies and individuals may be reluctant to use your code, because you don't grant them specific rights to do so.
Even when you put the code into the public domain, you are granting rights to use. So you might as well make a statement of acceptable use that is acceptable to you.
Without such a s... | The purpose of a copyright license is to give the licensee rights he would not otherwise have. Without a license, the only rights remaining are the ones guaranteed by copyright law, which are basically none.
Without a license, you aren't even allowed to download the code, since that implies making a copy, which is for... | https://softwareengineering.stackexchange.com |
64,019 | [
"https://mechanics.stackexchange.com/questions/64019",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/46081/"
] | My Renault Megane scenic 1.4 has a squeaky and chirp sound and it also drips power steering fluid under the car on the floor it's visible. The cambelt also have a 2cm Crack on the edge of the belt. I identified all these things that should be fixed but I am not really sure what the cause is of the squealing and chirp n... | In some manner, the 12v always-on power isn't always on.
Connect your voltmeter or test light to the always-on circuit, and leave it connected for the same length of time it takes for the head unit to lose memory. You'll probably find that he 12v always-on power goes away. Then you can search for the fault in the alw... | I had the same isue. Than it hit me. BMW utilizes a smart battery unit device on the positive terminal.
It's this device I think contain a relay off sum sort and gets timed out after set interval.
I just moved my Oterminal to behind it, and voila... Prob solved
Hope that helps guys
| https://mechanics.stackexchange.com |
399,711 | [
"https://softwareengineering.stackexchange.com/questions/399711",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/348779/"
] | <em>This question has been asked first on Stack Overflow, I've been told that this forum would be more appropriate</em>
I am wondering what's the recommended way to solve the following situation :
<ul>
<li>I have items, <em>Cars</em> which have a color and a car model property</li>
<li>When a customer choses a car ... | The answer to such a question is, it depends.
You need to ask yourself a series of questions:
<ol>
<li>How often are new colours added to the models?</li>
<li>Is the whole range of cars affected, or do some colours only exist for some models?</li>
<li>How easy is it to modify, retest and rerelease the software?</li>
... | Similarly to David, I would simply add a <code>colors_fallback</code> table.
This would be filled with mappings like <code>crimson -> darkred</code>.
Then check if there is an image with this car model/color, and if not, use the fallback color.
You can even do this recursively ;) If <code>model/darkred</code> isn... | https://softwareengineering.stackexchange.com |
439,897 | [
"https://electronics.stackexchange.com/questions/439897",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/222617/"
] | I would like to trip an input, similar to a button input to an microcontroller if the input voltage to the circuit/microcontroller is below a certain threshold. I have transistors and some zener diodes of different voltages but I am not sure how to connect them in order for the microcontroller to get an input when the ... | Thanks to those who posted suggestions, and particularly analogsystemsrf. The solution turned out to be a redesign of the driver board. Instead of a MOSFET switch, I went to a discrete bipolar design, basically because it turned out to be easier to achieve a controlled ramp up and ramp down of laser current. The curren... | Use a GROUND plane.
And make the enclosed area of M1, R1, D1 about 0.5 cm on a side. In other words, you should get the inductance, through which the switched-current flows, down down down to just a few nanoHenries.
Given Vspike = L * dI/dT,
with 100 nanoHenries inductance (4", 100mm, of wire NOT over a plane) and 0... | https://electronics.stackexchange.com |
1,891,691 | [
"https://math.stackexchange.com/questions/1891691",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/229328/"
] | Is $3x + 4x^2 + y$ a valid linear combination of $x$? I'm not sure if "a linear combination of $x$" allows for other terms than $x$ which would just be treated as constants, or it should strictly contain $x$.
| For the latter, you have $\dfrac{1}{a}\cdot d\left(\dfrac{u}{a}\right)= \dfrac{du}{a^2}$ while the former you have $\dfrac{du}{a} = d\left(\dfrac{u}{a}\right)$. The $\dfrac{1}{a}$ shows up to balance the $a^2$ at the denominator.
| I think the most intuitive way to think of this is in terms of dimensional analysis.
Since $u^2$ and $a^2$ are added, they must have the same dimensions, so $u$ and $a$ must as well. Say, for specificity, that $u$ and $a$ are both lengths.
Then the left integrand is dimensionless (as $du$ and $\sqrt{a^2-u^2}$ are bot... | https://math.stackexchange.com |
458,503 | [
"https://physics.stackexchange.com/questions/458503",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/208833/"
] | I am learning about the time translation invariance of the Hamiltonian. I read that
<blockquote>
the time translation invariance is already manifest in the fact that our
Hamiltonian is chosen an instantaneous function of time—we have assumed that the dynamics only
depends on position and velocity at the current... | Here is the setting. You have a system that is described by some generalized coordinates <span class="math-container">$q_i$</span> and some generalized momenta <span class="math-container">$p_i$</span> (in the Lagrangian formalism, you'd use <span class="math-container">$\dot{q_i}$</span> instead). Hamilton's equations... | If a function (of time) is invariant under time translation then it is constant. In other words the Hamiltonian does not depends explicitly on time.
The (admittedly a bit sibylline) paragraph that you cite wants to stress the fact that, from the get go, the Hamiltonian is assumed to be <em>time local</em> i.e., to be ... | https://physics.stackexchange.com |
1,037,804 | [
"https://math.stackexchange.com/questions/1037804",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/53265/"
] | I would like to show that $\Bbb{R}^2$ and $\Bbb{R}^2\setminus \{0\}$ are not homeomorphic without using Algebraic Topology. Is there an elementary way to do this?
| The answer is yes, but you must to add the hypothesis $A\not= \emptyset$, therefore exist an element $a\in A$. In the case $A=\emptyset$ is not true, take $\emptyset\times \{1,2,3\}=\emptyset\subset \emptyset\times \{1\}=\emptyset$ and $\{1,2,3\}\not \subset \{1\}$. Now take $A\not =\emptyset$. Take any element $b\in B... | Assume that $A \not=\emptyset$. Note that for any $y \in B$, Pick $x \in A$ such that $(x,y) \in A\times B$. Since $ A\times B \subset A \times C$, then $(x,y)\in A\times C$, and hence $y\in C$.
| https://math.stackexchange.com |
10,033 | [
"https://mathoverflow.net/questions/10033",
"https://mathoverflow.net",
"https://mathoverflow.net/users/2821/"
] | Is there a standard example of two abelian varieties $A$, $B$ over some number field $k$ which are $k_v$-isomorphic for every place $v$ of $k$ but not $k$-isomorphic ?
| (If you upvote this answer, please consider upvoting the answers by Felipe Voloch and David Speyer too, since this answer builds on their ideas.)
The smallest examples are in dimension $2$. Let $E$ be any elliptic curve over $\mathbf{Q}$ without complex multiplication, e.g., $X_0(11)$. We will construct two twists o... | If $A,B$ are as stated, then $B$ must be a twist of $A$ which is everywhere locally trivial, so $B$ gives a class in $H^1(k,G)$ (where $G$ is the automorphism group of $A$), which is everywhere locally trivial. So, pick a group $G$ that you know has everywhere locally trivial but globally non-trivial class in $H^1(k,G)... | https://mathoverflow.net |
300,701 | [
"https://softwareengineering.stackexchange.com/questions/300701",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/161954/"
] | Must all the controllers, models and views be placed in the private folders? If so, what are the roles of the public folder? What are the correct terminologies for such roles in computer science? How does the GUI relate with such concepts?
Otherwise, does the controllers and/or views have to be distributed across the ... | As long as all values used in the function are defined solely by its parameters, it's a pure function.
The facet that output is the same each time for the same input is controlled by whether the parameters are pure. If you assume the parameters (like a function argument) are also pure, then it is pure.
In a language ... | <blockquote>
Since the purity of an input parameter is an unknown until runtime, is a function immediately considered impure if it takes a function as an input parameter?
</blockquote>
Technically, yes, unless there is some way in your language to guarantee that the input function is also pure.
<blockquote>
if a ... | https://softwareengineering.stackexchange.com |
2,091,392 | [
"https://math.stackexchange.com/questions/2091392",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/392234/"
] | Prove that a group with more than one element contains an element of prime order.
| Choosing a specific subset $I$ and showing that there's no coordinate chart for $I$ is not enough. However, you are indeed very close with your idea.
If $S$ is a manifold, then by definition the point $p=(0,0)$ must have a neighborhood which is homeomorphic to $\Bbb{R}^n$. However, $\Bbb{R}^n$ is path connected, and a... | The topologist's sine curve $S$ is a subspace of $\mathbb{R}^2$ meaning that it is a subset of $\mathbb{R}^2$ and inherits its topology from the topology of $\mathbb{R}^2$. In order to be a manifold it must be locally Euclidean in the inherited topology. That means that it must also be locally connected but, as noted, ... | https://math.stackexchange.com |
85,490 | [
"https://mathoverflow.net/questions/85490",
"https://mathoverflow.net",
"https://mathoverflow.net/users/20528/"
] | Let $F:\mathbb{R}^2 \rightarrow \mathbb{R}$ be three times continuously differentiable in some open neighborhood $\mathcal{U}$ of $(0,0)$. Suppose that $F(0,0) = F_x(0,0) = F_y(0,0) = F_{xy}(0,0) = 0$ and that $F_{yy} \not = 0$ and $F_{xx}(0,0)/F_{yy}(0,0) < 0$.
Obviously I cannot apply the implicit function theore... | This is a job for the Morse lemma. The second degree Taylor polynomial of $F(x,y)$ has the form $ax^2+by^2$ where $ab<0$. (You said $>0$ but that can't be what you meant.) The Morse Lemma says, for a sufficiently smooth function of several variables, that if it has zero constant and linear parts and a nondegenera... | Scketch of an elementary proof. Assume <span class="math-container">$F\in C^2(\mathbb{R}^2, \mathbb{R})$</span> and
(changing sign to <span class="math-container">$F$</span> if needed)
<span class="math-container">$$F(0,0)=F_x(0,0)=F_y(0,0)=F_{xy}(0,0)=0\, ,$$</span>
<span class="math-container">$$F_{xx}(0,0) <0\, ,... | https://mathoverflow.net |
222,548 | [
"https://softwareengineering.stackexchange.com/questions/222548",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/109856/"
] | Aren't both just a set of instructions that get executed by some interpreter? (contrary to a full-fledged program, which runs (gets executed) at some computer system?
| Basically the difference is just one of degree. A script tends to be a lot more formally defined, with a well-defined scripting language behind it, while a macro is usually something more simple and ad-hoc. Macros are frequently "recorded" by telling a program "watch everything that I do" and then performing the acti... | Macros come in different flavors.
The original kind was macros in assembly language, where a group of instructions could be named and each time the name was used the instructions were substituted for the name.
C and C++ also have macros that work this way.
But as with most things in IT, why have one term or one m... | https://softwareengineering.stackexchange.com |
310,264 | [
"https://softwareengineering.stackexchange.com/questions/310264",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/13030/"
] | When you sign a SOAP message with a private key, how does the server know your public key in order to verify it?
I am connecting to a Datapower instance that sits in front of our actual Java web service. I keep getting signature not valid errors from the server, so it leads me to believe it has something to do with ke... | I would think about it this way: how is it determined which Operation causes which Trigger to be triggered. It has to be an algorithm which can change over time, or evolve into multiple algorithms. Putting it in either Trigger or Operation classes imply that those classes will be able to handle such scenarios in the fu... | Been there, done that.
<strong>Option #3.</strong>
I don't know what language you will be using but I will use a pseudo-code that is very simillar to Java. If your language is C# you probably have similar interfaces and structures.
Have a Mapping class or interface:
<pre class="lang-java prettyprint-override"><code... | https://softwareengineering.stackexchange.com |
115,948 | [
"https://softwareengineering.stackexchange.com/questions/115948",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/16851/"
] | I started using Github now that I'm working on a project with some guys. And I started to wonder if I should use it in my personal projects too. I'm not sure if this will help me in some way or if it is unnecesary?
| There's a distinct difference between using GitHub and using {your favorite version control software}.
If you're working on a project, it should be under some kind of version control somewhere, preferably off your local machine, so it's stored in two places should something happen to one machine. Putting your code on ... | I use BitBucket for my personal projects but any other online source control can work for you.
This way I'm sure that I always have a backup of my projects. Plus, <strong>it's free</strong> so why wouldn't you use some form of Source Control?
| https://softwareengineering.stackexchange.com |
2,850,011 | [
"https://math.stackexchange.com/questions/2850011",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | I want to compute this integral
$$I=\int_{0}^{1}\ln x\ln\left(\frac{\ln x}{\ln x-1}\right)\mathrm dx$$
Applying integration by parts
$u=\ln\left(\frac{\ln x}{\ln x-1}\right)$
$\mathrm du=\frac{\ln x-1}{\ln x}\left(\frac{1}{x(\ln x-1)}-\frac{\ln x}{x(\ln x-1)^2}\right)dx$
$\mathrm dv=\ln x \mathrm dx$
$v=x\ln x-x ... | Your last line is not quite right. It is true that
$$ \int \log x \log\left(\frac{\log x}{\log x - 1}\right) \, dx
= x(\log x - 1)\log\left(\frac{\log x}{\log x - 1}\right) + \int \frac{dx}{\log x} $$
on $(0, 1)$. But you cannot simply plug $x = 0$ or $x = 1$ since the RHS has singularity at those points. Thankfully... | Make the change of variable $x\mapsto e^{-x}$ and integrate by parts two times to arrive at
$$I=\int_{0}^{1}\ln x\ln\left(\frac{\ln x}{\ln x-1}\right)\mathrm dx=-\int_0^{\infty} \log(x) e^{-x}\mathrm dx=\gamma.$$
| https://math.stackexchange.com |
337,419 | [
"https://mathoverflow.net/questions/337419",
"https://mathoverflow.net",
"https://mathoverflow.net/users/124654/"
] | Recently, I formulated the following conjecture which seems novel.
<strong>Conjecture</strong>. For any positive odd integer <span class="math-container">$n$</span>, we have the identity
<span class="math-container">$$\sum_{j,k=0}^{n-1}\frac1{\cos 2\pi j/n+\cos 2\pi k/n}=\frac{n^2}2.\tag{1}$$</span>
Using Galois theo... | The following argument is very short, but bit tricky, so I remain it along with the previous answer.
Actually this sum quickly factorizes: using <span class="math-container">$\cos (x+y)+\cos (x-y)=2\cos x\cos y$</span> and denoting <span class="math-container">$(j+k)\cdot \frac{n+1}2=a,(j-k)\cdot \frac{n+1}2=b$</span>... | Let <span class="math-container">$T_n$</span> be the <span class="math-container">$n$</span>-th Chebyshev polynomial, so that
<span class="math-container">$$T_n(x)-1=\prod_{j=1}^n(x-\cos 2\pi j/n).$$</span>
Taking a logarithmic derivative we have
<span class="math-container">$$\sum_{j=0}^{n-1}\frac{1}{x-\cos 2\pi j/n}=... | https://mathoverflow.net |
421,720 | [
"https://softwareengineering.stackexchange.com/questions/421720",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/384259/"
] | I read/watch about Uncle Bob's clean code and when he talks about exceptions he suggested to have a well named exception over a description which make perfect sense for the programmer, for decoupling the main application form the business logic and for the tests.
But what about the user? I have an application that take... | <blockquote>
<ol start="2">
<li>Mega switch in the catch</li>
</ol>
</blockquote>
You lost me at "mega switch". Mega switches are common bad practice. They're textbook OCP violations, on top of generating near monolithic code, and often indicative of a design that's not promoting reusability to the degree tha... | Why do Exceptions have <em>Names</em>/Types?<br />
Because we have code constructs to <em>catch</em> Exceptions <em>based on</em> their Type.
Why do Exceptions have <em>Custom Properties</em>?<br />
So that the code <em>catching</em> the Exception has all the information it needs to <em>handle</em> the Exception delive... | https://softwareengineering.stackexchange.com |
295,088 | [
"https://mathoverflow.net/questions/295088",
"https://mathoverflow.net",
"https://mathoverflow.net/users/88385/"
] | Let $S$ be a irreducible scheme over a field $k$ (for example a smooth projective curve over algebraically closed field). Denote by $k(S)$ its field of fractions. Let $K$ be a(n algebraically closed) field. Given an embedding $k(S) \hookrightarrow K$ we can construct a $K$-point of $S\otimes K$ in the following way: ch... | So I am at the airport with some time to kill, so I will answer this question when $S$ is the spectrum of $k[t]$, i.e., is the affine line. The base change is the spectrum of $K[t]$. A $K$-point of this is given by an element of $K$, namely the image of $t$ under the corresponding $K$-algebra map. Now I leave it up to ... | Let $\eta \to S$ be the generic point of $S$. The function field of $S$ is exactly the residue field of $\eta$. Let $k(S) \subset K$ be a field extension (no need to assume that $K$ is algebraically closed).
The map $\mathrm{Spec}\, K \to S$ factors as $\mathrm{Spec}\, K \to\mathrm{Spec}\, k(S) \to S$. The image of $... | https://mathoverflow.net |
1,376 | [
"https://softwareengineering.stackexchange.com/questions/1376",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/71/"
] | I have a tester that while testing will have an error occur (ok so far), but then he frequently reports it right away. We (the developers) then later find that the tester has not tried to reproduce the issue and (when asked) cannot find a way to make it happen again.
Now these are still bugs, I don't want to ignore t... | A bug without context is not a bug, it's a fluke. The problem could be your code, it could be a third party library, it could be the hardware, or it could be solar radiation causing a single bit to flip on it's own. If you can't reproduce it with at least <em>some</em> regularity (even if only "it happens once every 10... | Ultimately if neither the developer nor the tester can reproduce the bug it should be closed but marked as such.
However, how long it takes you to get to that point is debatable.
Some people would argue that if it's not immediately reproducible then it should be closed forthwith.
I usually strive to try to get more ... | https://softwareengineering.stackexchange.com |
110,003 | [
"https://electronics.stackexchange.com/questions/110003",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/40024/"
] | Assuming that you are focusing on the choice that will give the best protection against spikes, would you go for an AC or a DC powered piece of electronics? Could you explain why and if this is your choice only in some given intervals in input (like only in X to Y Volts from a maximum of Z volts of tolerance)?
For exa... | I assume you're talking about how to physically write to the program memory, and not how to write programs using an assembler or compiler for a given micro.
Often this information is <strong>not</strong> in the datasheet or in the data that covers the instruction set, but in a separate document.
In some cases, unfo... | If the manufacturer does not provide programming documentation, but you have a working programmer, then you may simply have to reverse-engineer the programming protocol.
The first steps I would take are: determine if a special programming voltage must be present on some pin (like PICs and AVRs in some programming mode... | https://electronics.stackexchange.com |
18,259 | [
"https://mechanics.stackexchange.com/questions/18259",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/11074/"
] | When I was learning to drive, I took my lessons in a manual car. On one of the lessons I was performing a reversing maneuver and I controlled the car on both the clutch and the brake pedal... which I now understand was stupid...
The instructor explained to me that controlling the car with the clutch and brake pedals a... | Your instructor properly objected because the clutch/engine and the brakes do opposite things: the former adds energy to the car (speeding it up) and the latter removes energy (slowing it down). If you use both at the same time, then you're just pumping energy from the engine into the brake pads, to no good end. If at ... | The torque converter in a "standard" automatic transmission does the job of a clutch, and it doesn't really wear out in the same way as a clutch does - a clutch is two dry pieces of material holding each other through friction (kind of like 2 sheets of sandpaper pressed against each other), and each time the clutch is ... | https://mechanics.stackexchange.com |
145,741 | [
"https://mathoverflow.net/questions/145741",
"https://mathoverflow.net",
"https://mathoverflow.net/users/12996/"
] | (This is a reformulation of an earlier unanswered question. I would like to thank Ian Agol for pointing out to me Walter Parry's characterization of hyperbolic translation length functions.)
Let $G$ be a finitely generated group and $T_1$ and $T_2$ two minimal (and hence cocompact) metric simplicial trees that $G$ act... | Here is a counterexample (unfortunately):
We give the two graphs of groups the same metrics. Equivariantly folding the two branching edges in the Bass-Serre covering $T_1$ gives rise to an equivariant 1-Lipschitz map from $T_1$ to $T_2$ and hence we have $l_{T_2}(g)\leq l_{T_1}(g)$ for all $g\in G$. Now observe that t... | Suppose $S$ is a metric $G$-tree of covolume $1$ (ie the sum of the edge lengths in $G\backslash S$ is equal to 1). Now suppose that $T$ is obtained from folding together two adjacent edges in the same $G$-orbit, as follows:
Let $e$ be an edge of $S$ adjacent to a vertex $v$, and suppose that $g\in G$ stabilizes $v$ ... | https://mathoverflow.net |
63,214 | [
"https://dba.stackexchange.com/questions/63214",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/37124/"
] | I looked on the internet and I found your post on the stackexchange.<br>
I will try to explain in more detail what we really need<br>
I do not know well to write English<br>
It is a system for airporttaxi
<pre><code>id date time direction
7 2014-04-15 21:35:00 to
8 2014-04-15 15:20:00 f... | <h1>THIS IS THE QUERY YOU NEED</h1>
<pre><code>SET @GivenDate = '2014-04-15';
SELECT CONCAT(@GivenDate,' ',MAX(time)) INTO @LastDateTime
FROM airport_taxi WHERE date=@GivenDate;
SET @nexttime = 0;
SELECT
TIME(FROM_UNIXTIME(StartTime)) "start",
TIME(FROM_UNIXTIME(EndTime)) "end",
FLOOR((EndTime - StartTime)... | Uuse this to get duration between two times:
<pre><code>select startTime, duration, time, TIME_TO_SEC(TIMEDIFF(time,startTime)) as diff
from idling
limit 25;
select startTime, duration DIV 60 as duration, time, TIMESTAMPDIFF(MINUTE,startTime,time) as diff
from idling
limit 25;
</code></pre>
| https://dba.stackexchange.com |
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