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108,733 | [
"https://physics.stackexchange.com/questions/108733",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/41110/"
] | Can someone give a qualitative description on why the density of states for a two dimensional free electron gas is independent of energy while it is not in one and three dimensions? In one dimension it goes as $E^{-1/2}$ and in three $E^{1/2}$.
| <blockquote>
TylerHG: Yes it is easy to calculate the density of states. But what I'm really asking here is "why."
</blockquote>
Note that a thin circular ring in $\mathbf{k}$-space of thickness $dk$ has area $dA=2\pi k\,dk$ (by elementary geometry). In $E$-space, since $E\propto k^2$, that ring corresponds to a pat... | It is easy to show that the total number of electrons in a 3D fermi sphere is :
$$N(e)=\frac{V}{3\pi^2}*k_F^3$$
Where $k_F$ is the Fermi wave vector and $V$ is the real space volume of your sphere.
Now if you rearrange for $k_F$ in terms of the total number of electrons you'll get a particular equation.
It is know... | https://physics.stackexchange.com |
2,982,884 | [
"https://math.stackexchange.com/questions/2982884",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/40318/"
] | When applying L'Hospital's Rule to
<span class="math-container">$$\lim_{x \to \infty} (x - \ln x)$$</span>
I would have thought the answer to be
<span class="math-container">$$\lim_{x \to \infty} \left(1 - \frac{1}{x} \right) = 1$$</span>
But the answers I am seeing are shown as
<span class="math-container">\beg... | <strong>HINT</strong>
You can’t apply l’Hopital to the original limit which is in the form <span class="math-container">$\infty-\infty$</span>, use that
<span class="math-container">$$x-\ln x=x\left(1-\frac{\ln x}{x}\right)$$</span>
and apply l’Hopital to <span class="math-container">$$\lim_{x \to \infty}\frac{\ln ... | If the L'Hopital formalism <span class="math-container">$\lim_{x\to c}{f(x)\over g(x)}=\lim_{x\to c}{f'(x)\over g'(x)}$</span> when <span class="math-container">${f(x)\over g(x)}\to{\infty\over\infty}$</span> generalized to <span class="math-container">$\lim_{x\to c}(f(x)-g(x))=\lim_{x\to c}(f'(x)-g'(x))$</span> when <... | https://math.stackexchange.com |
194,858 | [
"https://dba.stackexchange.com/questions/194858",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/-1/"
] | In a Sql note "you should never put applications in each other's way if you do not have to.. Applications sharing memory, CPUs, internal tempdb can cause an issue. One application submitting a runaway query can cause server to go to single halt." (SQL Server 2012 Unleashed) (Rankins)
Our company has 5 different appli... | Unlimited everything?
Sure, buy five different physical boxes at $50,000 each hardware alone, use Core-based Enterprise licensing with SA, patch them all, administer them all, and pay annually for the licensing.
Realistically? In a way you can justify to accounting?
Use a physical box either directly running Window... | Separate SQL Server instances trivially allow distinct tempdbs, and allow limitations on the amount of server memory each instance is allowed. It may also be possible to restrict instances to specific CPUs. However, by the time you do that, the advantages of sharing a single server would tend to evaporate. If you wante... | https://dba.stackexchange.com |
189,378 | [
"https://physics.stackexchange.com/questions/189378",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/74685/"
] | Professor Susskind proved conservation of energy in one of his lectures by taking the classical definition of force ($F = ma$) and by showing that its time derivative is zero. How can we do that with the modern, relativistic definition of force ($F = dP/dt$)? Can we do it the same way, or do we need to use a different ... | <strong>SECTION A :</strong> <em>Non-relativistic conservation of energy</em>
The work done by the non-relativistic force $\:\mathbf{f}\:$ per time unit, that is the power produced or consumed, on a particle moving with velocity $\:\mathbf{v}=d\mathbf{r}/dt\:$ is
\begin{align}
\dfrac{dW}{dt}=\mathbf{f}\circ \mathb... | I will try to give you here the proof of work-energy theorem.
We have that $\bar p = {m \bar u \over \sqrt{1- {u^2 \over c^2} } } $ the relativistic momentum. We define the force as $\bar F ={dp \over dt} $. So the work is: $W= \int \bar F \cdot d \bar l =\int {d \bar p \over dt} \cdot d \bar l = \int {d \bar p \over... | https://physics.stackexchange.com |
31,998 | [
"https://security.stackexchange.com/questions/31998",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/6004/"
] | I am being asked to perform a forensics investigation on a network.
The reason being that for the past few months strange things are happening on the network. VLANs are being deleted, configuration changes are being made, and all sorts of modifications are occurring on the network devices. IT ranges from the Firewalls,... | OK. I promise I'm not trying to be mean about this.
To paraphrase the question: <em>how do I go about performing forensics with no prior experience?</em>
<strong>You don't.</strong> Seriously. There are dummies books available that will touch on the basics and there are a number of websites that will walk you throug... | A forensic investigation involves preserving data for future law enforcement, it doesn't sound like you are being asked to do that, rather it sounds like you are being asked to investigate and remediate security issues on your network that are causing problems. That's just an plain old review. If they do expect you to ... | https://security.stackexchange.com |
62,500 | [
"https://security.stackexchange.com/questions/62500",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/29409/"
] | I am needing to run scans from one computer/laptop/device to another computer in a protected system. I am worried that plugging in my scanning computer to the protected computer might cause some risk in that a virus may be transferred.
To reduce the risk I wish to use incompatible architectures, specifically I wish ... | It might, different architectures may stop binaries compiled for one architecture being executed on another. On the other hand, if the virus does not depend heavily on the host architecture (eg. it uses interpreted code such as shell scripts) or is designed to work across multiple architectures then it may not help.
A... | It is theoretically possible to write multi-architecture binary malware. In practice, it's very difficult, belonging to the realm of proofs of concept rather than actual malware in the wild.
Script-based malware is a very different story. A well-written shell script, for example, could infect any Unix-like operating... | https://security.stackexchange.com |
389,712 | [
"https://mathoverflow.net/questions/389712",
"https://mathoverflow.net",
"https://mathoverflow.net/users/145920/"
] | Let <span class="math-container">$A$</span> be an Artin algebra, <span class="math-container">$\text{mod}\,A$</span> the category of finitely generated <span class="math-container">$A$</span>-modules and <span class="math-container">$\text{Ab}$</span> the category of abelian groups. Is every additive, covariant, left-e... | One can prove the following result: let <span class="math-container">$F \colon \mathrm{Mod}_R \to \mathrm{Mod}_S$</span> be left-exact and preserve small products (equivalently, a continuous functor). Then <span class="math-container">$F$</span> is of the form <span class="math-container">$\mathrm{Hom}(M,-)$</span>.
Le... | Here is a (stupid in some sense) counterexample.
Suppose <span class="math-container">$A$</span> is not trivial and <span class="math-container">$\kappa$</span> is a cardinal greater than the cardinality of <span class="math-container">$\textrm{Hom}_A (X, M)$</span> for all f.g. <span class="math-container">$A$</span>-... | https://mathoverflow.net |
153,026 | [
"https://physics.stackexchange.com/questions/153026",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/40214/"
] | In a world without air, I understand they definitely would. However, with drag taken into account, I think they wouldn't. Since the drag force varies proportional to the square of speed (ignoring the change in coefficient of drag with speed), wouldn't the total impulse due to drag on a bullet fired from a gun from a sp... | Just based on the quadratic drag of air, yes, the fired bullet would take longer to hit the ground.
Just consider the vertical force caused by the air friction:
$F_y = - F_{\rm drag} \sin \theta = - C (v_x^2 + v_y^2) \frac{v_y}{\sqrt{v_x^2 + v_y^2}} = - C v_y \sqrt{v_x^2 + v_y^2}$
Where $\theta$ is the angle above ... | I don't deal with drag forces often, but I think the equation for drag is
$$F_D=Cv^2,$$
where $F_D$ is in the same direction as $v$, and $C$ contains all the various things – density of air, cross-section, drag coefficient, etc. Importantly, $C$ <strong>depends on the orientation of the object</strong>. What I ... | https://physics.stackexchange.com |
540,946 | [
"https://electronics.stackexchange.com/questions/540946",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/266913/"
] | I am designing a board for my Bachelors thesis. The board takes 12V - 24V as an input. The dimensions are 100mmx100mm. It will be a 4 layer board and has an onboard antenna, multiple sensors and a Raspberry Pi compute module 4 as well. I am thinking of reserving 1 layer as a power rail and 1 layer for GND. My question ... | A single regulator for 5 volts and a single regulator for 3.3 volts is all you'll need by the sounds of it. Use your power layer to route 5 volt and 3.3 volt supplies but, try and place those regulators reasonably close to where they are needed. Don't forget to calculate heat dissipation of these regulators if linear t... | Just a single 5V and 3V3 regulator, as Andy suggested. 5V is often better as a switcher unless current is very low, because a linear 5V regulator will have 19V across it, which tends to make it get hot. Decoupling caps are key - look at the data sheets for your chips. More is better, you can always leave a few unfitted... | https://electronics.stackexchange.com |
163,766 | [
"https://dba.stackexchange.com/questions/163766",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/95236/"
] | I am receiving the Log shipping Primary Server Alert warning in error
In <code>msdb.dbo.log_shipping_monitor_primary</code> the last backup time is 1215 however on that machine there are .trn files that have been created since
In <code>msdb.dbo.log_shipping_monitor_secondary</code> the destination is updating as I'd ... | You are correct the last backup time is showing <code>2017-02-09 12:15:02.010</code> while last copied and restored are much greater <code>2017-02-09 16:32:02.397</code> and <code>2017-02-09 16:34:11.477</code>. So I believe somehow the catalog is not getting updated. There can be two reasons:
<ol>
<li>This can be a b... | This might be a permission issue. I am assuming that the monitor server is not on the primary and the issue could be that the primary server is not able to update the values in the monitor server.
Can you check where your monitor server is and make sure primary server service account has permission to update the mo... | https://dba.stackexchange.com |
89,057 | [
"https://security.stackexchange.com/questions/89057",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/69998/"
] | Suppose I have a protocol like the one below between users A and B;
-A sends random nonce G1 to B
-B sends back hash_k(G1) and random none G2
-A verifies hash_k(G1), then sends back hash_k(G1 | G2)
-B verifies and a connection is made
Would this protocol be safe from eavesdroppers and other attacks like man in ... | <strong>Your protocol is not safe by any means!</strong>
Example for MitM:
<ul>
<li>A sends random nonce G1 to B</li>
<li>C intercepts and sends nonce G1c to B</li>
<li>B sends back hash_k(G1c) and random nonce G2</li>
<li>C intercepts and sends hash_k(G1) and nonce G2c to A</li>
<li>A verifies hash_k(G1) (is OK), th... | No. Stick to known protocols such as TLS, Kerberos, SSH & IPSec for key exchanges.
Try researching Diffie-Hellman key exchanges and ECDH (Elliptical Curve Diffie-Hellman, the new method of key exchanges like your example).
| https://security.stackexchange.com |
104,463 | [
"https://dba.stackexchange.com/questions/104463",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/67028/"
] | I wish to keep local variable from stored procedure even if rollback occurres.
In following procedure, i "register" numbers to users. The procedure takes user_id, value_from and value_to as arguments. When I call the procedure with user_id = 1, value_from = 10, value_to = 20, it registered numbers 10, 11, ..., 19, 20 ... | It's not because of <code>ROLLBACK</code>. Value is reset because error is thrown later during <code>INSERT</code>. Handling the error inside procedure body (for instance, with <code>DECLARE CONTINUE HANDLER FOR SQLEXCEPTION</code> ) will give you what you want. Alternatively, in your case you can just <code>LEAVE</co... | Plan A: Put it in a MEMORY or MyISAM table.
Plan B: Have two connections; use the second to store the value you don't want rolled back.
| https://dba.stackexchange.com |
145,005 | [
"https://mathoverflow.net/questions/145005",
"https://mathoverflow.net",
"https://mathoverflow.net/users/41401/"
] | I am looking for a proof (or a reference to a proof) of the following theorem:
Let $X$ be a compact metric space with metric $d$, endow $X$ with the Borel $\sigma$-algebra and a probability measure $\mu$. Let $T\colon X\to X$ be a continuous map which is $\mu$-preserving. Then for $\mu$-almost every $x\in X$ there is ... | Let $\mathcal P_1,\mathcal P_2,\ldots$ be a sequence of refining partitions of decreasing diameter (converging to 0) of $X$. Let $\delta_n$ be the diameter of $\mathcal P_n$. Let $\mathcal P_n(x)$ be the element of $\mathcal P_n$ containing $x$.
For $\mu$-a.e. $x$, $\mu(\mathcal P_n(x))\ne 0$.
Now the Poincaré recurr... | This is part of the statement of Proposition 4.1.18 in <em>Introduction to the Modern Theory of Dynamical Systems</em> by Katok and Hasselblatt. As far as I am aware there is essentially only one proof, which is that given by Anthony. It generalises to the following result which I have found useful once or twice but wh... | https://mathoverflow.net |
246,339 | [
"https://physics.stackexchange.com/questions/246339",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/107112/"
] | I know that electric current is the flow of electrons but electrons have a very slow drift speed (about 2mm/s). How is it that electric current reaches its destination at almost the speed of light?
| The speed at which the signal in a circuit travels is not the speed of drifting free electrons but rather the speed of electromagnetic waves whose velocity factor is usually $0.5$ to $0.99$(50%-99% $c$).
Therefore, the speed of electric signal is the speed of the propagation of the electric field waves of the conducto... | Drift speed is the average speed of the average electron. Electric current reaches the destination because the wires that carry the electrons are already filled with loose electrons (because the wires are metals) so the electrons do not actually travel from source to destination but rather provide a push to the ones at... | https://physics.stackexchange.com |
897,113 | [
"https://math.stackexchange.com/questions/897113",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/166713/"
] | Does there exist a non trivial function $f:\mathbb{R}\to\mathbb{R}$ such that $f^{-1}(x)=\frac{1}{f(x)}$ ?
| At least the functional equation can't hold for every $x \in \mathbb{R}$. Note that plugging in $x = 0$ implies
$$
f^{-1}(0) = \frac{1}{f(0)} \Rightarrow 0 = f\left(\frac{1}{f(0)}\right)
$$
so in particular $f$ attains value $0$ for some $c \in \mathbb{R}$. But now
$$
f^{-1}(c) = \frac{1}{f(c)} = \frac{1}{0},
$$
so the... | Suppose $f$ continuous. Then $f$, being invertible, is increasing or decreasing. If it is increasing, $f^{-1}(x)$ is increasing, while $[f(x)]^{-1}$ is decreasing. If it is decreasing, $f^{-1}(x)$ is decreasing, while $[f(x)]^{-1}$ is increasing. So there exists no solution among continuous functions. In fact, any solu... | https://math.stackexchange.com |
111,232 | [
"https://mathoverflow.net/questions/111232",
"https://mathoverflow.net",
"https://mathoverflow.net/users/9970/"
] | Let $P\rightarrow M$ be a principal (right) $G$-bundle, where $G$ is a Lie group. Given a finite-dimensional representation of $G$, $V$ say, we can define the associated bundle $P\times_{G}V\rightarrow M$. This is a vector bundle over $M$ defined as the quotient of the (free, right) action of $G$ on $P\times V$ - $(p... | Yes, there is such a universal coefficient theorem.
$$0 \to Ext(H^{q+1}(X,R), G) \to H_q(X, G) \to Hom(H^q(X, R), G) \to 0$$
see Theorem 6.5.12 in Spanier's textbook "Algebraic Topology". It's on page 248.
| The proof of Spanier's 6.5.12 starts from a free chain complex with homology of finite type and replaces it by a quasi-isomorphic free chain complex of finite type. Then the conclusion reduces directly to application of the usual universal coefficient theorem for computing cohomology of chain complexes from homology. ... | https://mathoverflow.net |
427,256 | [
"https://math.stackexchange.com/questions/427256",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/81552/"
] | This is a homework problem, but I don't want the answer, just a little guidance:
<blockquote>
Prove that the additive inverse of an odd integer is an odd integer.
</blockquote>
When approaching a problem like this, how much is it safe to assume? Is it safe to assume that "the additive inverse of an integer is an i... | <blockquote>
An integer $n$ is odd if and only if there exists an integer $k$ such that $n = 2k+1$. (Note, $2$ does not divide $n = 2k + 1$: $2$ divides $2k$, but not $1$, hence $n = 2k + 1$ is not even, therefore is odd).
</blockquote>
So let $n$ be an arbitrary odd integer; i.e. $n = 2k+1$ where $k$ is some intege... | The sum of an odd and an even integer is odd, since $(2m + 1) + 2n = 2(m + n) + 1$.
Let $x$ be any odd integer. Then $-x$ is odd, since otherwise, $x + (-x) = 0$ is odd.
| https://math.stackexchange.com |
61,810 | [
"https://physics.stackexchange.com/questions/61810",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/-1/"
] | I know perfectly well how to derive the magnitude of the electric field near a conductor,
$$E = \frac{\sigma}{\varepsilon_0}$$ and near a sheet of charge, $$ E = \frac{\sigma}{2\varepsilon_0} .$$
In fact, I can explain with clarity each step of the derivation and I understand why is one two times larger than the othe... | <strong>(original poster here)</strong>
Thank you all for posting your answers! You all helped me to develop an intuition which I think illustrated what the problem really is, so I think this qualifies as an answer, although it's my own.
The problem with my intuition was that I viewed the conducting surface in the <e... | The answer is simple. It is all in the definition of sigma. In the case of a non-conducting sheet sigma means entire charge in a given area of the sheet meaning both surfaces and everything between them. In the conducting case it is just easier to think of sigma as being the charge on one surface not the sum of both as... | https://physics.stackexchange.com |
71,623 | [
"https://softwareengineering.stackexchange.com/questions/71623",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/3214/"
] | Firstly, a disclaimer. This question is not because I'm a disgruntled employee planning to hide some malicious code which I can later blackmail my employer with. I actually quite like the people I work with - just simply curious.
So if an old employer needed a password for some resource that only I had. Would I be obl... | If they are a previous employer, meaning the work you did for them belongs to them, then <strong>yes</strong>, you should give it to them. In fact, you might be legally obliged to. Ideally you should have given them everything before you left. Even more ideally, before you left you should have given them everything ... | When I was let go from a recent place of employment, I took the approach of telling them straight up which systems I had access to, and demanded they deny me access immediately. I had them change passwords, told them locations of all secret keys, deactivate my door entry codes, etc, and preemptively prevented the probl... | https://softwareengineering.stackexchange.com |
81,243 | [
"https://physics.stackexchange.com/questions/81243",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/31255/"
] | I'm originally from stack overflow, so this is my first foray into this particular community.
I've read in numerous places that the Large Hadron Collider is capable of accelerating protons at 0.999999991 <em>c</em>, which mathematically works out to being 3 metres per second slower than the speed of light. That seems ... | Reaching the speed of light requires infinite energy.
Let a proton of mass $m_p$ have a velocity $v$. Then by the energy-mass equivalence:
$$E = \frac{m_pc^2}{\sqrt{1 - \frac{v^2}{c^2}}}$$
Which goes to infinity as $v$ approaches $c$. Since you can't supply infinite energy to the proton, reaching $c$ is impossible.
... | <blockquote>
What is preventing the LHC from achieving 1 c, in terms of how fast it
can propel these protons?
</blockquote>
The nature of the spacetime within which we exist.
There are several ways to look at this and here's just one. According to Special Relativity, the momentum of a proton with velocity $\vec ... | https://physics.stackexchange.com |
498,591 | [
"https://electronics.stackexchange.com/questions/498591",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/197433/"
] | I am a beginner in microprocessors. Apologies if my question is too naive.
The memory section of the <strong>8086</strong> processor is divided into two segments: even and odd to allow the CPU to fetch <strong>16</strong> bits in <strong>one clock cycle</strong>. When a <em>16</em> bit word is to be read from memory a... | Word-wide memory is addressed with a single address bus. When a word is read from an odd address, one byte is in one word, the other byte in another. You cannot address both bytes at the same time. Even if the memory had a separate address bus for each byte, the processor does not have the pins to address both address ... | The book is slightly wrong, but let's try to clear it up. The thing is, you <em>can</em> put 125 on the address bus. But you <em>cannot</em> read addresses 125 and 126 in a single clock by doing so. Basically this is because the numbers 125 and 126 differ in bits other than the last bit.
The 8086 has 20 address lines (... | https://electronics.stackexchange.com |
332,305 | [
"https://mathoverflow.net/questions/332305",
"https://mathoverflow.net",
"https://mathoverflow.net/users/141013/"
] | I have wondered for a while if there are any interesting rational solutions to <span class="math-container">$a^b = b^a$</span>. I have tried but cannot find any solutions other than <span class="math-container">$a=2$</span> and <span class="math-container">$b=4$</span>, or vice versa.
Thank you in advance.
| There is an infinite number of rational solutions
<span class="math-container">$$a=\left(\frac{n+1}{n}\right)^n,\;\;b=\left(\frac{n+1}{n}\right)^{n+1},\;\;n\in\mathbb{Z},\;\;0\neq n\neq -1.$$</span>
For a proof that these are <em>all</em> the rational solutions of <span class="math-container">$a^b=b^a$</span> with <sp... | There are infinitely many solutions.
For example, take any positive integer <span class="math-container">$n$</span> and set <span class="math-container">$c=(n+1)/n, a=c^n, b=c^{n+1}$</span>.
Then
<span class="math-container">$$
a^b=c^{nb}=c^{nc^{n+1}}
$$</span>
and
<span class="math-container">$$
b^a=c^{(n+1)a}=c^{(n... | https://mathoverflow.net |
51,989 | [
"https://mathoverflow.net/questions/51989",
"https://mathoverflow.net",
"https://mathoverflow.net/users/12203/"
] | I'm a beginner to this. Can anyone please point me to any resources for studying about equational logic, preferably with some example proofs to wet my feet in?
Thanks in advance!
| Beyond what is taught in (American) high school algebra courses, I don't really know any beginner level treatments on equational logic. The link Ricky Demer provides a brief bibliography which includes a book on Universal Algebra and one on Mathematical Logic; browsing through your local university math library should... | Look up:
<ul>
<li>Term Rewriting systems and Knuth-Bendix completion</li>
<li>theory of equations and solving systems of multivariate polynomials (Groebner Bases)</li>
</ul>
Though the latter subject, solving systems of equations over more concrete domains like fields, sounds very different from the purely syntactic ... | https://mathoverflow.net |
329,465 | [
"https://physics.stackexchange.com/questions/329465",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/106205/"
] | Suppose there are $4$ points on a straight line from left to right - $A$, $B$, $C$, $D$
$A$ and $D$ are stationary wrt one another and so have same frame of reference. $A$ & $D$ are sufficiently far apart.
$B$ & $C$ are part of a lab which is moving from left to right (in the direction $A$ to $D$) at a signif... | <blockquote>
the rest mass energy of a nucleus is smaller than the rest mass energy of its constituent nucleons in free state
</blockquote>
This sentence speaks about $protons$ and $neutrons$ as the constituents. In your equation you have three nuclei and the sentence is valid for each of the three nuclei separately... | The BE per nucleon is higher in the region of mass number where you have the two fission fragments as compared to the region of mass number 236. Therefore you get the answer. Higher BE lower rest mass.
| https://physics.stackexchange.com |
480,949 | [
"https://physics.stackexchange.com/questions/480949",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/169802/"
] | In my understanding, quasi-particles were just some real world particle (like electrons) but in different environment i.e. an electron in a crystal.
But
Recently, I have started studying Spintronics/Magnetism and I see words like "quasi-particle", "excitations" and "elementary excitation" a lot. For example Spinons, ... | There are basically two types:
<ol>
<li>Quasiparticles, if they are related to fermions</li>
<li>collective excitations if they are related to bosons</li>
</ol>
Now there are four main differences between real elementary particles (or real particles and composite particles) and quasiparticles:
<ol>
<li>real particle... | They have kinetic energy and momentum, some have rest energy. They only live in a medium but some would argue that space-time itself is a medium. Let me reverse the question: why not ?
| https://physics.stackexchange.com |
24,485 | [
"https://physics.stackexchange.com/questions/24485",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/8913/"
] | If we have a one dimensional system where the potential
$$V~=~\begin{cases}\infty & |x|\geq d, \\ a\delta(x) &|x|<d, \end{cases}$$
where $a,d >0$ are positive constants, what then is the corresponding classical case -- the approximate classical case when the quantum number is large/energy is high?
| To prove that experimental Physics is alive and well, I used my kitchen scales to measure the force needed to click the button on my mouse, and it turned out to be 100g i.e. 1 N plus or minus about 10%. The distance the button moves is about a millimeter i.e. 0.001m, plus or minus 20% (OK - you try measuring it without... | It very much depends where you begin. If it is before a person presses a mouse button to after it - it is very small. If it is all the work neccessary before you get to press that button, say from Charles Babbages Difference Engine to what we have now - a great deal of work.
| https://physics.stackexchange.com |
224,774 | [
"https://softwareengineering.stackexchange.com/questions/224774",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/93656/"
] | I am working on a highly customized shop software, based on a open-source one, written in PHP and usual web techniques (CSS, HTML, JS).
I did a lot of customization in the past months/years and developed specific, individual features which are necessary to be successful at our branch.
The shop and the project files a... | You <em>will</em> encounter encoding problems. You can handle Swedish and English with ISO-8859-1, but not Polish. You could use ISO-8859-2 for Polish, but then you would need to use other encodings when moving to languages that are not covered with ISO-8859-1 or ISO-8859-2.
So it is best to internationalize the softw... | Converting from a single-byte to a multi-byte encoding (which utf-8 is) is far from trivial. It will require you to touch almost every string in your application (or at least examine it), and every function that manipulates it. Bear in mind that PHP 6 and Perl 6 have been held up for years because of this. I would not ... | https://softwareengineering.stackexchange.com |
5,917 | [
"https://dsp.stackexchange.com/questions/5917",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/3233/"
] | I have the following sourcecode. For all I know, it should be a Bessel LPF at 10Hz. Is there a way to get the sampling frequence out of this?
<pre><code>double LPF(register double value) {
static double buf[4];
register double tmp, fir, iir;
tmp= buf[0]; memmove(buf, buf+1, 3*sizeof(double));
iir= val... | In digital signal processing, everything is normalized to and therefore independent of the sampling frequency. The sampling frequency is not defined before the digital signal is converted to an analogue signal.
Suppose a digital lowpass filter has a 3dB cutoff frequency of 0.2 and the filtered signal is converted to a... | I don't think Deve's answer is correct. The coefficient determine the transfer function in normalized frequency. You can determine at what relative frequency the transfer function hits -3dB. Let's say it's 0.01. Since you know this is corresponds to 10 Hz you can infer that the sample rate is indeed 1000Hz.
This code... | https://dsp.stackexchange.com |
304,432 | [
"https://electronics.stackexchange.com/questions/304432",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/148694/"
] | I am taking a course in digital electronics at university. I am second year mechanical engineering student, but I felt like it was important to understand some electronics. The lecturer in one of the slides, when talking about making a binary synchronous counter out of T-flip flops, said
<blockquote>
It is undesira... | By "modify", I'm pretty sure he meant some sort of gating controlled by feedback from the counter outputs. The problem with doing this is that you must be very, very careful that your gate signal is clean. If you are, for instance, turning off the clock when two outputs change, the two may not change at exactly the sam... | There are probably more reasons, but I will list three that come to mind:
As you stated, the propagation time of the gates will affect the placement of the clock edges, and can reduce timing margin of the circuit. This may not matter with slower clocks like the few MHz you mentioned, but will matter if running in a f... | https://electronics.stackexchange.com |
149,926 | [
"https://softwareengineering.stackexchange.com/questions/149926",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/54723/"
] | Is it a good idea to use CouchDB for a web application that is going to be platform agnostic (from tablet to PC):
<ul>
<li>The app is a big form which I need to be able to modify at will. </li>
<li>I also need to scan the results to retrieve the data to send some file
to help the client.</li>
<li>I need to be able to... | CouchDB has been running on iOS ( iPhone and iPad ) for a while now. An app using it was even accepted to the AppStore. There is a version for Android as well. That said, your question points to a lack of fundamental comprehension of database theory, I would not start off with CouchDB if I was you.
Google <em>"CouchD... | <strong>No</strong>
The choice of database depends largely on the nature of the data and how its being used. But you leave that information completely out of your question, and for some reason include the type of hardware the web client would run on (which is largely irrelevant, as Frustrated points out). I'm interp... | https://softwareengineering.stackexchange.com |
310,907 | [
"https://physics.stackexchange.com/questions/310907",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/97850/"
] | Magnitude of instantaneous velocity = $|dr/dt|$ but this is not always equal to $ds/dt$ where ds is the infinitesimally small change in <em>distance</em> in the interval $dt$
Which one is speed?
<img src="https://i.stack.imgur.com/Uhjcs.jpg" alt="enter image description here">
| For both you are only travelling an infinitesimal amount.
The difference between $\Delta r$ and $\Delta s$ goes away as $\Delta$ becomes smaller. In this case it's $ds$ and $dr$ which is the limit where they become equal.
You can look at your drawing and imagine as you decrease r and s, the space between the line ... | More precisely,
\begin{align*}
s &=\int |\mathbf{v}| \, dt \\
\frac{ds}{dt} &= |\mathbf{v}| \\
&= \left| \frac{d\mathbf{x}}{dt} \right| \\
ds &= |d\mathbf{x}| \\
&= |\mathbf{v}| \, dt
\end{align*}
For linear motion without <em>"U-turn"</em>, we can mix the usage of $s$ and $x$.
| https://physics.stackexchange.com |
3,533,807 | [
"https://math.stackexchange.com/questions/3533807",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/577174/"
] | <span class="math-container">$$f(x) = \frac{1}{e^x+e^{-x}}$$</span>
<span class="math-container">$$I_n=\int_n^{n+1}f(x)dx\,\,\,\,\, n\in\mathbb N^*$$</span>
Show <span class="math-container">$f(n+1) \le I_n \le f(n)$</span>.<br>
I don't know what to do here. I realized that <span class="math-container">$I_n$</span> can... | Your function is decreasing on <span class="math-container">$[0,\infty)$</span>. Therefore, for each <span class="math-container">$x\in[n,n+1]$</span>, <span class="math-container">$f(x)\leqslant f(n)$</span> and so<span class="math-container">$$I_n=\int_n^{n+1}f(x)\,\mathrm dx\leqslant\int_n^{n+1}f(n)\,\mathrm dx=f(n)... | using the inequality <span class="math-container">$m(b-a)\leq \int_a^b f(x)dx\leq M.(b-a)$</span>, here <span class="math-container">$a=n,b=n+1$</span> , and the function is decreasing, hence <span class="math-container">$m=f(n+1),M=f(n)$</span> .
Here <span class="math-container">$m,M$</span> as usual gobal minimum... | https://math.stackexchange.com |
249,980 | [
"https://security.stackexchange.com/questions/249980",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/258331/"
] | If a website emails a password in cleartext when you use the "forgot password" function, is there any possibility that the password is hashed? It does generate a different password if you reset it again, but it always gets emailed in cleartext.
Is it possible to reset a user's password, proceed to email it in... | "If a website emails a password in cleartext when you use the "forgot password" function, is there any possibility that the password is hashed?"
Yes, of course. The site might (it actually <strong>ought to</strong>) store the password in hashed form, and send it to you when it is generated in plain ... | Absolutely. This is quite common for initial passwords. You set up a new account, the system generates a password, emails it to the user, but it is properly hashed and stored server-side. Microsoft 365 does this, for example. It usually means that the emailed password triggers a reset once you use it.
It might be <em>b... | https://security.stackexchange.com |
14,916 | [
"https://security.stackexchange.com/questions/14916",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/9701/"
] | I'm new to commercial SSL certificates and would like to know if a CSR that I generate is safe to send via email?
| Sure but I don't think that the certificate authority should receive CSRs by email unless they are employing a mechanism (like signing your email with PGP/GPG) to ensure that the CSR came from you (rather than someone pretending to be you).
| As the CSR only contains your public key: No, it's not dangerous to send it by email or publicize it in any other way. The only thing you can basically do with it is to generate certificates for that public key. <strong>Don't send your private key to anyone however.</strong>
Also, I'm not aware of any commercial SSL p... | https://security.stackexchange.com |
1,828 | [
"https://mathoverflow.net/questions/1828",
"https://mathoverflow.net",
"https://mathoverflow.net/users/966/"
] | Suppose w^(2n)=1 (w is a complex number).
For which n (if any) \sqrt(w) \in Q(w) ?
| The key point is to understand the field Q(w) for w a primitive kth root of unity. Call this field Q<sub>k</sub>. In particular, you want to know that Q<sub>4n</sub> \neq Q<sub>2n</sub>.
The key fact here is that the field extension Q<sub>k</sub>/Q has degree phi(k), where phi(k) is the Euler phi function, and phi(4k)... | If w is a primitive 2n-th root, then the answer is "none". If w is not primitive, then Q(w) has a square root of w if and only if and odd power of w is 1.
| https://mathoverflow.net |
565,170 | [
"https://physics.stackexchange.com/questions/565170",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/141925/"
] | The principle of relativity suggests that all inertial frames are indistinguishable. But if I consider the rest frame of a photon (i.e. an IRF traveling at speed <span class="math-container">$c$</span> w.r.t. me), then won't it be distinguishable from my rest frame? And both are IRFs.
For example, time is not well-defi... | Inertial frames in special relativity are limited to sub-light-speed. The concept only applies, and only makes sense, for frames that could in principle be the rest frames of things like rods and clocks.
So light signals simply do not have a rest frame.
The relative speed between a photon and any inertial frame is <spa... | A photon doesn't have a rest frame.
| https://physics.stackexchange.com |
69,226 | [
"https://mathoverflow.net/questions/69226",
"https://mathoverflow.net",
"https://mathoverflow.net/users/3818/"
] | The following inequality is from page 125 of D.S. Mitrinovic, J. Pecaric, A.M. Fink, Classical and new inequalities in analysis, Kluwer
Academic Publishers, Dordrecht/Boston/London, 1993.
If $a_i>0$, $b_i>0$ for $i=1,\cdots, n$ and $A=\frac{\max a_k}{\min a_k}$, $B=\frac{\max b_k}{\min b_k}$ with $\frac{1}{p}+\f... | The following proof was inspired by Fedor Petrov's and Gjergji's Zaimi's argument, but it is simpler.
By a scaling argument we may assume $a_i\in[1,A]$, $b_i\in[1,B]$.
The inequality can be rewritten as
$$x^{1/p}y^{1/q} \leq (A^pB^q-1)\sum_{i=1}^n a_ib_i,$$
where
$$x:=p(AB^q-B)\sum_{i=1}^na_i^p\qquad\text{and}\qquad
y... | sorry, it is not full proof, but too long for comment
By scaling argument, we may suppose $a_i\in [1,A]$, $b_i\in [1,B]$.
Note that the difference LHS-RHS is convex function in each $a_i$ and $b_i$ (the function $f(x)=(x^p+M)^{1/p}$ for constants $M>0$ and $p>1$ is convex on $(0,\infty)$). Hence it maximum on a ... | https://mathoverflow.net |
553,382 | [
"https://electronics.stackexchange.com/questions/553382",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/148673/"
] | So I'm a newbie to Verilog. I decided to purchase a nice board (a Terasic DE0-CV) and teach myself some Verilog. And I'm seeing some strange behavior that I can't explain.
I lifted some code out of an example that drives the 7-seg LEDs. And I have them tied to a register. Whatever the register holds, the LEDs displ... | Learning Verilog is one thing. It also good to learn digital design. Modern designs use clocks. The synthesis tool doesn't want to make something sensitive to two different edges.
You should us a clock and either count or reset depending on the input. You should digitally detect the edge to count up.
<pre>
<code>
... | Assuming:
<ul>
<li><code>KEY</code> input is already de-bounced</li>
<li>Your board has flip-flops with asynchronous set/reset</li>
<li>No other edge events will be added</li>
</ul>
Then you could do this:
<pre><code>always @(negedge KEY[3], negedge KEY[2]) begin
if (!KEY[2]) begin // async reset
mSEG7_DIG <= ... | https://electronics.stackexchange.com |
59,322 | [
"https://mathoverflow.net/questions/59322",
"https://mathoverflow.net",
"https://mathoverflow.net/users/13809/"
] | Let $(S,\cdot)$ be a semigroup and $W\subseteq S$ be a subset. Let me call $W$ "tile" if the following property is satisfied: there exist $s_1,...s_k\in S$ such that the sets $s_i\cdot W$ are pairwise disjoint and cover $S$. For instance, tiles for the semigroup $(\mathbb N,+)$ are given by the multiples of some fixed ... | Prime factorization provides an isomorphism between the semigroup $(\mathbb N,\cdot)$ and an infinite direct sum of copies of $(\mathbb N,+)$.
So you can reduce your problem to the case that you already know how to solve.
<i>Warning:</i> my first $\mathbb N$ does not contain the element zero, whereas my second $\mathb... | Consider $W$ consisting of those natural numbers in which a given prime $p$ occurs in the prime factorization with exponent divisible by some constant $k$. For example, if $W$ is numbers in which the exponent of 2 is even, then $W \cap 2W = \emptyset$ and $W \cup 2W = \mathbb{N}$.
| https://mathoverflow.net |
174,796 | [
"https://electronics.stackexchange.com/questions/174796",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/52949/"
] | I am currently analyzing the raw measurements of a sensor with the sampling frequency of 400[hz]. According to Nyquist theorem, the bandwidth must be less than a half of the sampling frequency, which means less than 200 [Hz]. What will happen if I select the bandwidth as 50 [Hz] or 100 [Hz]?
(I have already tried to... | The FIR filter can only be applied to the signal after it has been digitized, which is too late. You have to apply the anti-aliasing filter before the ADC. That is to say, you need an analog filter.
As far as bandwidth, that depends on your other requirements.
If you know that there's only noise above 50Hz, then you... | <blockquote>
According to Nyquist theorem, the bandwidth must be less than a half
of the sampling frequency
</blockquote>
No, that is the wrong way to look at it.
This is the correct way to look at it: -
<h2>The sampling frequency must be at least twice the highest spectral content of the signal you are wishing... | https://electronics.stackexchange.com |
602,366 | [
"https://physics.stackexchange.com/questions/602366",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/264409/"
] | When I took field theory 25 years ago, I learned to do the mathematical manipulations very fluently but had no idea whatsoever what it all meant. Now, with a more mature understanding of quantum mechanics, the following issue confuses me.
My understanding at this point is that QFT is just quantum mechanics with fields ... | The construction you described is, indeed, possible, but this is not the standard way to do QFT. In standard wave functional approach, we work in some specific (but arbitrary) reference frame. In this reference frame we chacterize basis field state as a function of space variables only: <span class="math-container">$\p... | Let <span class="math-container">$M^4$</span> denote the four dimensional Minkowski spacetime.
<span class="math-container">$$\Phi(x)$$</span>
is the quantum field at the event <span class="math-container">$x\in M^4$</span> and here <span class="math-container">$x$</span> is nothing but a label.
Next there is the actio... | https://physics.stackexchange.com |
249,465 | [
"https://dba.stackexchange.com/questions/249465",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/155122/"
] | I have the scenario where I have to generate a backup of the database (SQL Server 2008) and restore into the new server (SQL Server 2016). While taking backup data should not be changed in any case. So I have two options to do that, but I am not sure how it will work. (Everything is using SQL Job only.)
<ol>
<li>Set a... | <blockquote>
→ Is it possible to restore read-only DB in the new server?
</blockquote>
<strong>Yes</strong>
An example:
<pre><code>CREATE DATABASE ReadOnlyDB;
GO
ALTER DATABASE ReadOnlyDB SET READ_ONLY;
BACKUP DATABASE ReadOnlyDB TO disk = '\\share\readonly.Bak';
</code></pre>
<blockquote>
destination serve... | Yoy have a couple of options in your situation:
<ol>
<li>Turn off applications that can change data in the database</li>
<li>Set the database in single user mode and back it up. Something like this: </li>
</ol>
<code>ALTER DATABASE [<DBName>] SET SINGLE_USER WITH ROLLBACK IMMEDIATE;</code>
<code>BACKUP DATABAS... | https://dba.stackexchange.com |
121,625 | [
"https://dba.stackexchange.com/questions/121625",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/11075/"
] | I have this issue. I have three types of posts a user can generate (some users can generate all some can only generate one type). I have the following types: main, employee and admin posts.
I expect to have to deal with millions of posts (all three types). But about 90% of all posts would be <code>main_posts</code>... | <pre><code>SELECT
t1.ID,
CASE
WHEN t1."name" != t2."name" then
t2.name
ELSE
t1.name
END as fname
FROM
t1 LEFT JOIN t2 ON t1.ID = t2.ID
</code></pre>
from comments - duplicates, mean You have duplicates (answer with <strong>coalesce</strong> return the same), variant for avoid duplicates:
<pre><co... | The cleanest has to be:
<pre><code>SELECT T1.id
COALESCE(t2.name, t1.name) as name
FROM t1
LEFT JOIN t2
ON t1.id = t2.id
</code></pre>
| https://dba.stackexchange.com |
32,789 | [
"https://physics.stackexchange.com/questions/32789",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/10531/"
] | In most basic level introduction to the quantum harmonic oscillator formulation of fields, it is assumed that the commuting variables for the fields $p_m$, $q_m$ are
$$ \lbrack p_m , q_n \rbrack = \delta_{m n} i \hbar $$
which seem to imply that each individual mode holds an uncertainty relation like $ \Delta p_m \D... | The fluctuation in a field at a point is infinite in any field theory, this is because of the reason you state. This is why you need to smear the field over a region with a test function for it to have finite fluctuation, and the reason that the fields are characterized as operator valued distributions.
If you look at... | EDITED: @ronmaimon comment is right, this answer does not apply to this question. The below applies if you were asking a different question, how does a single mode of the field have finite noise. But the actual question actually refers to a situation in which an infinite number of modes are present and the fluctuatio... | https://physics.stackexchange.com |
710,379 | [
"https://math.stackexchange.com/questions/710379",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/123465/"
] | I see this term often.
find $P(\min\{X,Y\}< z) $ where $X$ and $Y$ are independent random variable .
From the online source, I see people saying this as $P( \{X < z\} \cup \{Y< z\} )$, but I don't see why.
Can you guys please define what it means? Also for maximum?
| This is more a statement about sets than probabilities.
$\{\omega | \min(X(\omega), Y(\omega)) < z \} = \{\omega | X(\omega) < z \text{ or } Y(\omega) < z \} = \{\omega | X(\omega) < z \} \cup \{\omega | Y(\omega) < z \}$.
You can show this in the following way if you prefer, but it is essentially the ... | $P(\min{X,Y}<z)$ is the probability that a realization of $X$ and $Y$ from their distributions will be such that the minimum of those two numbers is less than $z$.
For instance, for two uniformly distributed random variables from $[0,1]$, how likely is it that their minimum is less than $1/2$? (Equivalently, how l... | https://math.stackexchange.com |
217,285 | [
"https://physics.stackexchange.com/questions/217285",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/59168/"
] | Say we have two gas containers of $N_{2}$ at the same temperature of $300 ~\text{K}$, one containing $10^{23}$ particles and the other containing $10^{13}$ particles. If we add a quantity of heat to both containers, and the volume of these containers remains constant, then the rise in entropy is given by $d\mathtt{S} =... | <blockquote>
Why does heat added to a system cause an increase in entropy that is independent of the amount of particles in the system?
</blockquote>
<strong>Short answer: it doesn't</strong>
The systems <em>won't</em> end up with the same entropy. Your intuition is correct that the change in entropy depends on the... | As march pointed out, your reasoning is incorrect for finite $Q$ and $\Delta S$. However, the equation is true for differentials, so we have to address that.
Possible 'conceptual' answers:
<ul>
<li>Stat mech: the increase in $S$ is related to how much extra disorder is produced. When there are less particles, they ea... | https://physics.stackexchange.com |
58,375 | [
"https://mechanics.stackexchange.com/questions/58375",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/24177/"
] | I recently flushed my non-ABS motorcycle rear brake line with new fluid using a little hand vacuum pump, but accidentally initially let the brake fluid drop below the "low" marker on the master cylinder/reservoir.
After bleeding and cycling in the new brake fluid a couple of times with a little vacuum pump, it seemed... | Looks like this is not due to the air or sponginess of the brake line, but the opposite. Having bled the brakes of air and other impurities (floating around the old brake fluid), my brakes now bite harder/sooner than what I was previously used to. For the rear brake, this makes it easier to lock up and skid the rear ti... | Im not a mechanic but any brake that is having issues may cause skidding (due to terrible brake performance), less stopping power and may hurt your brakes, rotors, calipers. My SUV Avura MDX I failed to change my brakes because they were so bad that when it came to emergency braking my screw that held my caliper came l... | https://mechanics.stackexchange.com |
370,252 | [
"https://softwareengineering.stackexchange.com/questions/370252",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/210153/"
] | I have a set of jobs that I need executed. The entity that manages these jobs is responsible for composing the jobs and starting them. When a job is finished, the job uses a callback function to inform the entity of the result of the job (Whether it threw an exception and such)
<pre><code>public class TaskSet {
Qu... | <blockquote>
I have a set of jobs that I need executed.
</blockquote>
Excellent. You have chosen a reasonable architecture to meet this need.
<blockquote>
When a job is finished, the job uses a callback function to inform the entity of the result of the job (Whether it threw an exception and such)
</blockquote>
... | Don't have a callback. or at least dont pass the job into it.
<pre><code>TaskSet.executeJob(Job j)
{
j.Execute();
//the job has finished
if(j.status == failed)
{
this.Queue.Add(j); //queue for retry
}
}
</code></pre>
| https://softwareengineering.stackexchange.com |
184,839 | [
"https://stats.stackexchange.com/questions/184839",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/96999/"
] | I have a series of 7 pairs with each pair adding up to 100%.
I now need to average column a and b to end up with a final pair totalling 100%.
Where do I start?
<pre><code>| | A | B | C
| Asus | 74.38%| 25.62%| 100%
| HTC | 0% | 100% | 100%
| LG | 0% | 100% | 100%
| Moto | 25% | 75% ... | <em>Any</em> linear combination of the rows will have this property if all row weights add up to one. The most sensible weighting scheme is one that takes into account the number of elements in each row (if there are twice as many Asus as LG elements in the total population, ideally the Asus row will count for twice as... | Are you interested in proportion of A among all Android <strong>phones</strong>? Or are you interested in proportion of A among all Android <strong>models</strong>? The difference between these two is that each model has many actual physical phones.
If your answer is "among models", then you do a simple average. If y... | https://stats.stackexchange.com |
86,146 | [
"https://chemistry.stackexchange.com/questions/86146",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/-1/"
] | Is there any reason why fluorescence spectroscopy is said to be more sensitive than Uv-Vis?
| Fluorescence is a 'zero background' or absolute type of measurement meaning that single photons can be measured against a 'dark' background so the sensitivity is huge, and limited by the fraction of light caught and efficiency of the detector.
Absorption is a relative measurement as it is the difference in two large ... | Absorbance may occur from non-analytes, while fluorescence is often quite specific to an analyte or a fluorophore-tagged analyte. Also, instrumental setup could play an effect. UV-Vis specs are often horizontal in terms of incident beams, sample, and the detector. Fluorescence specs are often at 90 degrees to "extract"... | https://chemistry.stackexchange.com |
155,004 | [
"https://physics.stackexchange.com/questions/155004",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/-1/"
] | So I understand the concept of gravity, in that it's not actually a force, but more of a displacement in the spacetime grid. An object with a big enough mass will bend the spacetime, causing smaller objects to "attract" to it (not really, but good enough for what I am trying to figure out)
Ok so that makes sense, but... | Objects not being affected by forces move on straight lines in euclidean space.
If space is curved, there will not be straight lines, and particles follow trajectories that are the next best thing to straight, eg. paths whose length is the shortest. Knowing curvature, these can be calculated, for example by using vari... | On a practical level (baseball, soccer, tennis, ...) Newton's account of gravity as a force with particular behaviors is entirely satisfactory. It is even used for launching satellites (well most of them), and predicting artillery impact points. It was this last application that made warfare even more horrendous than i... | https://physics.stackexchange.com |
25,332 | [
"https://security.stackexchange.com/questions/25332",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/6253/"
] | This might be far too narrow, but it is a unique problem to ITSec professionals. A loved one is just starting out in a new programming career and I get the joy of watching her learn the most basic programming concepts from scratch. She is at the top of her class in each of her college courses, producing high quality wo... | I would say a great way to learn is for her to <strong>break</strong> the applications she has already written.
Assuming she is writing web applications, point her towards the OWASP Top 10. Have her see if she can find any of those flaws in her own code. There is no better way to learn about security concepts than act... | I'm going to take the position that may get me flambéed...
The problem I see, is that secure programming is taught as an add on. Best practices should be taught from the beginning (including security). The lie people are taught is that practice makes prefect. The truth is practice makes permanent. So if you are do... | https://security.stackexchange.com |
257,411 | [
"https://mathoverflow.net/questions/257411",
"https://mathoverflow.net",
"https://mathoverflow.net/users/48522/"
] | I could not decide if I should post this question in MO or Mathstackexchange, so feel free to downvote it if you think it does not belong here. I will delete my post and post it in MathSE in that case.
What I am wondering about is the following: Given a projective map $\pi:X\to B$, where $B$ is integral and $\mathscr{... | The existence of a proper class of Woodin cardinals and a proper class of strong cardinals is strictly stronger in consistency strength over ZFC than the existence of a cardinal $\lambda$ that is a limit of Woodin cardinals and a limit of $<\lambda$-strong cardinals. The former implies the consistency of the latter ... | I think I got a solution. If you find any mistakes or if anything is unclear, please let me know!
Firstly, recall that the $\Omega>0$-hypothesis (or sometimes stated as $\theta_0<\Theta$-hypothesis), is the statement that there is a $\lambda$ which is the limit of Woodins and there is a $\kappa<\lambda$ which... | https://mathoverflow.net |
723,128 | [
"https://physics.stackexchange.com/questions/723128",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/332808/"
] | When we dip a spoon (<em>stainless steel</em>) into ice cream, does it becomes cold or stay the same temperature? If it does, can we say that heat(?) can flow from cold to hot objects?
<ul>
<li>Is this the reason that we are given wooden(or plastic) spoons to eat ice cream?</li>
</ul>
| Heat is energy transfer due solely to temperature difference. Heat "flows" naturally from higher to lower temperature. In order for heat to flow from low to high temperature (e.g. a refrigerator or air conditioner), work must be done (by a compressor).
Since the initial temperature of the spoon (room temperat... | Heat is transferred from the spoon to the ice cream thereby reducing the internal energy and hence the temperature of the spoon. A metal spoon is more effective at this heat transfer than a wooden spoon.
To effect heat transfer from a cold to a hot object you need a refrigeration cycle (pump, evaporator, compressor, t... | https://physics.stackexchange.com |
33,879 | [
"https://stats.stackexchange.com/questions/33879",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/13130/"
] | Using <code>mgcv</code> I have made 5 models, each one using a subset of my data, defined by quantiles. The response/explanitory variables are the same in each case, but the deviance explained by each in each model differs, along with the P values.
Is there a clever way of comparing the contributions of each explanito... | I just used a table in the end, with colours to make picking out high values easier. <img src="https://i.stack.imgur.com/MpzU2.png" alt="enter image description here">
| I am not sure I follow your quartile models but within each model set you can compare models using AIC wieght ($w_i$) for each model $i$:
$$
w_i = \frac{e^{(-0.5 * \Delta AIC_i)}}{\Sigma e^{(-0.5*\Delta AIC_i)}}
$$
which can be interpreted as the weight of evidence for each model.
If each variable is equally represe... | https://stats.stackexchange.com |
625,355 | [
"https://physics.stackexchange.com/questions/625355",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/256066/"
] | In spherical coordinates the acceleration can be written as
<blockquote>
<span class="math-container">$$\textbf{a} = \dot{\textbf{v}} = \ddot{r} \hat{\textbf{r}} + \dot{r} ( \dot{θ} \boldsymbol{\hat{\theta}} + \sin θ \dot{\phi} \boldsymbol{\hat{\phi}}) + \dot{r} \dot{θ} \boldsymbol{\hat{\theta}} + r \ddot{\theta} \bold... | By definition, if the acceleration points towards the origin it is centripetal. So if <span class="math-container">$\ddot{r} - r \dot{θ}^2 - r \sin^2 θ \dot{\phi}^2<0$</span> then the radial component is centripetal.
You are most likely getting confused with introductory systems of uniform circular motion where we s... | The two negative terms combine to give a resultant, -r<span class="math-container">$ω^2$</span>.
| https://physics.stackexchange.com |
631,203 | [
"https://electronics.stackexchange.com/questions/631203",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/213323/"
] | There is an option in many terminal programs or ICs to set 1 stop-bit, 2 stop-bits or 1.5 stop-bits. Is there any standard that says the RS-232 can have only 1/1.5/2 stop-bits but can't have 0.5 or 3 stop-bits, for example?
| Nothing, there is no such standard that says how data should be sent over an RS-232 interface.
RS-232 is only a physical, mechanical, and electrical standard; it does not even define a protocol for to sending bits on the data wires. It also does not define at what rates data should be sent, other than it is intended fo... | In asynchronous UART communications - as typical over RS-232 - the only real constraint is that the transmitter must be able to send <strong>at least</strong> as many stop bits as the receiver needs. That's all.
If the receiver is configured to expect 1 stop bit, the transmitter can be configured for 1, 1.5, 2, 10, 100... | https://electronics.stackexchange.com |
76,837 | [
"https://mechanics.stackexchange.com/questions/76837",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/57491/"
] | I have a small crack in my oil pan, I have ordered the new one but it will take a few days to come in. Is there a to band aid it temporarily until the new one arrives and is installed?
| If you have the pan off, clean and dry there's a few things you can try, depending on the size and shape of the crack. Keep in mind all of these must be on the outside of your pan, not the inside, the last thing you want is for any of this to get into your engine:
<ul>
<li>Waterproof, heat resistant tape: okay, duct t... | I have seen some cracks that were drilled, threaded and bolts put in - takes careful work and they need to overlap. Can also be welded.
But usually means it has to be removed.
May be easier just to wait as the labor required is usually significant.
| https://mechanics.stackexchange.com |
724,344 | [
"https://physics.stackexchange.com/questions/724344",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/262541/"
] | I can't understand one thing in the definition of the resolving power of a spectrometer:
Let the resolving power be defined as: <span class="math-container">$R=\frac{\lambda}{\Delta \lambda}$</span> where <span class="math-container">$\Delta \lambda$</span> is the separation of two resolved spectral lines and <span cla... | Because resolving power is unitless. By definition.
You <em>can</em> define something called the "resolution" of a spectrometer, which is just <span class="math-container">$\Delta \lambda$</span>, which has units.
Why is a unitless resolving power useful? One reason is that it arises naturally in the properti... | <span class="math-container">$\dfrac{\Delta \lambda}{\lambda}$</span> is the fractional change which can be resolved and the reciprocal is the resolving power which has no units.
So in terms of a fractional change <span class="math-container">$\dfrac{1\,\rm nm}{400\,\rm nm}$</span> is the same as <span class="math-cont... | https://physics.stackexchange.com |
320,579 | [
"https://electronics.stackexchange.com/questions/320579",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/149466/"
] | If I terminate a line with an open circuit, I'll get reflections of any incoming signals with the same phase (a reflection coefficient of 1). If I terminate the line with a short circuit, I'll get reflections of any incoming signals with opposite phase (a reflection coefficient of -1).
What can I terminate the line wi... | Terminate with an inductor or capacitor.
$$\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}$$
\$Z_0\$ is char. impedance i.e. 50 Ohm. \$Z_L\$ is complex if you use inductor or capacitor; thus \$\Gamma\$ is complex.
| You wanted the reflection coefficient to have magnitude = 1. That means pure reactive load. A piece of transmission line with open or shorted termination can itself be any reactace - capacitive or inductive. You need no other components such as capacitors. Smith's diagram is the legacy tool to find the needed line len... | https://electronics.stackexchange.com |
46,012 | [
"https://mechanics.stackexchange.com/questions/46012",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/-1/"
] | When I drove off in my 1996 Legacy I used the brakes very gently a couple times then saw the Brake indicator lamp and pushed the brake pedal... It went easily to the floor and then I got about 5 percent of normal braking action. So it seems that a problem affects both sides of the dual system. There is still fluid in t... | I would have it towed to a repair shop. But it sounds like the master cylinder is bad. If the reservoir is full and there are no signs of leaks, its most likely the master. But you do need to have this checked by a qualified mechanic.
| Yes a tow is expensive, $100 or thereabouts, so, you don't want to fix it yourself and you can't afford a tow either? Then to keep the cost at a bare minimum, maybe I'd find somebody who's a mechanic and/or has done brake work before, and who is willing to come to where your car is and fix it there?
Just for kicks I ... | https://mechanics.stackexchange.com |
127,605 | [
"https://physics.stackexchange.com/questions/127605",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/44726/"
] | Back in my undergrad I had a course on classical electrodynamics where the fields had values in the space of tempered distributions. In this way one could correctly treat self-interaction and effectively solve the differential equations involved in the distribution sense.
Unfortunately the few notes that I have are in... | You have $2$ atomic ensembles.
For each atomic ensemble, the ground state is $|\psi_0\rangle$, and the excited state is $|\psi_1\rangle$. The excited state is signaled by an idler photon.
Without beam splitter, if you detect only one photon (for the whole set), you know which atomic ensemble is excited, so you have t... | I feel the original question from the post is two-fold: (1) why the beam splitter changes the photon basis, and (2) why does a measurement in such a basis project and create entanglement?
The second question is answered well by others. The answer to the first question is actually a direct generalization of the classica... | https://physics.stackexchange.com |
3,277,870 | [
"https://math.stackexchange.com/questions/3277870",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/557708/"
] | Is <span class="math-container">$\mathbb{Z} [i] $</span> is field ? yes/No
yes, I thinks it will field because it is integral domain
Is its True ?
| No, it is not a field. For instance, <span class="math-container">$2$</span> has no inverse in <span class="math-container">$\mathbb Z[i]$</span>.
| Well, <span class="math-container">${\Bbb Z}[i] = \{a+bi\mid a,b\in{\Bbb Z}\}$</span> is a subset of <span class="math-container">${\Bbb C}=\{a+bi\mid a,b\in{\Bbb R}\}$</span> and so an integral domain. But its clearly not a field.
| https://math.stackexchange.com |
128,537 | [
"https://softwareengineering.stackexchange.com/questions/128537",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/17887/"
] | I got this question in one of the interviews I took. I started by explaining that it is like a set of rules that should be followed while moving resources around, but I did not feel convinced. Is there any other analogy or take on this question ?
| One reasonable analogy might be speaking to someone on the phone. After all TCP/IP is very similar to phone call. So something along the lines of:
<blockquote>
HTTP is like a language that computers on the internet use to talk to each other.
If you want something from somebody on the other end of the world, you phone t... | <blockquote>
it is like a set of rules that should be followed while moving resources around
</blockquote>
Although not entirely inaccurate, that sounds closer to a description of REST to me.
I would describe HTTP as a communication protocol, originally designed for Hypertext (hence the name) but extended over time... | https://softwareengineering.stackexchange.com |
85,823 | [
"https://softwareengineering.stackexchange.com/questions/85823",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/552/"
] | When would you keep simple to medium-complex personal calculations in a spread sheet (Excel etc) and when would you write a small program or script for it?
For example when you want to calculate what size of mortgage you can afford to buy a house.
I could create a spreadsheet and have a nice tabular representation. On... | You really have to ask yourself "Am I going to re-use this enough to justify the cost (or time)?". Many times it's not worth the cost of a full app or in the broader sense a tool.
I know too many developers that get all caught up in building "tools" and never actually accomplish the task at hand, in your case calcula... | It depends on what you plan to do. For a mortgage calculations for my personal use, I used a spreadsheet. It was very easy to copy/paste cells with formulas and compare different sets of data and calculated results. This solution worked well because I was the only one using the spreadsheet (so the "UI" (cell layout) wa... | https://softwareengineering.stackexchange.com |
54,483 | [
"https://stats.stackexchange.com/questions/54483",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/20316/"
] | I have yearly data for the import of fish products to Portugal (years 2000 to 2011) per country. There are some trends that I would like to visualize, for example some countries drop out and enter in the top 5 as exporters.
Now I'm not really sure how I can do it in a non-confusing way that highlights the changes but ... | You can definitely use line charts to communicate change. You should <em>not</em> use pie charts. As the great John Tukey said "there is no data that can be displayed in a pie chart that cannot better be displayed in some other type of chart".
Line charts <em>do</em> seem to me to imply connectedness - but your data ... | You're on the right path with the line chart (as Peter Flom mentioned, AVOID PIE CHARTS, especially comparing two of them). As for the line chart, consider the message you're trying to convey and use the chart's formatting to reinforce the message.
Here's an example where perhaps you want to emphasize the strongest... | https://stats.stackexchange.com |
1,824,228 | [
"https://math.stackexchange.com/questions/1824228",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/347469/"
] | <blockquote>
Consider the equation $$z_1 + z_2 + z_3 + z_4 + z_5 + z_6 = k$$ For:
$i = 1, \dotsc,6$ $z_i$ is a positive natural number and they must
satisfy the following: \begin{align} z_1 & \ge 4 \\ z_2 & \ge 10 \\
z_3 & \ge 12 \\ z_4 & \ge 18 \\ z_5 & \ge 6 \\ z_6 & \ge 30
\end{align}... | It is clearly false because $\log(1+\exp(x))$ exists for all real $x$ while $-\log(1-x)$ does not.
| The relation surely doesn't hold for every $x$: for instance, if $x=-1$ you have
$$
-\log(1-(-1))=-\log2<0,
\qquad
\log(1+e^{-1})>0
$$
It may be interesting looking for what values of $x$ equality holds. First we have to assume $1-x>0$, that is, $x<1$, for the left-hand side to exist. Once ensured this, we... | https://math.stackexchange.com |
332,421 | [
"https://electronics.stackexchange.com/questions/332421",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/4541/"
] | I have done a fair amount of soldering with SMD components now. I can solder an 0402 size component I prefer not to. I can solder an 0603 just fine and anything bigger. But my question is, what diode packages are similar in size to an 0603 but no smaller. I am not very familiar with diode SMD packages. I am aware diffe... | <blockquote>
I will need a small signal diode package(s) in the 0603 size range. I will also need the around about recommended package(s) for dissipating ~100 mw continuously
</blockquote>
SC-70 is about the same size as 0603, but it is fussier to solder because there are up to 6 pins on it. To dissipate 100 mW you'... | SOD123, which you mentioned above, and SMA, are probably the most common, and fairly easy to hand solder.
There is also the D2PAK (TO-263-3), which is easy to solder but possibly too big for your purposes.
You can find some diodes in standard 0805/1005/1206 packages but these are more rare and often a bit pricier th... | https://electronics.stackexchange.com |
61,502 | [
"https://softwareengineering.stackexchange.com/questions/61502",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/16707/"
] | Have an Excel spreadsheet that summarizes various key metrics for the business. The source data for these metrics comes from various sources including other spreadsheets, and web/ERP systems.
A suggestion was made to completely automate the whole data retrieval/population into the spreadsheet.
For this type of tool ... | You should make a list of all the anticipated risks and costs (in time, resources, cash) on one side, and the perceived benefits (time savings etc.) on the other.
If the sum total of the first list is noticeably larger than that of the second, you have a convincing argument towards management.
My addition to your lis... | Doing it manually is great to start out so users can find out what they need, how to do it, what problems to look out for, etc., but at some point you have to ask yourself the question WTF did they hire a programmer for? A programmer should have an aversion to watching users copy an paste data and doing things manually... | https://softwareengineering.stackexchange.com |
344,892 | [
"https://softwareengineering.stackexchange.com/questions/344892",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/266599/"
] | Introduction
I'm working at a software company building software as a service. Currently we have version x.y in production and x.z on acceptance. Let's say we find a critical bug on production and want to test the fix on an acceptance environment. I don't know how to work with this.
Options
<ol>
<li>Create a second ... | This all depends how easy it is to change the acceptance environment.
<strong>If deployment on acceptance environment is easy...</strong>
...for example if you just have to replace the executables, you may consider <strong>option 2</strong>:
<ul>
<li>overwrite current acceptance</li>
<li>make the test</li>
<li>depl... | This is always a question of money. If it isn't a question about money, frame it terms of money anyway.
If your cost of making an environment is low, e.g. spinning up dozens of virtual machines from build scripts, then make them whenever needed. The cost is low and pays for the benefits.
If you cost of making an en... | https://softwareengineering.stackexchange.com |
542,783 | [
"https://math.stackexchange.com/questions/542783",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/97221/"
] | I am once again stuck on a question about geometry, this problem is about altitudes that crate right triangles:
<strong>Let there be a triangle that has side lengths of 13, 20, and 21. Given this, find the length of the altitude drawn to the side of length 21.</strong>
I have drawn the following picture to make my un... | Let $a$ be altitude, and x be the base of the right-most right triangle.
$$20^2 = a^2 + x^2$$ $$13^2 = a^2 + (21 -x)^2$$
$$a^2 = \color{blue}{\bf 20^2 - x^2 = 13^2 - (21-x)^2 }$$
$$400 - x^2 = 169 - (441 - 42x + x^2) = -272 + 42x - x^2$$
$$672 = 42x \iff x = 16.$$
$$x = 16 \implies a = \sqrt{400 - x^2} = \sqrt{400... | <strong>Hint</strong>
You can also find altitude from area of triangle, recall that if $a,b,c$ are the sides of $ABC$ then the area of $ABC$ can be calculated by
$$\sqrt{u.(u-a).(u-b).(u-c)}$$
where $2u=a+b+c$.
So from the following you can find the height
$$\sqrt{u.(u-a).(u-b).(u-c)}=\frac{h.21}{2}$$
| https://math.stackexchange.com |
115,190 | [
"https://softwareengineering.stackexchange.com/questions/115190",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/11340/"
] | Are there any general guidelines to understand when I should consider using (as a eat-your-own-dogfood) and providing web services, and when I shouldn't worry about them? Or, any reading materials you can point me to?
My project is in PHP but I don't mind if it's language agnostic.
| Basically, you should implement a web service when you have functionality that, for whatever reason, requires centralization on a computer you control, but that needs to be directly accessed by consumers on many machines that you may not control.
Typically, the reasons for such centralization are:
<ul>
<li>The functi... | There is no all encompassing answer to this - it depends on many variables.
In general, one should implement a web service when there is a need to provide an API (internally and/or externally) that will have multiple users (as in different applications) and that should be consumed over the network (using HTTP).
If th... | https://softwareengineering.stackexchange.com |
82,986 | [
"https://mathoverflow.net/questions/82986",
"https://mathoverflow.net",
"https://mathoverflow.net/users/7868/"
] | Suppose that we have a closed embedding $G_1\hookrightarrow G_2$ of reductive groups (say over $\mathbb{Q}$), and suppose that we have a <em>maximal</em> parabolic sub-group $P_2\subset G_2$, and a <em>minimal</em> parabolic $P_1\subset G_1$. Is it possible to have two different maximal parabolic sub-groups of $G_1$ co... | If $P$ is a parabolic subgroup of a reductive group $G$ and $H$ is a closed subgroup of $G$ containing $P$, then $G/H$ is a quotient of $G/P$, so it is projective, and $H$ is parabolic. It follows that a maximal parabolic subgroup is maximal among all proper closed subgroups, which would seem to imply that the answer t... | There are several questions being asked (and an unexplained reference to a field of definition), but the answer to at least one of them is no: Take $G_1 = G_2 = \mathrm{SL}_3(\mathbb{C})$, with a given minimal = maximal parabolic subgroup involving a single simple root subgroup relative to some choice of positive roots... | https://mathoverflow.net |
1,260 | [
"https://mechanics.stackexchange.com/questions/1260",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/756/"
] | Ford Focus 1.8 Ghia '99 V
A little while ago a large section of the aluminium heat shield above my exhaust came loose. Being in a situation where I needed to keep going and without tools, I crawled under and ripped the whole thing off. Now (a few months later) there is a suspiciously aluminium like rattle coming fro... | Should be able to remove the bolts as long as they don't hold anything to them. Even if there is something attached you can remove them to detach the rest of the shield and then reattach. Either way, removing your heat shield is not what you should focus on, but instead installing a new one. Your exhaust gets really... | I had a mechanic remove mine on my focus (2003) (it was half off anyway) and it's been fine since winter. I think the car got louder though.
You might want to check your front coil springs. I thought the problem was the heat shield, but it turned out I had a broken front right coil spring. Fixing that got rid of t... | https://mechanics.stackexchange.com |
562,886 | [
"https://physics.stackexchange.com/questions/562886",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/268776/"
] | How does the observation of strangeness enhancement in high-multiplicity proton–proton collisions explain the existence of quark-gluon plasma?
| In complement to answer 1 we should also remember that in QGP the threshold of strangeness production is lower compared to confined phase. Moreover, the color degeneracy factor 3 helps to enhance the yield above confined phase. In relativistic heavy ion collisions we see strong evidence for QGP based on observed patter... | The temperature of the quark-gluon plasma (above <span class="math-container">$150$</span> MeV) is significantly higher than the mass of the strange quark (which is <span class="math-container">$101$</span> MeV). This means that it's hot enough that "extra" strange quarks can be produced via thermal fluctuati... | https://physics.stackexchange.com |
16,346 | [
"https://security.stackexchange.com/questions/16346",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/1211/"
] | I am wondering if a biometric device can be used to generate some entropy or key material that would be stable across each authentication.
The contrary would be for biometric devices to act like an oracle, with a simple YES/NO answer to any authentication request.
What I am thinking about would be to gather the resu... | No,
If a device sequenced the user's genome from a skin cell, the results would be the same each time. Assuming the skin cells have replicated without errors. But it seems what you are asking is impossible, since you have to have a database somewhere that stores the answer to your key.
| <em>Entropy</em> means that you get a different and unpredictable result each time through. So using a source that is even a little bit stable somewhat defeats the purpose.
If instead you mean using biometrics as a means for verifying identify, then now we're talking about old news -- that's exactly what biometrics ar... | https://security.stackexchange.com |
195,081 | [
"https://softwareengineering.stackexchange.com/questions/195081",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/57792/"
] | There is a new hype with the long awaited lambda expressions in Java 8; every 3 day another article appears with them about how cool they are.
As far as I have understood a lambda expression is nothing more than an anonymous inner class with a single method (at least at the byte-code level). Besides this it comes wit... | <strong>tl;dr:</strong> while it's <em>mostly</em> syntactic sugar, that nicer syntax makes lots of things practical that used to end in endless, unreadable lines of braces and parentheses.
Well, it's actually the other way around as lambdas are <strong>much</strong> older than Java. Anonymous inner classes with a sin... | For Java, yes, it's nothing more than a better way of creating an anonymous inner class. This is because of the fundamental decision in java that every bit of byte code has to live within a specific class, which cannot be changed now after decades of legacy code to consider.
However, that is not what lambda expression... | https://softwareengineering.stackexchange.com |
270,950 | [
"https://softwareengineering.stackexchange.com/questions/270950",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/73526/"
] | I'm implementing a simple <code>Quota</code> object which determines a usage percentage based on the <code>maximum</code> and the <code>used</code>.
<pre><code>private int maximum;
private int used;
public Quota(int used, int maximum) {
this.maximum = maximum;
this.used = used;
}
public double getUsagePercen... | If you set the one value (maximum) and then update the next (used) (and even more often - think of a progress bar) you are doing the calculation twice and even worse, nobody asked for the values between the setXY-operations. Why spend time for things nobody actually requests. Thus it is perfectly fine to use the getter... | One common guideline is that for any fact known by your system, there should be only one place where it is stored. In a database, this is known as normalisation; in object design the principle is sometimes referred to as "Single Point of Truth", or the acronym SPOT. This is closely related to the principle "don't repe... | https://softwareengineering.stackexchange.com |
540,695 | [
"https://electronics.stackexchange.com/questions/540695",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/272916/"
] | I have a 250W solar panel connected to a 12V 100A (SLA) battery through a charge controller (12 and 24V). A 1100W inverter is also connected to the battery.
Now I want to connect a 12V 180A battery in parallel to it. Is it possible? What effect will it have on the batteries?
| It depends a lot on what the charger is doing. The simplest ones don't attempt to fully charge the battery and instead provide a constant voltage of 13.8 (or sometimes 13.6) volts, which will mostly but not fully recharge a lead acid battery of most any type. If your "charge controller" is that type then put... | Lead acid batteries don't have a very predictable voltage to charge-level relationship so paralleling dissimilar units is likely to end badly.
| https://electronics.stackexchange.com |
147,792 | [
"https://security.stackexchange.com/questions/147792",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/135770/"
] | I'm building a system where security is important and I'd like to store session details on the server in a session table. However, I'm concerned that if an attacked got hold of the database, they would have access to all session details for all logged in users.
How do I get round this - is it just to keep the session ... | In general session information is not usually encrypted on the server side. It is generally assumed, as part of the threat model, that if an attacker gains access to the server then they already have everything they need. Session data itself is usually not any more sensitive than the rest of the contents of the databas... | The standard approach to this is to make sessions have a relatively short lifetime - the most common way for an attacker to gain access to the database is through obtaining backups and those sessions would be expired in that backup.
If an attacker has live access to the live database, they likely don't need sessions, ... | https://security.stackexchange.com |
123,535 | [
"https://mathoverflow.net/questions/123535",
"https://mathoverflow.net",
"https://mathoverflow.net/users/2095/"
] | Hello,
Let $X$ be a smooth variety in char. 0. Let us call a $D$-module on $X$ constant, if it is isomorphic to a finite direct sum of the $D$-modules $O$ (the sheaf of regular functions with the usual $D$-module structure).
Then a subquotient of a constant $D$-module should be constant. But how to show it?
The reason... | If your base field $k$ is algebraically closed (or if $X$ has a rational point), then this follows directly from the fact that the category of $\mathcal{O}_X$-coherent flat connections is neutral Tannakian, i.e. equivalent to the category of finite dimensional $k$-representations of some affine $k$-group scheme $G$. Fo... | Suppose that $E\subset\mathcal O_X^n=V\otimes\mathcal O_X$, where $V$ is a finite-dimensional vector space, is a D-module. It is well known that both $E$ and $V\otimes\mathcal O_X/E$, being both D-modules and coherent $\mathcal O_X$-modules, must be locally free. Hence, if $\mathrm{rank}E=k$, there exists a morphism $f... | https://mathoverflow.net |
10,850 | [
"https://chemistry.stackexchange.com/questions/10850",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/5540/"
] | I've been studying organic chemistry and I think trans fats are actually more stable than cis fats. This is because the steric hindrance (for trans) would be lower and the molecule energy would be lower too (more stable).
Why are unsaturated fats usually cis at room temperature if the steric hindrance is higher?
Is ... | Nature produces cis fats exclusively (someone may prove me wrong with some weird example.) This is why unsaturated fats such as vegetable oil have lower melting point than saturated fats of the same MW (the cis double bonds lead to a more compact molecule which gets less tangled with its neighbours.)
The reason nature... | But in biological systems, polyunsaturated fatty acids are cis, and for an animal to oxidize them, they first have to be converted to trans, which takes energy. So I think it depends on whether you are discussing in vitro or in vivo polyunsaturated fats.
| https://chemistry.stackexchange.com |
78,625 | [
"https://physics.stackexchange.com/questions/78625",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/29282/"
] | I read the following problem: Prove that a dielectric medium for which $\varepsilon \to \infty$ behaves as a perfect conductor in the presence of static electric fields.
So, the easy part is that the normal component of an electric field is cancelled when entering the medium, as in a conductor. We also know that the t... | Before proving anything, one should remember that the most general boundary conditions are:
<img src="https://i.stack.imgur.com/aNDY1.png" alt="enter image description here">
For the perfect conductor case, we know <strong><em>intuitively</em></strong> that the electric field is zero. So by setting the electric field... | Inside a perfect conductor, the (static) electric field is zero.
Simply consider Gauss law : $\nabla E=\frac{\rho}{\epsilon}$ so with $\epsilon\rightarrow\infty$ there is no $\nabla E$ : the electric field, if it exists, must be divergence-free.
However, if you don't have any variable magnetic field, $\nabla \times ... | https://physics.stackexchange.com |
54,572 | [
"https://security.stackexchange.com/questions/54572",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/43117/"
] | Is IIS and ASP vulnerable to the same Host Header Attacks we have seen on Apache and Nginx? Specifically the attacks that use the HTTP Host Header to reset a password or implement web-cache poisoning. These specific attacks use the SERVER and HOST variables.
Any insight or help is greatly appreciated.
| In addition, I would add that most attackers don't directly log in via the console or ssh, but instead use bugs in other software in order to break out into a root shell or elevate their privileges. Adding 2-factor authentication or other mechanisms will definitely improve your security posture, but must be viewed as p... | This is called two-factor authentication. It already exists, though it may not be trivial to implement in all cases.
| https://security.stackexchange.com |
37,797 | [
"https://stats.stackexchange.com/questions/37797",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/4701/"
] | I am having some difficulties interpreting the results of an analysis perfomed using lme. I conducted an experiment where the subjects had to estimate the time elapsed in a task involving a spatial measure (e.g. subjects watched a video game where a car travels a certain distance). My goal is to determine if there is a... | I will assume that the modeling assumptions you made are correct and you ran the program properly since your question only addresses the interpretation of the output.
In a linear model involving a single covariate, you can test for a linear association either by testing whether the slope coefficient is 0 or not or by t... | A further clarification. Your question asks whether this test provides evidence of a <em>linear</em> relation. In fact, linearity is an <em>assumption</em> of the model - the model does not test for linearity. Therefore, it is possible that the relationship between variables is, in fact, non-linear. In this case, the m... | https://stats.stackexchange.com |
350,067 | [
"https://physics.stackexchange.com/questions/350067",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/37286/"
] | The usual Klein-Gordon Lagrangian reads
\begin{equation} \mathscr{L}= \frac{1}{2}( \partial _{\mu} \Phi \partial ^{\mu} \Phi -m^2 \Phi^2) \, . \tag1\end{equation}
Without additional symmetry beyond Lorentz symmetry, nothing forbids an additional linear term:
\begin{equation} \mathscr{L}= \frac{1}{2}( \partial _{\m... | Hint 1. What happens if you take the vacuum expectation value of equation $(3)$? You should find $\langle\Phi\rangle\neq 0$. Why is this a bad thing?
Hint 2. (pretty much the same thing as Hint 1 actually) What happens under the field redefinition $\Phi\to\Phi+C/m^2$?
| One obvious change is as well that the usual Klein-Gordon Lagrangian
\begin{equation}
\mathcal{L} = \frac{1}{2}(\partial_{\mu} \Phi^{\dagger} \partial^{\mu} \Phi - m^{2} \Phi^\dagger\Phi)
\end{equation}
has a $U(1)$ symmetry if you consider the field $\Phi$ to be complex.
Meaning that $\mathcal{L}$ is invariant under... | https://physics.stackexchange.com |
306,118 | [
"https://mathoverflow.net/questions/306118",
"https://mathoverflow.net",
"https://mathoverflow.net/users/126661/"
] | I was able to conclude, numerically, the following:
$$s(n) = \sum_{j=0}^n\frac{(-4)^j}{(2j+1)!}\left(\sum_{k=j}^n\frac{(k+1)(2k+1)(k+j)!}{(k-j)!}\right)x^{2j+2}\le n+1$$
for $x\in[0,1]$. For example
\begin{array}{ccc}
n& max(s(n)) & n+1 \\
0& 1.00& 1\\
1& 1.53& 2\\
2& 2.07... | The alternative form can be expressed as
$$ \frac{1}{2} - \frac{n+1}{2} \cos (n+1)t
+ \frac{1}{2}\bigl( \cos nt + \cdots + \cos t \bigr).$$
The third summand has period $2\pi$ and takes its maximum value of $n/2$ when $t=0$. So just from the triangle inequality we get
$$s(n) \le \frac{1}{2} + \frac{n+1}{2} + \frac{... | For $t\in[0,\pi]$,
\begin{align}
s(n)&=\sin(t/2)\sum_{k=0}^n(k+1)\Im(e^{\mathrm{i}(k+1/2)t})\\
&=\Im\left(e^{-\mathrm{i}t/2}\sin(t/2)\sum_{k=0}^n(k+1)e^{\mathrm{i}(k+1)t}\right)\\
&=\Im\left(e^{-\mathrm{i}t/2}\frac{e^{\mathrm{i}t/2}-e^{-\mathrm{i}t/2}}{2\mathrm{i}}\frac{1}{\mathrm{i}}\frac{\,d}{\,dt}\frac{e... | https://mathoverflow.net |
193,473 | [
"https://security.stackexchange.com/questions/193473",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/179932/"
] | Say you just want to encrypt a number. For example, say the number could be any <code>double</code>. A double in C# and Java is 8 bytes.
If you were to encrypt a double using AES:
<pre><code>var cypherText = AES.Encrypt(123d); // 8 bytes
</code></pre>
would that be trivial to crack? If not would it at least be sign... | <h1>No, Unless...</h1>
It shouldn't be possible to "brute-force" possibilities unless you provide an oracle for an attacker to utilize.
An oracle in this case allows the attacker to feed as much chosen plaintext as they'd like into your encryptor, and receive encrypted output to check against.
If this is the case, ... | <h1>Yes, if...</h1>
<ol>
<li>You are stupid enough <strong>not</strong> to fill the remaining 64 bits of the block with random data</li>
<li>The attacker knows that you're doing this (which is generally an assumption one should make according to Kerckhoff).</li>
</ol>
The reason is obvious: AES operates on 128-bit bl... | https://security.stackexchange.com |
218,867 | [
"https://mathoverflow.net/questions/218867",
"https://mathoverflow.net",
"https://mathoverflow.net/users/69078/"
] | Let $l=\min\{s\in \mathbb{N}|0\in s\cdot (\mathbb{F}_{p^n}^\times)^k\}$. Is any information known about this number already as a function of $k$? Any reference would be greatly appreciated!
| Write $q=p^n$. You can replace $k$ by $gcd(k,q-1)$ and assume $k | (q-1)$. Now, trivially, $l=2$ if and only if $(-1)^{(q-1)/k} = 1$. If $l >2$, then $l=3$ if the Fermat curve has a point which, from the Weil bound, happens if $k < q^{1/4}$, approximately. For larger $k$ you may want to use techniques from additi... | Adding on to Felipe's answer, here is a nontrivial example (obviously you can take $k=p^n-1$) where we can make $\ell$ as large as we want if $k$ is very large (almost $p^n-1$). We can assume $k|p^n-1$ and write $d=(p^n-1)/k$.
If $d$ is prime, $d|p^n-1$ but $d\nmid p^m - 1$ for any $m<n$, and the cyclotomic polyno... | https://mathoverflow.net |
312,028 | [
"https://dba.stackexchange.com/questions/312028",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/140660/"
] | I have code to query partition data.
But when the database creates an index, my query does not work because of the LCK_M_SCH_S lock, although I use the NOLOCK hint for all tables.
Is there any way around this lock or get this data without lock?
<pre><code>SELECT DB_NAME() AS DatabaseName
, OBJECT_SCHEMA_NAME(p.... | Try using an EXCEPT…
<pre><code>SELECT InstNum
FROM Table
WHERE Profile = 'C'
EXCEPT
SELECT InstNum
FROM Table
WHERE Profile <> ‘C'
GROUP BY InstNum;
</code></pre>
| <pre><code>WITH sameProfiles AS
(
SELECT
COUNT([instNum]) cnt
, [instNum]
FROM
(
SELECT distinct
[instNum]
, [Profile]
FROM
myTable1 ) q
GROUP BY
[instNum]
HAVING
COUNT([instNum]) = 1)
SELECT DISTINCT
[instnum]
FROM
myTabl... | https://dba.stackexchange.com |
50,123 | [
"https://math.stackexchange.com/questions/50123",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/5032/"
] | I am trying to figure out what is going on in this Surface Area problem:
<img src="https://i.stack.imgur.com/siDqx.png" alt="enter image description here">
As I attempted to illustrate above, It seems like the formula has been applied incorrectly. Where x has been placed should be f(x) which in this problem is (x^(1/3... | The key is that the commutator subgroup is not merely the collection of all commutators, but rather the subgroup <em>generated</em> by these products. That is,
$[HK,L]$ is not equal to the set $\{[hk,\ell]\mid h\in H, k\in K,\ell\in L\}$, but rather is the subgroup generated by such products. Because of that, you easil... | In this same spirit, and Arturo's answer and remark on commutator identities, try to prove the <em>Three Subgroups Lemma</em>: if $N$ is a normal subgroup of $G$, and $K, L, M$ are subgroups of $G$ such that $[K,L,M] \subset N$, and $[L,M,K] \subset N$, then $[M,K,L] \subset N$. Here $[*,*,*] = [[*,*],*]$.
| https://math.stackexchange.com |
687,450 | [
"https://physics.stackexchange.com/questions/687450",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/306598/"
] | Here is the problem I confused a lot.
<blockquote>
A uniform rod of mass m and length l is hanging vertically from the pivot O. A horizontal force F acts at the lower end of the rod. If F always remains horizontal then maximum angular displacement of the rod is______.
</blockquote>
First I thought the problem is of sta... | This is a dynamics problem. You have to equate the work done by the force <span class="math-container">$F$</span> with the energy gained by the rod when it has reached its maximum angular displacement from the vertical. Note that at this point of maximum displacement the angular speed of the rod is zero, so its kinetic... | I will offer some general advice on how to handle ambiguity in a question, and how to tackle hard problems.
First, if you think a question is ambiguous, meaning you think the words have more than one legitimate interpretation in terms of what is physically going on and what is being asked, then a correct scientific res... | https://physics.stackexchange.com |
502,811 | [
"https://physics.stackexchange.com/questions/502811",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/242426/"
] | Given
<span class="math-container">$$
L=L(t, x_i, \dot x_i)
$$</span>
as a function of generalized coordinates/velocity, and
<span class="math-container">$$
p_i:=\frac{\partial L}{\partial \dot x_i},
$$</span>
how can we calculate
<span class="math-container">$$\frac{\partial L}{\partial p_i}?$$</span>
| How light is a feather? Light = color, or light = weight? Those are different questions, so of course they have different answers. When I skim through the literature about how Lorentz trnasformations affect temperature, I see people arguing about how light a feather is.
We can debate how the words should be used, but t... | As the excellent answer by Chiral Anomaly already states, QFT takes no particular stance on the issue unless you specify more precisely what you mean by "temperature". I'll just elaborate on why this would be a hard thing to do.
A very good definition of temperature is that it's whatever a thermometer measure... | https://physics.stackexchange.com |
86,624 | [
"https://stats.stackexchange.com/questions/86624",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/21793/"
] | I would like to get the standard error on a prediction. Using R <code>glm</code>, I can get the SE of the fit for a specific prediction:
<pre><code>mod <- glm(y~wa_WSI, data=mydata, family=gaussian(link="identity"))
predict.glm(mod,newdata=newdata, type="response", se.fit=T)
</code></pre>
But when I compare the pr... | It is hard to answer without knowing more about what <code>mod</code> is. That is why we suggest a reproducible example.
If <code>mod</code> is a glm fit with a 'gaussian' family (the default) then it is just a linear model and you can use <code>predict.lm</code> instead which has the <code>interval</code> argument t... | I would like to throw in a comment for non-normal distributions and non-identity link functions.
<code>se.fit=T</code> yields standard errors of the prediction, i.e. a measure of uncertainty for the predicted value. This prediction, by one of the Central Value Theorems, can be assumed to be normally distributed at the ... | https://stats.stackexchange.com |
271,911 | [
"https://electronics.stackexchange.com/questions/271911",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/131404/"
] | This is my first attempt at working with electronics so sorry for the potentially stupid question!
For my A-Level Product Design project I am making a digital photo frame with built in bluetooth speakers. I am using a Raspberry Pi as the controller for the photo frame and plan to use a bluetooth amp for the speakers. ... | It's best to avoid working in mains power at all: Safety; ease of getting UL/CSA listing; and ease of changing power supplies if you change countries.
You may have noticed that almost every small machine uses a <em>wall wart</em> power supply. This is why. They can focus on their low-voltage gadget and not worry ab... | Are you using splitting the mains power to go to the RPi and speakers?
If so, what I would recommend you do is find out how much voltage and current is required for the bluetooth speakers.
If the bluetooth speakers need just 5 Volts DC, then it becomes easy in the sense that you just need to invest in an (off the she... | https://electronics.stackexchange.com |
324,366 | [
"https://softwareengineering.stackexchange.com/questions/324366",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/233551/"
] | I am writing a program that looks for a solution of a diophantine equation. The program is cycling
<pre><code>for (int d = 0; d <= max; d++) {
for (int c = 0; c < d; c++) {
boolean possibleSolution = true;
possibleSolution =test1(c,d);
if(possibleSolution) {possibleSolution =test2(... | Based on your question <code>break;</code> will NOT work for you. You want to skip all further steps in this cycle, do <code>c++</code> and go for a new round. This is done using <code>continue</code>. Break will end inner cycle do <code>d++</code> and start inner cycle from 0.
| Measure first instead of making assumptions.
The sequence of 30 tests "if (possibleSolution)" is something that a good compiler should be able to optimise away. At the first "if (possibleSolution)" a good compiler would generate code that doesn't only skip one call, but figures out immediately that the second, third,... | https://softwareengineering.stackexchange.com |
1,755 | [
"https://mechanics.stackexchange.com/questions/1755",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/966/"
] | The check engine light just came on in my 2000 Camry. It has 180,781 miles, and the 180k mark is the recommended timing belt replacement mark. I've checked the tightness on the gas cap, and I'm hoping that the light is on just to get me to take my car in to have the timing belt changed. Does Toyota do this? It seem... | There's no immediate way to just know what's wrong, what you need to do is take the car to an advance auto or autozone and have them hook up and ODB-II code reader. They will tell you what code is coming up and can clear the code for you if you'd like. This is a free service, and they will often be able to recommend pa... | I don't think it's got anything to do with the timing belt change interval, but the only way to find out is to hook up the car to an appropriate code reader to find out what code is triggering the check engine light.
| https://mechanics.stackexchange.com |
18,351 | [
"https://bioinformatics.stackexchange.com/questions/18351",
"https://bioinformatics.stackexchange.com",
"https://bioinformatics.stackexchange.com/users/4595/"
] | I have PBMCs of some patients who have recovered from cancer, and I want to look at their immune landscapes.
I got confused when looking at available kits: can I use 5' end kits from 10x instead of 3' end based?
I tried to search for information on this, but I am not clear if 5' can substitute for 3' or if they are jus... | Perhaps you could use this (assuming <code>hg38</code>) as a lookup table:
<pre><code>$ wget -qO- https://ftp.ebi.ac.uk/pub/databases/gencode/Gencode_human/release_39/gencode.v39.annotation.gff3.gz \
| gunzip -c \
| grep -wF exon \
| convert2bed --input=gff --attribute-key="gene_name" - \
>... | I'd use biomart on ensembl.org
| https://bioinformatics.stackexchange.com |
13,050 | [
"https://electronics.stackexchange.com/questions/13050",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/2285/"
] | I am developing a portable device embedding sensors, Bluetooth, MCUs etc.. At present I'm at a prototype stage and I'm starting thinking about production.
What kind of certification steps do I need to follow in order to produce and sell it?
| The more I learn about regulatory compliance, the more I'm convinced that I don't know anything. This is a giant maze of international laws that is difficult to navigate. What I wrote below is just a rough guide, to be taken with a grain of salt. But my last paragraph is very important: You need professional help (... | It depends where in the world you are trying to sell your device. In Europe for example, there are numerous EEC directives affecting the sale of electronic goods, the most well known being :-
<ul>
<li>Electromagnetic Compatability Directive (EMC)</li>
<li>Low Voltage Directive (LVD)</li>
<li>Waste Electrical and Elect... | https://electronics.stackexchange.com |
207,987 | [
"https://softwareengineering.stackexchange.com/questions/207987",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/9346/"
] | Bugs happen and sometimes data has to be fixed in production. What is the safest way to go about this from a big company standpoint? Are there tools that can help? Here are some considerations driving this requirement...
<ol>
<li>We need to log who ran the query and what they ran</li>
<li>Ideally we need to give the p... | <strong>Never ever update production databases manually.</strong>
Write scripts.
Triple check them, and have multiple people do that, not just a single person doing it three times.
Include post-change validation queries in those scripts.
Whenever the situation allows, test the whole change within a transaction whic... | The answer by Marjan Venema is technically valid and should be followed when possible. Alas, <strong>Marjan answers from the point of view of a <em>theorist</em>, or a <em>purist database administrator</em> who likes to make things cleanly. In practice, sometimes business constraints make it impossible to do things in ... | https://softwareengineering.stackexchange.com |
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