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396,921 | [
"https://electronics.stackexchange.com/questions/396921",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/198923/"
] | Is it possible to view and edit avr which is programmed by manufacturer.
| <blockquote>
but I have talked to a few people who are experienced in electronics, and they have suggested using mosfets or PWM for powering long nichrome wire coils.
</blockquote>
That is what I would use to design a circuit to control a nichrome coil too.
But as your instructor already mentioned, you don't need t... | MOSFETS are too complicated (relatively) for a simple task like this, all you need for a heating element is an SCR control, hence the suggestion of a "dimmer" because most dimmers are just a simple triac device. You could also use a simple cheap SSR (Solid State Relay) that does Time Proportional Control, that way you ... | https://electronics.stackexchange.com |
1,577,853 | [
"https://math.stackexchange.com/questions/1577853",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/223701/"
] | The question states to calculate the image of the line $\{\text{Re}(z)=c\}$ under the Mobius transformation $z\mapsto \frac{z+1}{z-1}$. For $c=1$, it can be shown that the line is invariant under the transformation (in the extended real sense, i.e. $1\mapsto \infty$ and $\infty\mapsto 1$. For $c\neq1$, I have a hint sa... | Every non-singular Möbius transformation maps circles and lines to circles and lines. Equivalently, circles on the Riemann sphere are maps to other such circles. That does not mean that circles are mapped to circles or lines are mapped to lines.
For $c\ne 1$, the line where $-\infty < v < \infty$ is mapped ... | As you said, $\infty \mapsto 1$, and you can see that $c\mapsto \frac{c+1}{c-1}$. A Moebius transformation takes a circle to a circle or a line, but you can see that $c+i$ goes to a point outside the real line, so in this case it will be a circle.
Also, this Moebius transformation has real coefficients, so it maps the... | https://math.stackexchange.com |
228,202 | [
"https://physics.stackexchange.com/questions/228202",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/81827/"
] | I have a question about how to move a trapped bubble in a tube.
If we assume to have a horizontal tube, with water on each side of the bubble. The point to the left of the bubble is point 1, while the point to the right is point 2.
The capillary pressure equation is: $\Delta P_{cap}=\frac{2\cdot \sigma \cdot cos(\th... | The stuck bubble problem happens when there is static equilibrium between the circumferential contact forces of surface tension and the forces due to the pressure difference upstream and downstream of the bubble. The gas pressure inside the bubble is uniform and between the upstream and downstream liquid pressures. In ... | From an engineering POV you could use sound, either in the audible or ultrasonic range to disrupt the adhesion. A transducer could be placed in the liquid at one end of the tube and arranged so some sound travels along the tube. If that is not possible them maybe attach a transducer to the outside. All experimental tho... | https://physics.stackexchange.com |
204,445 | [
"https://dba.stackexchange.com/questions/204445",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/149513/"
] | I have a table containing a part time working pattern consisting of start and end dates together with hours values.
Where there is an gap between an end date and a subsequent start date, this indicates the member was full time.
Where the most recent entry has a NULL end date, this indicates the part time work is st... | Assuming hours are added to table in chronological order. If not, sort them before inserting them!
First we need to relate each record to successive records.<br>
You say that is done by matching their hrs1 & hrs2, and making sure the initial record ends one day before the next starts.
Your sample data only include... | SQL Server 2008. So far I've been able to fill in the enddtes by joining on ROW_NUMBER -1 and using COALESCE
<pre><code>;with cte as (
select row_number() over(order by startdte) as ROWNO, * from #hours)
Select c1.STARTDTE, coalesce(c1.enddte, c.startdte -1)
From CTE c1
LEFT JOIN CTE c
On c1.ROWNO = c.ROWNO - 1
</... | https://dba.stackexchange.com |
43,006 | [
"https://security.stackexchange.com/questions/43006",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/31285/"
] | True Scenario:
Representatives from the GCHQ went into The Guardian newspaper corporate office in London and demanded top secret data (leaked by Edward Snowden) be returned to authorities. The senior editor did not comply with the request to turn over the laptop, but agreed to have the machine symbolically destroyed b... | If the laptop was only used to handle sensitive information, then destroying HDD is enough, if the laptop has been without any power for a few hours. However, if the laptop is used to hide sensitive information, there are many places where such information could be stored. Such places include: HDD/SSD, NIC boot ROM, BI... | <strong>You grind the whole laptop and incinerate the remains.</strong>
<hr>
But as you mention focusing on the laptop is <em>merely symbolic</em>.
Any and all data could have been transferred off the laptop prior to destruction. Indeed unless the newspaper is willing lose unrelated information from the laptop, so... | https://security.stackexchange.com |
210,834 | [
"https://dba.stackexchange.com/questions/210834",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/154279/"
] | I have a production database on SQL Server. I want a copy of that database to use for running tests. I'd like the test copy to be updated nightly, so that I always have access to fresh data every morning.
The simple solution would be a full backup and restore each night, and then run the latest migration scripts on ... | The answer to your question is NO. Even rolling back all changes to the test database, you are still stuck with the fact that you cannot restore a differential backup to a database that is already recovered. You will always have to start over by restoring the latest FULL backup, then restore the latest DIFF that belong... | If it runs overnight, until you get into the 12 hour range, who cares how long it takes? As Kevin says, the first diff will work, but the second will fail because you've used your migration scripts to alter the database.
Surely you are already taking full backups of your primary database? You don't need a full backup... | https://dba.stackexchange.com |
498,588 | [
"https://physics.stackexchange.com/questions/498588",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/143993/"
] | When an object is experiencing free fall, it has a constant acceleration and hence an increasing velocity (neglecting friction). Thus its momentum is increasing. But according to law of conservation of momentum, shouldn't there be a corresponding decrease in momentum somewhere else ?
Where is it ?
| Linear Momentum is conserved only in systems with <strong>net external force</strong> equal to <strong>zero</strong>. For a body falling on Earth, it experiences Earth's gravitational force so its linear Momentum increases.
But if you include Earth in your system then definitely, momentum is conserved, as an equal amou... | Linear momentum of a system remains conserved unless an external force acts on it. Since during free fall, a gravitational force acts on the body, it's momentum will not remain conserved. However, if we change the reference in such a manner that the gravitational force becomes an internal force of the system, i.e. rega... | https://physics.stackexchange.com |
57,460 | [
"https://electronics.stackexchange.com/questions/57460",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/17853/"
] | I have to build a counter using only D Flip-Flops and wires. What's the approach?
| D Flip-Flops are great for applications like counters because they "remember" their own states. They are sequential based components instead of combinational.
<img src="https://i.stack.imgur.com/fCzlY.jpg" alt="enter image description here">
As you can see in the picture, for each bit, a D Flip-Flop is required.
NOT... | The key to your homework question is "Build a 4-bit synchronous <strong>direct</strong> counter using ONLY D Flip-Flops and wires." note: <strong>emphasis</strong> mine.
This might be solved using either a "one hot" or a "thermometer code" style counter, which are sometimes used in FPGA designs. Not the most efficie... | https://electronics.stackexchange.com |
289,084 | [
"https://mathoverflow.net/questions/289084",
"https://mathoverflow.net",
"https://mathoverflow.net/users/60732/"
] | Is 47 the largest number which has a unique partition into five parts (15, 10, 10, 6, 6), no two of which are relatively prime?
| Yes. Suppose $n>47$.
If $2\mid n$, we can take $(n-8,2,2,2,2),(n-10,4,2,2,2)$, which are distinct partitions for $n\geq 14$.
If $3\mid n$, we can take $(n-12,3,3,3,3),(n-15,6,3,3,3)$, which are distinct partitions for $n\geq 21$.
If $n\equiv 1\pmod 6$, we can take $(n-37,15,10,6,6),(n-43,15,12,10,6)$, which are d... | Here is a quick demonstration that it is effectively solvable for any number of parts.
A sufficiently large number that has exactly one partition with the property must be prime. Otherwise we can write it as $n = a \cdot b$ with $1 < a \leq b$ and partition $b$ into $k$ parts in two different ways, then multiply t... | https://mathoverflow.net |
121,686 | [
"https://dba.stackexchange.com/questions/121686",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/70410/"
] | Oracle Recommended Patches -- Oracle Database (Doc ID 756671.1) says the following 4 patches are recommended to be applied to 11.2.0.4 database.
Note:21744343.8 Combo of 11.2.0.4.5 OJVM PSU and 11.2.0.4.8 DB PSU (Oct 2015)
Note:21555791.8 Oracle JavaVM Component 11.2.0.4.5 Database PSU (Oct 2015) (OJVM PSU)
Note:213... | You should download only the combo patch - in essence it is the two other - DB and OJVM patches bundled in one archive. After that the patching sequence is as follows:
<ol>
<li>Apply the DB PSU, but do not run any post-install steps</li>
<li>Apply the Java PSU</li>
<li>Run the Java PSU post-install steps</li>
<li>Run ... | Install the first one, that is a combo patch that includes the latest versions of all the others. The others are just the seperate PSU or OJVM patches.
| https://dba.stackexchange.com |
30,154 | [
"https://softwareengineering.stackexchange.com/questions/30154",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/5940/"
] | I see mostly everyone distinguishes between being a Software Engineer and being in IT on the jobs sections of their site. What are the definitions of each, and the distinction between them?
| IT (Information Technology) is a catch-all for the industry at present, any job that is primarily to do with the operation of computers or developing for them is within the "IT industry/sector".
However, within the UK at least, job descriptions and adverts tend to reserve "IT" within a job <em>title</em>, as administr... | In Morocco, to be an IT, it's required to study programming in general for 2 years.. but to be an engineer , you must study at least for 5 years , technically... the software engineer is somemone who not only writes programs, but he takes security, flexibility and reusibility in mind. An IT can only be specialized in e... | https://softwareengineering.stackexchange.com |
598,806 | [
"https://electronics.stackexchange.com/questions/598806",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/7910/"
] | I have a 80's Christmas incandescent light string without any kind of electronics. They are static until you replace one normal bulb with one marked bulb.
It seems to be that the marked bulb (that actually also works) has some kind of special filament that burns out but still works, with some different resistance. Then... | The red-tipped bulb is a "twinkle bulb" with a bimetallic strip inside which will disconnect the circuit when the bulb heats up. Then the bulb will cool down and the bimetallic strip will close the circuit again. Because all the bulbs are wired in series (fixed ones + twinkle), the whole string will blink off... | I don't know how your specific light works, but one way to do this without complex circuitry is using a polyfuse, e.g. a polymeric positive temperature coefficient device (PPTC).
Such a polyfuse trips once its temperature exceeds a certain threshold.
Initially, the temperature of the PPTC is low, but once current flows... | https://electronics.stackexchange.com |
601,223 | [
"https://stats.stackexchange.com/questions/601223",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/253215/"
] | I hope someone can clear the doubt on conditional distribution, in a text, it mentions:
To interpret
the notation <span class="math-container">$f_{X \mid Y}(x \mid y)$</span>, we need to be clear that there is nothing random about
the random variable <span class="math-container">$Y$</span>, since it is already fixed. W... | Without getting very much into measure theory,consider the random vector <span class="math-container">$(X,Y)$</span> with density <span class="math-container">$f_{X,Y}(\cdot,\cdot)$</span> (wrt a dominating measure <span class="math-container">$\text d\mu(x,y)$</span>) decomposed as
<span class="math-container">$$f_{X,... | If you take the expectation of <span class="math-container">$X$</span>, it's not a function of <span class="math-container">$X$</span>. You integrate <span class="math-container">$xf(x)$</span> over <span class="math-container">$x$</span>, and so <span class="math-container">$x$</span> is gone, producing only a number.... | https://stats.stackexchange.com |
446,964 | [
"https://physics.stackexchange.com/questions/446964",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/75502/"
] | We know light travels in a straight line but spacetime around an object with mass is curved, anyhow I'm wondering gravitational wave going at speed of light could also be bended by stars and probably black hole?
I read some similar question and the answer are mixed but mostly yes, is it because that's the way how all ... | Of course no experiment or observation has yet been done that can answer your question, so any answer you're given will be based on theory -- and not everyone adheres to the same theory.
There is pretty convincing evidence that gravitational waves carry positive energy (e.g., the "chirp" observed in gravitational wave... | This is worked out to some degree in detail in MTW Gravitation, a classic text. There is a whole section on the bending of GW paths in the presence on a strongly curved background. The basic approach is the same as for GW on a flat BG. You assume a BG metric and do perturbation theory with a weak fluctuation on top ... | https://physics.stackexchange.com |
89,611 | [
"https://mechanics.stackexchange.com/questions/89611",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/72191/"
] | Instead of a compressive force, it would work in a tensioning force (like an elastic), so you can imagine how different it would be installed on the vehicle.
The idea is that the liquid - inside an apparatus similar to a shock absorber - would be in a vacuum, like a tendon, the vacuum on the liquid would pull the liqui... | What you propose is possible in theory but impractical in practice.
When using a compressed gas, say nitrogen which is commonly used in shocks and such, the limit of pressure is the lesser of the point at which the gas liquifies or the structural integrity of the unit itself. But in practice pressure of 1000 psi or mo... | If the only thing you are wanting to do for some type of shock device to pull rather than to push, the simple solution is not to use vacuum, but instead, push from the other direction. Shocks are designed to push out (extend the rod). This is done with (IIRC) nitrogen gas. To get it to retract, it would just need to be... | https://mechanics.stackexchange.com |
52,816 | [
"https://softwareengineering.stackexchange.com/questions/52816",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/4091/"
] | I find working in isolation, on a piece of code that won't be seen by anyone else for weeks, draining. I'm looking for ideas to try to keep myself productive and motivated.
What do you do to remain motivated and productive, when given a long term programming task, and working on your own (for example, from home, witho... | <strong>Maintain a balance.</strong>
Given something novel (e.g. playing a game, having a beer, etc.), we're able to focus and do that one thing for an extended amount of time. The only way to power through a mundane task (without overdosing on <em>coffee</em>) is to <strong>maintain a balance</strong>. <em>I say 'mun... | Setting targets is the only way for me to work...
By the end of the hour, I want to have X done. And by lunch I want Y & Z finished. At lunch you review where you have got to, then set new targets. For completing each target, there is a reward; the reward might be a cuppa, or a walk from your desk or 10 mins on s... | https://softwareengineering.stackexchange.com |
1,741,616 | [
"https://math.stackexchange.com/questions/1741616",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/129566/"
] | I want to do the following integral using the change variables $u = x+y$ and $ v= y/x$:
$$\int_U \frac{1}{x^2}\,{dU}$$
where $U$ is the region such that $1 \le x+y \le 2$ and $x \le y \le 2x$, and given that $x >0$.
I calculated the Jacobian $\displaystyle \begin{vmatrix}
\dfrac{\partial (u, v)}{\partial (x, y)}... | You need to solve $u=x+y$ and $v=y/x$ for $x$ by eliminating $y$. For example, $y=vx$ and thus $u=x+vx$, so $x=u/(v+1)$. Of course you get the same result using the equations the other way around: $y=u-x$ and thus $v=(u-x)/x$, so $x=u/(v+1)$.
| Now what you need to do is to represent x and y in terms of u and v.
Since you have two equations: $x+y=u$ and $\frac{y}{x}=v$, you can write $y=xv$ and plug this into the first equation. You'll get $x+xv=u$, rewrite this as $x=\frac{u}{1+v}$.
Then you substitute every x and y in the integral with u and v, and try to... | https://math.stackexchange.com |
375,763 | [
"https://softwareengineering.stackexchange.com/questions/375763",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/311258/"
] | I'm currently building an opensource administration interface to help people easily manage their contens/websites.
Application built on Vue.js and heavily rely on json configurations for which fields to show or which data to send server via forms.
Since rest apis doesnt have popular standardization (besides jsonapi) ... | There has been quite a lot written on the subject of api pagination. I think it's fair to say that the naive approach 'get page 5, 20 items per page' doesn't work well with changeable data, filters, sorting etc.
ie,
<ul>
<li>if I get 20 items and then apply a filter, i don't have 20 items</li>
<li>if I request page ... | Paging in multi-user environments where data can change between requests for pages is tricky at best, pointless at worst.
Say your first request got you a page of 10 items out of a total of 100. Now you do another request for the second page of 10 items, but the number of items has changed to 101, with the new item ex... | https://softwareengineering.stackexchange.com |
33,162 | [
"https://electronics.stackexchange.com/questions/33162",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/4330/"
] | I'm trying to monitor the health of my duracell consumer battery.
Is there an IC that can do this? I've come across the MAX6775, but it appears that you have to use a voltage divider. Isn't this drawing excessive current? This is for a ultra low power application.
Ideally, I'd like to switch on and off the monitor IC... | The context of the statement in the question is the important thing. In simple terms, current takes the path of least resistance (actually current is inversely proportional to resistance, so a less resistive path will draw more current for a given potential). The statement in question was made in the context of an ungr... | My father received a 33 kV shock and survived due to this effect. He was working at an electrical substation and climbed up a ladder to work on a transformer. Unfortunately the wrong transformer was switched off and he climbed up a live one instead. As he reached up, a spark went into his palm and out through his forea... | https://electronics.stackexchange.com |
112,564 | [
"https://mathoverflow.net/questions/112564",
"https://mathoverflow.net",
"https://mathoverflow.net/users/27990/"
] | Hello,
suppose $R$ is a non-commutative ring of finite (left) global dimension and $M$ is a finitely generated (left) $R$-module.
So we know that there is a projective resolution of $M$ of finite length. The first term $P_0$ of the standard resolution will be finitely generated free. However, the next step would tak... | Let me start recalling the Schanuel's Lemma:
<em>If $M$ is a module and $P,P'$ are projective modules, then for every short exact sequences $0\to K\to P\to M\to 0$ and $0\to K'\to P'\to M\to 0$, there is an isomorphism $K\oplus P'\cong K'\oplus P$.</em>
So, if you have a short exact sequence $0\to K\to P\to M\to 0$ w... | The last sentence in the answer by Simone Virili can easily be generalised as follows:
If $R$ is left coherent, then a left $R$-module has a projective resolution whose components are of finite type if and only if it is of finite presentation.
| https://mathoverflow.net |
1,818 | [
"https://mathoverflow.net/questions/1818",
"https://mathoverflow.net",
"https://mathoverflow.net/users/763/"
] | As someone whose knowledge of cohomology is patchy and picked up on a need-to-know basis, and whose algebraic geometry is even worse, I wondered if someone could help with this question. (I ran into it a while back while trying to answer some questions about the <em>Fourier</em> algebras of compact Lie groups.) The sol... | I am very far from an expert on the subject, but I think the Hochschild homology of the coordinate ring should be the algebraic de Rham complex of your variety SO(n, R)--<em>not</em> the cohomology of the complex, just the groups in the complex (with 0 differential if you like). This is the Hochschild-Kostant-Rosenber... | First some general references on homology of lie groups:
<ul>
<li>Cohomology Theory of Lie Groups and Lie Algebras
Claude Chevalley and Samuel Eilenberg
Transactions of the American Mathematical Society, Vol. 63, No. 1 (Jan., 1948), pp. 85-124</li>
<li>Borel, Armand
Homology and cohomology of compact connected Lie gro... | https://mathoverflow.net |
139,009 | [
"https://mathoverflow.net/questions/139009",
"https://mathoverflow.net",
"https://mathoverflow.net/users/4649/"
] | Let $H^*$ and $K^*$ be two cohomology theories, and $X$ a reasonable spectrum. Here, I'm thinking that $H^*$ is singular cohomology (and for my purposes, rational cohomology will suffice), and $K$ is a Morava K-theory.
Suppose that I understand $H^*(X)$ and $K^*(X)$. Is there any way to use this (or other!) informat... | I'm going to phrase in terms of $H$-homology instead of cohomology.
<ul>
<li>If $H$ is a finite spectrum, smashing with it always commutes across the limit. More generally, if you express $H$ as a (homotopy) colimit of finite spectra -- eg, by taking a cellular filtration -- then you get a directed system of spectral... | $\newcommand{\Q}{\mathbb{Q}}\newcommand{\S}{\mathbb{S}}\newcommand{\sm}{\wedge}\newcommand{\Z}{\mathbb{Z}}$Let me give not an answer but instead a long comment that says this is a hard question to which we do not know the answer in at least one "basic" case: $K = K(n)$, $H = H\Q$, $X = \S^0$. This should at least lend... | https://mathoverflow.net |
343,337 | [
"https://physics.stackexchange.com/questions/343337",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/138929/"
] | For instance, take a light bulb. It has voltage and power rating, like "230V, 60W". However, for alternating current, which they usually consume, power is alternating too. Then, the most reasonable power ratings to use are the average power, which can be used to calculate heat, and the peak power, which can be used to ... | Almost all AC devices have their voltage and current capabilities stated in RMS terms (root mean square), which make it easy to calculate power with equations like $P=VI$. For a sinusoidal, this is $V_{rms}=\frac{1}{\sqrt 2}V_{max}$.
You could state the power terms in either notation, but history has shown that the R... | <blockquote>
Then, the most reasonable wattage ratings to use are the average wattage, which can be used to calculate heat, and the peak wattage, which can be used to calculate electrical stuff. So, which one is rated?
</blockquote>
The average is typically specified.
The instantaneous power is not as useful as you... | https://physics.stackexchange.com |
29,578 | [
"https://mathoverflow.net/questions/29578",
"https://mathoverflow.net",
"https://mathoverflow.net/users/7094/"
] | Given a conflict graph $G = (V, E)$, a man has to transport a set $V$ of items/vertices across the river. Two items are connected by an edge in $E$, if they are conflicting and thus cannot be left alone together without human supervision. The available boat has capacity $b\geq 1$, and thus can carry the man together wi... | Maybe there is a solution. But, for that I assume there is an upper bound in the number of rounds needed, say n, and that the value b is fixed upfront. Then, there is the following EMSO formula,
$\exists L_{1} \exists B_{1} \exists R_{1} ... \exists L_{n} \exists B_{n} \exists R_{n} \phi(L_{1},B_{1},R_{1} ...,L_{n},B_... | I am not sure whether it is definitely expressible in MSO or not.
| https://mathoverflow.net |
166,190 | [
"https://mathoverflow.net/questions/166190",
"https://mathoverflow.net",
"https://mathoverflow.net/users/16702/"
] | For each commutative monoid $M$, there exists a "groupification" $\widehat{M}$, i.e. an abelian group that satisfies an obvious universal property.
I tried to prove the following: If in the diagram of Monoids
$$ L \stackrel{j}{\rightarrow} M \stackrel{i_1, i_2}{\rightrightarrows} N$$
the morphism of monoids $j$ is an ... | Let $A=\mathbb{N}\cup\{\infty\}$, considered as a monoid under addition. Let $M=\mathbb{N}$, $N=A\oplus A$, $i_1(n)=(n,0)$ and $i_2(n)=(0,n)$. Then the equalizer of $i_1$ and $i_2$ is $0\to\mathbb{N}$. But $\widehat{N}=0$, so $0\to\widehat{M}\to\widehat{N}$ is not exact.
If you want an example without absorbing ele... | This is not true. This might be a simple example. Let $M=(\mathbb{N}, +)$ be the monoid of natural numbers (under addition) and let $N=(\mathbb{N}, \times)$ be the monoid of natural numbers under multiplication. Put $i(n)=2^n$ and $j(n)=3^n$. Then $L=0$. However, the groupified sequence will be $0\to\mathbb{Z}\to 0$.
... | https://mathoverflow.net |
382,667 | [
"https://physics.stackexchange.com/questions/382667",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/182004/"
] | This might be a stupid question but I could not find the answer in my textbook or on the internet with a few searches.
So I believe when an atomic electron moves down to a lower energy level it emits radiation in the process. However since the energy levels are discrete, the photons released have specific energies an... | In liquids and solids the difference in energy between energy levels becomes very small, due to the electron clouds of several atoms bein in very close proximity of one another. These similar energy levels will form 'bands' of indistinguishable spectral lines.
In gases however, atoms will be spaced loosely enough suc... | You see line spectra usually only in gases because there the interaction between the atoms can be neglected. In gases with high pressures you get the so-called collision broadening of the lines which eventually become bands. Similarly, in liquids and solid the atoms are so close that the interaction between them leads ... | https://physics.stackexchange.com |
69,378 | [
"https://physics.stackexchange.com/questions/69378",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/26373/"
] | I have a doubt over the Kelvin and Planck's statement of thermodynamics' second law, in particular applied to a cycle. Let's take a Carnot cycle as an example, and let's call the first two transformations (the isotherm and the adiabatic) done. Now, isn't it obvious that the machine has to give up heat to go back to the... | Every thermodynamic system satisfies the first law during a given process;
$$
\Delta E = Q-W
$$
Here $\Delta E$ is the change in its internal energy, $Q$ is the heat transferred to the system, and $W$ is the work done by the system. For a system undergoing a cyclic process, namely one for which it starts and ends in... | in simple words we can expain kp statement as it is impossible to completely convert heat into work in a cycle(*).
while in case of a single process say a isothermal expansion as dt=0 therfore ineternal energy which is a function of temp so dE = 0 neglecting k.e and p.e so from 1st law we get dq = dw .
in isotherma... | https://physics.stackexchange.com |
338,182 | [
"https://physics.stackexchange.com/questions/338182",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/158550/"
] | I was looking for a quantum physics section but I did not find any, so I decided to post my question here.
I've been reading articles and watching some videos on quantum computing these last few days, but there's something that I cannot understand. Qubits can have the state of 1 and 0 like binary computers, but they ... | The two possible basis states are $\vert 0\rangle$ and $\vert 1\rangle$. $0$ and $1$ are in fact any two binary values, and could be understood as <em>spin up</em> or <em>spin down</em>, or as <em>right polarized</em> or <em>left polarized</em> or any other two properties of a quantum system that are mutually exclusiv... | A qubit will generally be in a state
$$ |\psi\rangle = \alpha |0\rangle + \beta |1\rangle $$
where $\alpha, \beta \in \mathbb{C}$ and $|\alpha|^2 + |\beta|^2 = 1$. This is a superposition of the $|1\rangle$ and the $|0\rangle$ state. The system being in a state 1 is only a special case of this, namely $\alpha = 1, \bet... | https://physics.stackexchange.com |
389,231 | [
"https://softwareengineering.stackexchange.com/questions/389231",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/306685/"
] | <strong>Background</strong>
While writing a new component, I m in middle of making a decision of SQL/NOSQL database (Mongo vs Mysql) for my storage layer. As of today, mysql seems to be a perfect fit for my use-case (6-7 domain entities, closely related to each other). Still, I want to keep my integrations with the da... | The best way to guarantee that you stay reasonably decoupled from the database, but at the same time remain free to use any feature of it, is to <strong>not create an abstraction layer for the database</strong>. (Well, unless you have the explicit requirement <em>now</em>, that you need to support multiple databases. O... | Why would you wish to create such a degree of abstraction?
The very reason that different storage technologies exist, is because each have a matrix of advantages and disadvantages that cannot be made general - not even by the best minds of the industry and the economic resources of massive corporations.
If any stora... | https://softwareengineering.stackexchange.com |
628,794 | [
"https://physics.stackexchange.com/questions/628794",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/102501/"
] | Different units for the same quantity usually only differ by a multiplicative constant, such as inch and meter, but °F and K differ by both a multiplicative constant and an additive one. Does that make them so different they're really measuring different quantities?
If they're the same quantity, then where's the limit ... | Fahrenheit and Centigrade scale are related via a linear transformation:
<span class="math-container">$$
T_F = a T_C + b \Leftrightarrow T_C=\alpha T_F + \beta,
$$</span>
so they are really the: once we know one, we automatically know the other. Note that this would be also true for more general functions (e.g., if we ... | Here is one way of looking at this which might help you.
Imagine an old-school bulb-type thermometer. You stick the bulb into the water bath, the red alcohol meniscus inside the thermometer takes up a position along the length of the thermometer, and then you read the temperature of the bath off the scale point nearest... | https://physics.stackexchange.com |
96,154 | [
"https://softwareengineering.stackexchange.com/questions/96154",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/12879/"
] | I have more than 6 years of experience with .Net (C#/ASP.NET) and Visual Studio. But recently I'm tempted to switch to Linux/Mono. It comes up with some questions:
<ol>
<li>Can I run my old Visual Studio projects and source codes in Mono correctly?</li>
<li>My clients use IIS for ASP.NET and MS Windows for Winform and... | <blockquote>
Can I run my old Visual Studio projects and source codes in Mono correctly?
</blockquote>
For the most part. There's some magic that you can do the mono compiler won't handle. Most people don't do that though. MonoDevelop will open up VS solution and project files normally without a hitch.
<blockquote... | The thing to know about ASP.Net and Mono is that Mono just dropped support for ASP.net.
<code>***</code> Note: hmm.... I could have sworn I heard this on a Jeff & Joel podcast, but Googling around doesn't give me a good reference.
| https://softwareengineering.stackexchange.com |
130,843 | [
"https://dba.stackexchange.com/questions/130843",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/32863/"
] | I have a quick question regarding the cleaning of no longer necessary transaction log backups. So, I have a database in FULL recovery mode. Every night, at 10 PM a backup takes place (via maintenance plans). Now, I also have a different maintenance plan, that takes a transaction log backup every 15 minutes during the w... | Have you verified the backup you took passes a VERIFYONLY, along with a full restore on a separate server passing a CHECKDB evaluation? I ask this because I've seen situations where a person took a backup, they found out that it later failed a CHECKDB, but they had removed the transaction logs for that day, so they co... | I would play it safe and delete files older than three days, a week, two weeks, what have you. You never know when management will run to you saying someone accidentally deleted data yesterday or before the weekend and they need to restore to a point in time. Make sure you retain enough data to cover all of those possi... | https://dba.stackexchange.com |
240,676 | [
"https://security.stackexchange.com/questions/240676",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/72327/"
] | Instead of collecting various logs into the SIEM, can a full packet capture solution be better in terms of having to manage so many log sources?
| Just by capturing the full traffic the task does not magically gets less complex. The traffic still needs to be processed and the information extracted. In contrary, capturing all traffic inside a huge network and processing it in real-time is a huge task: there is lots of bandwidth needed to collect all traffic in the... | There is solutions such as moloch, or commercial ones that can store pcaps at big scale, however as Steffen mention the system will need a lot of resources, what the majority of the systems do is use netflow data is you can summarize as an abstraction of the flow and send this data to the SIEM system. Of course if you ... | https://security.stackexchange.com |
142,663 | [
"https://cs.stackexchange.com/questions/142663",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/139900/"
] | I understand uncountably many such languages exist, and the rationale for it is clear to me.
I just cannot think of one trivial, easy-to-prove example.
For instance, the complement of a^nb^nc^n is CF, and for more complicated examples I'm often not even sure what the complement is.
| In one of your comments you remark that for unary languages this is more intuitive for you. We can simply adapt a unary language to suit your goals by adding the other letters to the alphabet.
Consider <span class="math-container">$L_1 = \{a^{n^2} \mid n\ge 1 \}$</span> to be a non-regular language over <span class="ma... | Let <span class="math-container">$L = \{a^n b^n c^n \mid n \ge 0\}$</span>.
Define <span class="math-container">$L' = \{aw \mid w \in L \} \cup \{ bw \mid w \in \overline{L}\} \cup \{\varepsilon\}$</span>.
Notice that <span class="math-container">$L'$</span> is not context free by an application of the pumping lemma on... | https://cs.stackexchange.com |
81,764 | [
"https://cs.stackexchange.com/questions/81764",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/77464/"
] | I'm given that two sets, $A$ and $B$ are enumerable. I have to show that there exist subsets $A \supset C$ and $B \supset D$ ($C$ and $D$ also enumerable) such that $C$ and $D$ are disjoint and $A\cup B = C \cup D$. I was thinking to take $A=B=\{0,1\}$ and define $C=\{n : f(n)=1\}$, $D=\{n: f(n)=0\}$.
Do you think thi... | This answer assumes that <em>enumerable</em> means <em>recursively enumerable</em>.
Here is an enumerator for $C$:
<ul>
<li>Run enumerators for $A,B$ in parallel.</li>
<li>Whenever the enumerator for $A$ outputs a word, check whether the word has already been enumerated by the enumerator for $B$; if it hasn't, output... | Can you use the fact that a subset of an enumerable set is enumerable? If you can, then you could choose $C=A$ and $D=B\setminus A$ (set difference). Then $C$ and $D$ are enumerable and it's easy to show that $A\cup B=C\cup D$.
| https://cs.stackexchange.com |
2,020,517 | [
"https://math.stackexchange.com/questions/2020517",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/343230/"
] | I`m trying to prove that the volume of the intersection of the $n$-dimensional euclidean unit ball with the slab $-1/2 < x_{1} < 1/2$ is greater than $0.96$ times the volume of the $n$-dimensional euclidean unit ball for large $n$.
In the middle of the computations, I got stuck on compute
$$
\lim_{n \to \infty}\... | For your question on the cosine integral, do you know Laplace's method? It will give you the limit.
I think there might be easier ways to go for your result though. Essentially you want to show
$$\tag 1 \frac{\int_{1/2}^1(1-x^2)^{(n-1)/2}\,dx}{\int_{0}^1(1-x^2)^{(n-1)/2}\,dx} \to 0$$
as $n\to \infty.$ I've used Fubi... | Rewrite the integral as
$$\sqrt{n} \int_0^{\pi/6} d\theta \, e^{n \log{(\cos{\theta})}} $$
We can apply Laplace's method here. There is a min at the origin; all contributions outside of this stationary point will be exponentially small. Thus, as $n \to \infty$, we may approximate the integrand with a Taylor series ... | https://math.stackexchange.com |
33,640 | [
"https://quant.stackexchange.com/questions/33640",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/27429/"
] | when comparing the Sharpe Ratio (SR) of two different funds, does it make a difference, whether I use excess returs (returns - risk free rate) or returns (without dedcuting the risk free rate, assuming the risk free rate is always 0%) in the numerator? Since I am subtracting the same risk free rate from the returns of... | No, this is not the same. For example, consider the scenario
$$
\begin{align*}
r_A &= 10\% \quad\quad \sigma_A = 10\% \\
r_B &= 1.5\% \quad\quad \sigma_B = 1\% \\
\end{align*}
$$
If $r_f=1\%$,
$$
\text{SR}_A=0.90 \quad\quad \text{SR}_B=0.50
$$
then $A$ has the higher sharpe.
Now if $r_f=0\%$,
$$
\text{SR}_A=1.... | A motivating idea for the Sharpe Ratio is that the measure is invariant to leverage. Let's say we lever up $\alpha$ on the excess return $r^A - r^f$ to have the excess return $r^x = \alpha \left(r^A - r^f \right) $
Trivially, the Sharpe Ratio is unchanged:
\begin{align*}
\mathit{SR} &= \frac{\operatorname{E}[\alp... | https://quant.stackexchange.com |
674,567 | [
"https://physics.stackexchange.com/questions/674567",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/312497/"
] | I had a problem in my book stating:
<blockquote>
A boy lifts a ball with mass <span class="math-container">$m$</span> with force of constant magnitude <span class="math-container">$F$</span> to a height <span class="math-container">$h$</span>.<br />
Calculate the force magnitude <span class="math-container">$F$</span> ... | This is a horribly written question (the exercise, not your post).
You are correct in your reasoning. If the ball starts at rest, the force <span class="math-container">$F$</span> needs to be larger than <span class="math-container">$mg$</span> in order for it to begin moving upwards.
However, if the ball already start... | Yes, a very poorly worded question. First of all, we must make some assumptions. So let’s assume
<ul>
<li>the force <span class="math-container">$F$</span> is applied directly to the ball</li>
<li>the ball is lifted vertically through a height <span class="math-container">$h$</span></li>
<li>the ball is stationary at t... | https://physics.stackexchange.com |
81,278 | [
"https://softwareengineering.stackexchange.com/questions/81278",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/3139/"
] | If I have <code>n</code> rows in a database, and I want to compare every row against each other, how many loops (or processes) would I need to do.
Is it <strong>n<sup>2</sup></strong>? (so if I have 30,000 rows then it would be 900,000,000)
Or is it:
<pre><code>for (i = 0; i < 30000; i++) {
for (j = i + 1; j ... | It's <strong>O</strong>(n<sup>2</sup>) by definition.
The distinction between n<sup>2</sup> and your n*(n-1)/2 algorithm doesn't matter.
You need to work out a far, far better algorithm for locating "duplicates". This "similarity check" business is probably the kind of thing that requires a much smarter algorithm th... | I think it's <code>((n*(n-1))/2)</code>
Your code loop is correct, It's (n*n-1) because you don't have to compare each row to himself and divided by 2 because row X compared to row Y is the same as row Y compared to row X
Looking at the following it should be clear (if you see it as table with n rows and n columns) :... | https://softwareengineering.stackexchange.com |
105,557 | [
"https://security.stackexchange.com/questions/105557",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/91274/"
] | Our password reset logic works as follows:
<ol>
<li>user clicks "forgot my password" and enters the email address of the account</li>
<li>email is sent with a cryptographically secure token, and the email address of the account for which the pw needs to be reset.</li>
<li>user clicks the link, and if the token matches... | <strong>EDIT - Two answers below.</strong>
<strong>Original Answer</strong>
You could embed another ID in the link which tracks back to the original Session ID.
Is the requirement that the email link, when clicked launches in the same browser or can it be on a different browser? What if it's on a different machine? ... | I really have to disagree with the other answers, when answering the question as asked.
First, let's assume the attacker knows your email address, and has a way of reading a reset email sent to you.
If the attacker can read the email sent to you, he almost certainly knows what your email address is. You also said th... | https://security.stackexchange.com |
125,883 | [
"https://chemistry.stackexchange.com/questions/125883",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/87336/"
] | The thermal stability of alkaline earth metals increases down the group for hydroxides i-e., Be(OH)2 is less stable than Ba(OH)2.
The solubility also increases down the group for these compounds i-e., Be(OH)2 is less soluble in water as compared to Ba(OH)2. Hence for Hydroxides of group 2 elements the solubility and th... | <blockquote>
How can an Organic chemist know the <em>chemical formula</em> of a natural product she isolated from an organism?
</blockquote>
There are two levels of answers. One is historical and one is modern. Historically, determining the chemical formula for had been a trivial job for most small or medium sized m... | This question cannot be answered in such a Forum. Because there is no unique answer. The method for giving a formula is a huge process. It takes hours and days and weeks to the university students and professors to be able to answer this question. Sometimes a given process is specific for a given product, and not appli... | https://chemistry.stackexchange.com |
202,227 | [
"https://physics.stackexchange.com/questions/202227",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/45907/"
] | Imagine the following scenario:
I have one lamp, and I would like to turn it stronger, like a strength of 2 lamps.
Can I do it, passing the light (for example) through one crystal or a special glass? If not, has another way to do it?
| Consider the angle between the two vectors as $\theta$ and the following rules
$$\begin{align}
\vec{a}\cdot\vec{b} &= \|\vec{a}\| \|\vec{b}\| \cos\theta &
\|\vec{a}\times\vec{b}\| & = \|\vec{a}\| \|\vec{b}\| \sin\theta
\end{align} $$
Now to construct the parallel vector use the direction of $\vec{b}... | When you calculate the "parallel" vector, you should not use the dot product of $a\cdot b$ but instead the <em>normalized</em> dot product $$\frac{a\cdot b}{|b|}$$ times the unit vector $b$. The projection of $a$ onto $b$ should always be independent of the length of $b$.
| https://physics.stackexchange.com |
581,564 | [
"https://physics.stackexchange.com/questions/581564",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/275423/"
] | <strong>Question:</strong>
Show that the line of action of the buoyancy force on an arbitrarily shaped body submerged in a liquid passes through its center of mass
<strong>My observation:</strong> I know that buoyant force is equal to the weight of the submerged body. But how can I proceed further?
| <blockquote>
My observation: I know that buoyant force is equal to the weight of
the submerged body. But how can I proceed further?
</blockquote>
Be careful.
The buoyant force is only equal to the weight of the submerged body if the submerged body is floating, that is, if the density of the submerged body is equal to o... | <blockquote>
But how can I proceed further?
</blockquote>
The buoyant force is actually a net force due to the interaction of each part of the surface of the object with each interface with the surrounding atmosphere (liquid or gas). On each little area of the surface there is a normal force contribution. The vector su... | https://physics.stackexchange.com |
246,600 | [
"https://security.stackexchange.com/questions/246600",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/253411/"
] | I've searched for it but couldn't find a direct answer to this relatively simple question. Most questions are about a 3rd party intersepting the HTTPS POST or GET parameters inbetween client and server.
But my question is: can a PC user somehow see the HTTPS POST or GET parameters?
My C++ application sends HTTPS POST a... | HTTPS protects against man in the middle attackes. Machines between the client and the server can not see HTTP GET or POST parameters.
However, other applications on the client (e.g. a TLS intercepting proxy) could potentially read the entire request, parameters included, in clear text. Or an application with sufficien... | Yes. The user could decompile your program, add some code that writes the parameters into a file, then compile it again and run it.
| https://security.stackexchange.com |
194,133 | [
"https://stats.stackexchange.com/questions/194133",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/94716/"
] | <strong>Binary logistic regression in R</strong>
I have derived the chi square statistic and degrees of freedom for my model (200.7839, 8, respectively) however, when I attempt to determine the probability associated with the chi square statistic, I get a value of 0 with no decimal places - just "0". I was expecting a... | That Chi-squared statistic is gigantic. With 8 degrees of freedom, the a chi-squared statistic of 21.96 is associated with a p-value of 0.005. So a very large statistic like 200, with 8 degrees of freedom has a p-value so small that R returns zero, (i.e. close to zero). It is certainly less than .05, the level you a... | It is customary for software packages to have cut out values. For instance, in many MATLAB tests anything below 0.005 is returned as 0, but usually with some kind of a warning
| https://stats.stackexchange.com |
94,536 | [
"https://dba.stackexchange.com/questions/94536",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/-1/"
] | I backed up a database a few weeks ago, then for comparison downloaded it again. It was about 10,000 kB larger. There should not be anyone writing to it, but there are lots of reads.
Will a static MySQL database become bigger in file size over time, even without anyone writing to it?
| Finally we have decided that the best way to do it is with the checksum. The reason is: we want to compare rows per rows for the specified columns (wich checksum lets you specify).
At first we wanted to go with hashbytes but as Mikael told me, it has a limit of 8000 bytes, and that is really too small.
And although ... | This may not be exactly what you are looking to do, but you could use the EXCEPT statement to get the rows that are different:
<pre><code>SELECT Column1, Column2 FROM DatabaseName1.dbo.TableName1
EXCEPT
SELECT Column1, Column2 FROM DatabaseName2.dbo.TableName2
</code></pre>
If you do it this way though, make sure you... | https://dba.stackexchange.com |
471,641 | [
"https://physics.stackexchange.com/questions/471641",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/217514/"
] | When I break down <span class="math-container">$\omega = \sqrt{\frac km}$</span> (angular velocity for a simple harmonic oscillator) into its units, I get:
<span class="math-container">$$\omega = \sqrt{\frac{kg * \frac {m}{s^2}}{kg *m}}$$</span>
which simplifies to:
<span class="math-container">$$\omega = \frac 1s$$... | "Angular velocity" can be used inter-changably with "angular frequency", but you want to distinguish clearly between those and "cyclic frequency" which is the thing usual just termed "frequency".
The angular quantities are measured in radians per second, while the cyclic frequency is "cycles per second" AKA hertz (Hz)... | Angular frequency (rad/s) and frequency (1/s=Hz) have the same physical dimensionality, as OP correctly figured out, but are related by a factor of <span class="math-container">$2\pi$</span>.
The inverse of this quantity is time, in one case the time it takes to go one radian in angle and in the other to go the full r... | https://physics.stackexchange.com |
3,342,719 | [
"https://math.stackexchange.com/questions/3342719",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/700585/"
] | Given that: <span class="math-container">$p$</span> is an odd prime, <span class="math-container">$a\geq 1,b \leq \frac{p-1}{2}$</span>, and that <span class="math-container">$a \not\equiv b \pmod p$</span>, show that <span class="math-container">$a^2 \not\equiv b^2 \pmod p$</span>
Based on the second condition, I kno... | If <span class="math-container">$p\mid a^2-b^2=(a-b)(a+b)$</span>, then <span class="math-container">$p\mid a+b$</span> or <span class="math-container">$p\mid a-b$</span> because it is prime. Now as you correctly noted <span class="math-container">$a+b\lt p$</span>, so <span class="math-container">$p$</span> cannot div... | If
<span class="math-container">$a^2 \equiv b^2 \mod p \tag 1$</span>
then
<span class="math-container">$a^2 - b^2 \equiv 0 \mod p, \tag 2$</span>
or
<span class="math-container">$(a + b)(a - b) \equiv a^2 - b^2 \equiv 0 \mod p; \tag 3$</span>
since
<span class="math-container">$1 \le a, b \le \dfrac{p - 1}{2},... | https://math.stackexchange.com |
307,656 | [
"https://math.stackexchange.com/questions/307656",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/62018/"
] | Suppose {$a_n$}$_{n=1}^{\infty}$ {$b_n$}$_{n=1}^{\infty}$ are sequences such that {$a_n$}$_{n=1}^{\infty}$ and {$a_n + b_n$}$_{n=1}^{\infty}$ converge. Prove that {$b_n$}$_{n=1}^{\infty}$ also converges.
I'm confused on a few parts. Taking what can be assumed, we know that for every $\epsilon$>0, there exists $N_1 \... | <strong>Hint:</strong> If $a_n\to a$ then $-a_n\to-a$. Show that $(a_n+b_n)+(-a_n)$ is the sum of two convergent sequences and so it converges as well.
| Use the fact that if $a_n\to a$ and $b_n \to b$, $a_n - b_n \to a - b$.
| https://math.stackexchange.com |
229,238 | [
"https://security.stackexchange.com/questions/229238",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/231442/"
] | Suppose I have a very simple PHP application that acts as a front-end for an SQL database. The user enters their query into a box, and the app shows the query results in a table.
To prevent a user from modifying the table, the SQL user only has permissions for read-only queries, i.e. if a user tries to enter somethin... | Is it bad if the app is vulnerable to SQL injection if it does not cause any harm - yes. Is it critical - probably not. Should it be fixed - yes.
Basically you are having a multilayered defense: the first layer is to make sure to not allow SQL injection while the second is to make sure (using database permission) that... | It is always a bad thing. Because it means the code is sloppy, even if the vulnerability is not directly exploitable but you never know, an experienced hacker could find a flaw that you just cannot see at the moment. Or even a smarter user who thinks outside the box could find something that you missed.
So you should n... | https://security.stackexchange.com |
3,096,045 | [
"https://math.stackexchange.com/questions/3096045",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/143228/"
] | The Fourier transform of a continuous-time function is defined if the function is absolutely integrable, otherwise it does not exist. What about the inverse Fourier transform? If I make up any function of <span class="math-container">$\omega$</span>, can I always invert it to some time-domain counterpart?
Would that ... | Well, you can't just make up <em>any</em> function of <span class="math-container">$\omega.$</span> Just as for the Fourier transform itself, the function has to be nice enough, e.g. <span class="math-container">$L^1$</span> or <span class="math-container">$L^2.$</span> Since the Fourier transform can be extended to te... | Yes the inverse Fourier Transform can always generate its time domain counterpart. The frequency domain representation is simply a set of bin frequencies each with its coefficient and phase which can always be parsed to synthesize a time domain signal
| https://math.stackexchange.com |
7,662 | [
"https://security.stackexchange.com/questions/7662",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/3639/"
] | I am evaluating a service provider that performs the following functions with email:
<ul>
<li>A user logs in to a web based application over an SSL connection.</li>
<li>Email is composed in a web browser over the SSL connection.</li>
<li>The email is then sent to the recipient's account with the same service provider.... | You have some great questions there already. Without knowing your depth of knowledge on the subject it is kinda difficult to say which questions could be added that can be of value to you. Instead, I would prefer to just make sure your evaluation process is good enough to find the solution that will work for your org... | This sounds like a closed communication system where the content isn't actually "sent" from their system. You mention that data is encrypted once entered and that:
<blockquote>
"All encryption keys (one-way) are stored on the service provider's
systems and are secured with the users' password."
</blockquote>
I'm... | https://security.stackexchange.com |
75,943 | [
"https://electronics.stackexchange.com/questions/75943",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/24239/"
] | I am building a solenoid based equipment. The problem with off the shelf solenoids I figure out is they become hot very quickly. & may burn soon too or at least are not optimal for a constant long term vibration solution. I have this question in mind that audio speaker also use a solenoid like mechanism but that do... | What you are asking makes little sense. Solenoids and voice coils are quite different things designed for different applications. Voice coils are bi-directional, try to have constant force over the range of travel for a particular current, but that force is usually weaker for a similar size.
Either of these can be m... | Food for thought: -
<ul>
<li>What distance does the "solenoid" need to move? <em>Let's say 5mm</em></li>
<li>What force is it acting against? <em>Let's say 10 newtons</em></li>
<li>Does the "return" journey need energy inputted to return it? <em>Let's say same as forward journey</em></li>
<li>How many times a second d... | https://electronics.stackexchange.com |
310,838 | [
"https://softwareengineering.stackexchange.com/questions/310838",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/31310/"
] | I've found a piece of code like this in one of our projects:
<pre><code> SomeClass QueryServer(string args)
{
try
{
return SomeClass.Parse(_server.Query(args));
}
catch (Exception)
{
return null;
}
}
</code></pre>
As far as I understan... | Imagine code with thousands files using a bunch of libraries.
Imagine all of them are coded like this.
Imagine, for example, an update of your server causes one configuration file disappear; and now all you have is a stack trace is a null pointer exception when you try using that class: how would you resolve that? It ... | There are cases where this pattern is useful - but they are typically used when the exception generated should never have been (ie when exceptions are used for normal behaviour).
For example, imagine you have a class that opens a file, stores it in the object that is returned. If the file does not exist, you may consi... | https://softwareengineering.stackexchange.com |
102,338 | [
"https://physics.stackexchange.com/questions/102338",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/41996/"
] | If free electrons don't interact with photons, why are free electrons accelerated by electromagnetic fields?
| The laws of conservations of momentum and energy combined forbid the reaction $$e^- + \gamma \rightarrow e^-$$
(Go ahead and do the math, is simple and enlightnening).
But a completely different story is:
$$e^- + \gamma \rightarrow e^- + \gamma$$
Where the incoming photon has a different energy that the outcomin... | Electron accelerators work the way that photons gave to the electrons some amount of their energy. In my understanding this are free electrons. And as long as the electrons could be accelerated more of the energy from the photons will be absorpt then emitted by the electrons.
| https://physics.stackexchange.com |
213,748 | [
"https://security.stackexchange.com/questions/213748",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/212601/"
] | I'm a dev working on an app that has a monolith DB (PostgreSQL). We are using a third party provider for AUTH and MFA, but want to move MFA back into our control. The plan was to move the encrypted MFA secrets into our monolith DB, but there was concern voiced by members of the team about storing something like an enc... | tldr: The approach you outline fails do address the actual problem, use a separate application and a secure authentication scheme.
While you are right that keeping account credential information with the rest of the lot (i.e. whatever your application actually uses when it's not authenticating users) is a bad idea (tm... | Storing OTP tokens (as well as password hashes) in a separate DB would be helpful against read-only SQL Injection vulnerabilities in the main database. This could be considered as a second layer security measure, last resort after your main DB was compromised, however I can’t decide for you whether it’s worth the added... | https://security.stackexchange.com |
1,794,044 | [
"https://math.stackexchange.com/questions/1794044",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/266135/"
] | <blockquote>
Problem : Given $f(x) = x + |x|$ for what values of $x$ is $f$ differentiable?
</blockquote>
For the sake of generality, let's assume that it is unknown to us that $|x|$ is not differentiable at $x = 0$
<strong>Attempted Solution :</strong>
Using the definition of differentiability, a function is diff... | <blockquote>
<em>Given $f(x) = x + |x|$ for what values of $x$ is $f$ differentiable?</em>
</blockquote>
One may observe that
$$
f(x) =
\begin{cases}
2x & \text{if $x\geq 0$,} \\[2ex]
0 & \text{if $x<0$.}
\end{cases}
$$ Then one may apply the definition of differentiability, obtaining easily that $f... | The best thing to do is to simplify the function before you use the definition of the derivative. For $x\geq 0$, $f(x)=x+|x|=x+x=2x$, while, for $x <0$, $f(x)=x-x=0$. Hence, we have the following:
$$
\begin{aligned}
f(x)=
\begin{cases}
2x &\text{ if }x\geq 0\\
0 &\text{ if }x < 0
\end{cases}.
\end{align... | https://math.stackexchange.com |
179,952 | [
"https://softwareengineering.stackexchange.com/questions/179952",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/16992/"
] | Is there a difference between
<pre><code>public class A extends AbstractB implements C
{...}
</code></pre>
versus...
<pre><code>public class A extends AbstractB
{...}
abstract class AbstractB implements C
{...}
</code></pre>
I understand that in both cases, class A will end up conforming to the interface. In the se... | It all depends on if <code>AbstractB implements C</code> semantically. I.e if it makes sense semantically for <code>AbstractB</code> to implement <code>C</code>, then go for it.
If we take concrete examples the semantic difference becomes clear.
If A = Dog , AbstractB = Animal, C = IBark
Only choice that makes sen... | It's not a "hidden" documentation purpose. It allows you to cast AbstractB and all of it's subclasses to C. There are actually three styles.
<pre><code>public class A extends AbstractB implements C
public class AbstractB
</code></pre>
I'd use this one if AbstractB didn't logically implement C. Even if it doesn't p... | https://softwareengineering.stackexchange.com |
501,076 | [
"https://physics.stackexchange.com/questions/501076",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/240744/"
] | I heard somewhere that when an object is in equilibrium on the flat bottom of a hill that it will require work proportional to <span class="math-container">$dx^2$</span> when moved the small distance of <span class="math-container">$dx$</span>. I thought that it would be proportional to <span class="math-container">$dx... | Assume a one dimensional case with <span class="math-container">$\hat x$</span> being the unit vector in the positive x-direction.
Suppose a body is the system under consideration and it is at position <span class="math-container">$x=0$</span>.
To be in a position of stable equilibrium a displacement of the body ... | The bottom of a hill is a position of stable equilibrium.If you will displace a point mass by a distance <span class="math-container">$x$</span> then the restoring forces behave like spring force.It is just an analogy.The work done in stretching the spring is proportional to <span class="math-container">$x^2$</span>,he... | https://physics.stackexchange.com |
46,185 | [
"https://mathoverflow.net/questions/46185",
"https://mathoverflow.net",
"https://mathoverflow.net/users/10859/"
] | The nodes of a surface are special cases of more general singularities. For example, the Cayley cubic has four nodes.
The full set of singularities of a surface can be characterized by finding all points where the partial derivatives are all zero. However, not all singularities are nodes. Some are cusps or other ki... | A node (as in Cayley's surface) is a double point with nondegenerate tangent cone.
To check whether a given point on a surface in A^3 is a node in this sense, change coordinates so that it is the origin and write the equation as 0=F_2+F_3+... with F_i homogeneous of degree i. The point is a node iff F_2 is irreducible.... | You should make this question more precise.
There is a notion of a <em>node</em> of a surface which is, I believe, just an $A_1$ singularity, that is, analytically isomorphic to the vertex of the cone $(xy-z^2)\subset \mathbb A^3_{x,y,z}$.
It seems that you want self intersections. Those are actually not normal sin... | https://mathoverflow.net |
453,991 | [
"https://electronics.stackexchange.com/questions/453991",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/156004/"
] | I would like to make several hundreds of a device with video out. It seems every video connector involves huge fees for licensing - is there anyway around this?
| For devices and endpoints the contemporary digital interconnects have an ensnarement with HDCP. This includes both HDMI and DisplayPort, which can support driving HDMI with an ‘active cable’ adapter.
Besides HDCP, HDMI has a ‘Logo License’ that costs serious money. DP doesn’t, though MPEG LA wants to charge a (dispute... | HDMI is out of question then, you need to pay member fees to legally manufacture devices with HDMI interface. Even worse if you need HDCP protection, but you don't need HDCP if your device is a camera or similar which is not meant to play NetFlix or Blu-Ray discs. That said, you can buy HDMI and DVI encoder chips, and ... | https://electronics.stackexchange.com |
37,724 | [
"https://dsp.stackexchange.com/questions/37724",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/-1/"
] | The frequency spectrum of a time domain signal x(t) can either be written as X(f) or $X(j\omega)$. But how is the later correct? I mean, the frequency spectrum is clearly dependant on the frequency, not on a $j\omega$ part, is it?
| Basicaly it's just a convention and the Fourier transform is a function of $\omega$ .
The imaginary argument $j\omega$ can be understood in different perspectives.
In one convention, for the class of signals $x(t)$ which are absolutely summable (i.e., stable) , the continuous time Fourier Transform (CTFT) $X(j\omega... | Both. For a strictly real time domain signal, $X(j\omega)$ is conjugate symmetric, and thus X(f) is identical to folding the complex range (with a scale factor of 2 pi thrown in).
| https://dsp.stackexchange.com |
295,324 | [
"https://electronics.stackexchange.com/questions/295324",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/143630/"
] | I have a 1200 Watt hot plate that works great when connected to the grid but I want to be able to run it off of two deep cycle batteries wired in series to an inverter in an off grid situation. The thing is i want to be able to limit the current going to this device to have more fine tuned control on my battery drain. ... | A heater is probably the most "tolerant" device to drop power to, because no matter how you "mangle" the AC waveform coming in, a heater (being a purely resistive load) won't be affected in any detrimental way, and will tend to 'average" whatever is fed to it. This is why most of them just have a thermal contact that o... | <strong>Short answer:</strong> A canister of butane/propane gas contains a lot more energy that batteries, is lighter, cheaper, etc. Use a gas cooking burner for camping.
<strong>Long answer:</strong>
Your inverter might not like such a large load with a triac to dim it, so please check its documentation to see if t... | https://electronics.stackexchange.com |
23,037 | [
"https://physics.stackexchange.com/questions/23037",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/8392/"
] | Assuming a frictionless / "perfect" environment, and given a ball held in an elastic sling (like a hand-held catapult) <em>where the pocket is lighter than the projectile itself</em>, <strong>what is the point at which the ball separates from the sling pocket? At the start of the shot, or as the sling-pocket passes thr... | In the Einstein convention, pairs of equal indices to be summed over may appear at the same tensor. For example, the formula ${A_k}^k=tr~A$ is perfectly legitimate.
But your formula looks strange, as one usually sums over a lower index and an upper index, whereas you sum over lower indices only, which doesn't make sen... | You could rewrite your equation as
$$
f(\vec a) = S_{ijkl} a_i a_j \delta_{kl}
$$
where $\delta_{kl}$ is the Kroneker Delta, if that helps. The last equation you've written is the right idea.
I would stress, though, that Einstein notation usually uses one upper and one lower index. This is partially so you can qui... | https://physics.stackexchange.com |
67,971 | [
"https://softwareengineering.stackexchange.com/questions/67971",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/22882/"
] | We have a lot of documents associated with our software development. These include things like requirements, design documents, external PDFs, customer files, testing instructions, etc. Currently, these documents are scattered all over the place (wiki, "some place on the network", a local developers hard drive(!), an... | I'm not sure if there's a single "best way", but from your description, it sounds like every developer is doing his/her own way of storing documentation. Get a buy-in from everyone on a single method of storing documentation, and it'll be a lot easier to keep it organized.
Besides the "find the best way for you" answe... | The solution that worked out the best at my company was to set up a wiki (we use mediawiki). It gets used in many ways, from the traditional wiki entries to a simple "This is a..." plus a Word document. It is very easy to search. We're gradually building up a very useful repository of information.
| https://softwareengineering.stackexchange.com |
467,006 | [
"https://physics.stackexchange.com/questions/467006",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/213949/"
] | The Bernoulli equation states the following
<span class="math-container">$$
\frac{1}{2}u^2 + \frac{p}{\rho}+ gz = \text{constant along a streamline}
$$</span>
where:
<ul>
<li><span class="math-container">$u$</span> is the fluid flow speed at a point on a streamline,</li>
<li><span class="math-container">$g$</span>... | Pressure work per unit mass of fluid.
| The Bernoulli equation may be derived from the conservation of energy. The conservation of energy is a wonderful property in physics where despite interactions where nearly everything we can measure changes: position, velocity, acceleration..., something stays the same over time. If something stays the same over time, ... | https://physics.stackexchange.com |
38,463 | [
"https://softwareengineering.stackexchange.com/questions/38463",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/12230/"
] | On the one hand, the agile approach stresses a tight-knit team that holds each other accountable and accepts collective ownership of the project.
On the other hand, companies use contract programmers so that they can manage the peaks and valleys of funding without laying off actual employees. If there is a shortfall ... | Many teams work only with agile contractors. Some companies like ThoughtWorks are based on the idea to "sell" agile teams.
We are a team of 10 contractors working for a big telco, all from the same contracting company.
Where I saw problems was when there were 2 body-rental companies in the same team... after a while t... | In response to your edit, there are different sets of eyes to look at the situation. So to help clarify any potential confusion, it helps to understand which perspectives apply.
From the development team perspective, there is no difference between contractor and employee. We are all on the same team, and we all have... | https://softwareengineering.stackexchange.com |
2,232,268 | [
"https://math.stackexchange.com/questions/2232268",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/423805/"
] | There are 10 kings in a chessboard. prove that there is one square in the board that creed with 2 kings.
| Divide the board into 3x3 regions as much as possible, like this:
<pre>a a a b b b c c
a a a b b b c c
a a a b b b c c
d d d e e e f f
d d d e e e f f
d d d e e e f f
g g g h h h i i
g g g h h h i i</pre>
Each region obviously can contain at most one king (else they would be attacking each other, or both cover the ce... | Hint: When you "place a king" on the board, instead of tracking exactly where you put the king, trace how many squares that king touches. Might need to handle some special cases when the king is placed in a corner or on an edge, but I believe that this approach might give you a solution.
| https://math.stackexchange.com |
249,695 | [
"https://physics.stackexchange.com/questions/249695",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/89498/"
] | Im in holidays in Malaysia and it is god damn hot. You cannot leave your hotel/car and go out. All this because of the sun.
I we were to capture all the heat (solar panels, ...) would this help make the Earth cooler / fight global warming ?
Edit : see one of my comments, I mean use the heat to conduct some endothermi... | Solar panels are dark (they have to absorb light) and they do decrease the Earth's albedo, i.e. they make the planet locally darker. This means that locally they will cause slight additional heating. Part of the energy that a panel absorbs gets converted into electrical energy, which can be transferred off-site, i.e. r... | There are several interacting effects.
And for global warming, we need to look at the whole earth.
First effect: putting the panels up may change the albedo - the reflectivity - of the surface. That will increase the amount of energy absorbed by the earth, rather than reflected into space. The effect would be a minis... | https://physics.stackexchange.com |
91,814 | [
"https://stats.stackexchange.com/questions/91814",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/16175/"
] | I am playing Warlight online (something like Risk), and my goal is to create a bot which plays against other bots.
<pre><code>n=100 attacking troops will on average kill 60 (p=0.6) troops each turn,
n=100 defending troops will on average kill 70 (p=0.7) troops each turn.
They attack exactly at the same time.
Binomial... | The basic rules of engagement provide the probability distribution for transitions from $a$ attacking armies and $d$ defending armies to $a^\prime$ attackers and $d^\prime$ defenders, where $0 \le a^\prime \le a$ and $0 \le d^\prime \le d$. Beginning with $A$ attacking armies and $D$ defending armies, there are theref... | I use this to simulate, perhaps it might give more insight and perhaps someone might come with an analytical solution.
<ul>
<li>When defender has equal to attacker, the defender "wins"</li>
<li>When the attacker didn't destroy defender by turn X, the attacker "loses"</li>
<li>When defender reaches 0 troops, attacker ... | https://stats.stackexchange.com |
724,463 | [
"https://physics.stackexchange.com/questions/724463",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/311011/"
] | Let us take a horizontal spring block system where the block is initially at the mean position. If we pull the block applying a force <span class="math-container">$F$</span>,then by Newton's <span class="math-container">$3rd$</span> law,the block will pull us by the exact same force. This force is the restoring force i... | Cyclic stressing of a piece of metal causes dislocations to walk back and forth within the lattice structure of each crystallite within the bulk. The grain boundaries where the crystallites meet confine the dislocations to the interior of each crystallite.
Those dislocations pile up against the grain boundaries on each... | There is a correlation between materials with strain hardening properties and the existence of a fatigue limit<span class="math-container">$^1$</span>.
The puzzling of fatigue is that the stresses are always clearly below the yield limit, and are so supposed to have an elastic behaviour. Nevertheless, if they break aft... | https://physics.stackexchange.com |
7,134 | [
"https://mathoverflow.net/questions/7134",
"https://mathoverflow.net",
"https://mathoverflow.net/users/1353/"
] | Why doesn't the join operation on the category of simplicial sets commute up to unique isomorphism? I mean, aren't products and coproducts commutative up to isomorphism? That leads me to conclude at first glance that the join is commutative, but it's not. Recall, given two simplicial sets $S$ and $S'$, we define the j... | Implicit in the index of the coproduct is that you're writing J as an ordered disjoint union of I and I', where I comes first.
EDIT: Some elaboration.
For a simplicial set $T$, let's write $T\_n$ for the "n-simplices", i.e. the value of on the ordered set $\{0,1,...,n\}$; these together with the maps between them det... | It might be helpful to work through some simple examples. You probably know that Δ<sup>n</sup> ★ Δ<sup>k</sup> = Δ<sup>n+k+1</sup>. This has to do with the ordinal sum: one way of defining joins is as a restriction of the monoidal structure on <i> augmented </i> simplicial sets, which are contra... | https://mathoverflow.net |
3,943,740 | [
"https://math.stackexchange.com/questions/3943740",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | I am trying to show that a linear map from a field to a vector space <span class="math-container">$W$</span> is bounded.
Here is is what I know:
Since <span class="math-container">$\mathbb{F}$</span> is either <span class="math-container">$\mathbb{R}$</span> or <span class="math-container">$\mathbb{C}$</span>, it is f... | I gave a solution years ago.
Denote <span class="math-container">$A = \sum_{k=1}^n \frac{1}{\frac{1}{k} + a_k}$</span>,
<span class="math-container">$B = \sum_{k=1}^n \frac{1}{a_k}$</span>.
The function <span class="math-container">$f(x) = \frac{1}{1 + \frac{1}{x}}, \ x > 0$</span> is concave. By Jensen's inequality... | I know the question has already been answered, and the answer is quite nice, but I was thinking along a different line:
<span class="math-container">$$\sum\limits_{k=1}^{n} \frac{\frac{1}{k}}{a_k\left(\frac{1}{k}+a_k\right)} \geq \frac{2}{n(n+1)}\left(\sum\limits_{k=1}^{n}\frac{1}{a_k}\right)\left(\sum\limits_{k=1}^{n}... | https://math.stackexchange.com |
61,342 | [
"https://dba.stackexchange.com/questions/61342",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/35905/"
] | I'm building a checkout system which needs to store the customer's shipping address in the database and am unsure the best way to go about it.
Currently I have an addresses table which stores each address with a unique ID:
<pre><code>id | address | city | division | country | postcode
------------------... | The Shipping Address is a property of each and every order, not of the customer. Even if it was a property of the Customer, it would be necessary to know where past orders were shipped to if a customer relocated.
Therefore all that can be stored as a Customer property are the <em>Default Shipping Address</em> (and <em... | Are you interested in auditing information? Solution 2 would allow you to easily track changes to customer addresses and allow you to determine who did it (and optionally why, if you include a reason field.)
I am presently working with a system that uses approach 1 and the database is <strong>filled</strong> with dupl... | https://dba.stackexchange.com |
25,702 | [
"https://scicomp.stackexchange.com/questions/25702",
"https://scicomp.stackexchange.com",
"https://scicomp.stackexchange.com/users/7808/"
] | Consider the 1D poisson equation
$$
\frac{d^2 u}{dx^2} = -\rho
$$
with Dirichlet boundary conditions $u(0) = u(l) = g$. Using a finite difference scheme, with a 5-point grid $u_1,u_2,u_3,u_4,u_5$ (excluding boundary points $u_0$ and $u_l$), we get the set of linear equations
$$
\left( \begin{array}{ccc}
2 & -1 &... | If you include the boundary condition directly in the matrix, you will only get the g value at the points where the boundary is prescribed. If we use 5 nodes with the following BCS:
$$u_1=g_1$$ and $$u_5=g_5$$
Then the matrix resulting from the finite difference will be:
$$
\left( \begin{array}{ccc}
1 & 0 & ... | Another option to the answer @BlaisB is to incorporate the constraints via Lagrange multipliers, which basically leads to a linear system which is augmented by two variables,
$$
\left( \begin{array}{ccc}
2 & -1 & & & & 1 &\\
-1 & 2 & -1 & & \\
& -1 & 2 & -1 &... | https://scicomp.stackexchange.com |
67,852 | [
"https://softwareengineering.stackexchange.com/questions/67852",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/20971/"
] | I'm a new programmer and want to ask senior programmers (programmers who have some experience in the real world).
I do my work and after coding, my project gets completed but honestly speaking I don't remember the code, classes and frameworks name and their properties.
Sometimes even I doubt myself that did I made th... | In all honesty, I don't know how one could be a long term developer without the ability to "forget" code you've worked on. The amount of projects and resulting code would eventually lead to pure information overload.
However, I see this situation as a supporting argument for clean, logical design. There will come a ... | Remembering all of your syntax and apis for a project are not as important as remembering the concepts that went into building it. Once you have experience you can read over the code and figure out the meaning just like reading these posts.
What you'll find over time, if you are actively improving, is that you'll see... | https://softwareengineering.stackexchange.com |
29,356 | [
"https://stats.stackexchange.com/questions/29356",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/11615/"
] | I would like to gain a conceptual understanding of Root Mean Squared Error (RMSE) and Mean Bias Deviation (MBD). Having calculated these measures for my own comparisons of data, I've often been perplexed to find that the RMSE is high (for example, 100 kg), whereas the MBD is low (for example, less than 1%).
More spe... | I think these concepts are easy to explain. So I would rather just describe it here. I am sure many elementary statistics books cover this including my book "The Essentials of Biostatistics for Physicians, Nurses and Clinicians."
Think of a target with a bulls-eye in the middle. The mean square error represent th... | RMSE is a way of measuring how good our predictive model is over the actual data, the smaller RMSE the better way of the model behaving, that is if we tested that on a new data set (not on our training set) but then again having an RMSE of 0.37 over a range of 0 to 1, accounts for a lot of errors versus having an RMSE ... | https://stats.stackexchange.com |
1,579,032 | [
"https://math.stackexchange.com/questions/1579032",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/225377/"
] | I have the following PFD:
$f_x(x) = \begin{cases}
bx^2, & 0 < x < 2, \\
5b-bx & 2 \le x < 3, \\
0 & \text{otherwise}.
\end{cases}$
I already calculated $b = \frac{6}{31}$.
However, I am unsure about the CDF:
$F_x(x) = \begin{cases}
0 & x \le 0, \\
0 + \int_{0}^{x} bt^2 dt = bx^3 *... | I cannot comment since I'm a new user, but yes, that looks correct. To verify further you can try to get the PDF back by taking a piecewise derivative.
| That's correct. You can verify by plugging in $x=3$ for the $2 \le x < 3$ case and noting that you successfully obtain $1$. However, in order to give a proper piece-wise function, you should substitute $b$ back into the equation and get it in terms of only $x$.
| https://math.stackexchange.com |
9,630 | [
"https://cardano.stackexchange.com/questions/9630",
"https://cardano.stackexchange.com",
"https://cardano.stackexchange.com/users/7939/"
] | <blockquote>
An asset is uniquely identified by an asset ID, which is a pair of both the policy ID and asset name.
</blockquote>
The Policy ID is a hash of the monetary policy script that was used to mint the token. What prevents someone from minting more of the same tokens using the same policy script?
| The thing that prevents other people to (successfully) use the same policy script is that the <strong>policy needs to be validated by the blockchain</strong>. Minting transactions where the policy script is not successfully validated do not go through. This can be used for instance to make the policy require that the m... | One part of the policy is usually about signatures associated with it.
If you want to use the same script again, you will need the corresponding private keys.
If you have them and policy permits it, you can mint more tokens.
This is very important, as if your "NFT project" does not have a policy that prevents... | https://cardano.stackexchange.com |
120,381 | [
"https://softwareengineering.stackexchange.com/questions/120381",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/3965/"
] | I've started to notice a recurring pattern to our team's burndown charts, which I call a "bowstring" pattern. The ideal line is the "string" and the actual line starts out relatively flat, then curves down to meet the target like a bow.
My theory on why they look like this is that toward the beginning of the story, w... | If you're still meeting the target, I don't necessarily see this as a problem. You met your target!
Part of the reason for short sprints/increments is to keeps the team focussed: this batch of work should be finished by the end of <em>this</em> sprint.
If you have a long iteration, the amount of time stretches out li... | If it is <strong>consistent</strong> and <strong>predictable</strong>, I don't see it as being a problem.
Unless you <strong>detect that this is causing your quantifiable project harm</strong>, are there not more pressing issues to spend time considering? If there are not, I am envious of you :)
| https://softwareengineering.stackexchange.com |
4,619 | [
"https://cs.stackexchange.com/questions/4619",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/903/"
] | Assuming P $\neq$ NP, NP-complete problems are "hard to solve, but have answers that are easy to check." Does it make any sense to consider the opposite, that is, problems for which it's easy to compute a correct answer, but hard to verify an arbitrary purported solution?
I think such a problem would imply either:
... | If you are fine with artificial problems, you can make plenty of them. Here are a few:
<ul>
<li><em>Given a positive integer <i>n</i> in unary, answer a satisfiable 3CNF formula in <i>n</i> Boolean variables.</em><br>
Giving one satisfiable 3CNF formula is easy, but deciding whether a given 3CNF formula is satisfiabl... | Although Tsuyoshi Ito's answer covers the "main" answer, there were two subtler notes I wanted to add.
<ol>
<li><strong>Even when the solution is hard to verify, checking the solution is still easy to check with a short proof string.</strong> That is, by extending the solution a bit with extra information, it... | https://cs.stackexchange.com |
26,774 | [
"https://dsp.stackexchange.com/questions/26774",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/17828/"
] | I am a student learning dsp. I like the subject. I could understand the discrete time signals. When I move into z transform. I could not understand it.
Z transform is the mapping from discrete signal to zplane to make frequency analysis easy. It would be great if users give some tips that could help me to clarify var... | Discrete-time signals are generally written in the time domain as $x[n]$ where $n$ is an integer.
We want to be able to see how discrete-time systems can be used to modify these signals:
$$
y[n] = \sum_{m=1}^M a_k y[n-m] + \sum_{p=0}^P b_p x[n-p]
$$
The above equation is a linear, constant coefficient difference equa... | Z is a complex exponential, a complex exponential is a compact form of representing a sinusoid.
What essentially happens in z transform is we take a discrete signal and try to project it on various sinusoids - various frequency.
We multiply our samples with exp(z,-n).
Sum up the values., essentially that's a measure... | https://dsp.stackexchange.com |
3,047,607 | [
"https://math.stackexchange.com/questions/3047607",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/-1/"
] | With the help of <em>Mathematica</em> we find
<span class="math-container">$$\int e^{\cos x}\cos (x+\sin x)\ dx = e^{\cos x}\sin (\sin x)$$</span>
But I tried normal method like integrating by parts, without success.
| You can do this with real methods using the sum-angle formula to write
<span class="math-container">$$e^{\cos(x)}\cos(x+\sin(x))\\=e^{\cos(x)}\cos(x)\cos(\sin(x))-e^{\cos(x)}\sin(x)\sin(\sin(x))$$</span>
This is now recognizably in the form <span class="math-container">$u'v+v'u=(uv)'$</span>, and you may use the produc... | Just to spell out achille hui's use of complex numbers, , your integral is <span class="math-container">$$\Re\int\exp(\cos x + i(x+\sin x))dx=\Re\int(\cos x+i\sin x)\exp(\cos x + i\sin x)dx\\=\Re(-i\exp(\cos x+i\sin x)+C)=\Im\exp (\cos x+i\sin x)+\Re C\\=\exp\cos x\cdot\sin\sin x+\Re C.$$</span>
| https://math.stackexchange.com |
141,428 | [
"https://chemistry.stackexchange.com/questions/141428",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/99864/"
] | I'm writing a computer program to help a factory produce bottles of mixtures. A mixture is usually alcohol, water and some other stuff, like menthol. But could also be water and NaOH and some oils (think schampoo).
My basic understanding is that if one bottle is 500 ml and contains:
50% water
45% alcohol
5% oils
I need... | The nonlinearity of volumetric additions can be addressed theoretically. However, in practice, calculating how much material is needed to fill 1000 500 mL bottles containing 50% water 45% alcohol 5% oils (% by weight), can be addressed for one formulation by making a small batch and determining specific gravity. In pra... | Mixtures are never linear. Mixing <span class="math-container">$50$</span> mL water + <span class="math-container">$50$</span> mL ethanol gives a <span class="math-container">$96$</span> mL mixture. The masses are additive, but not the volumes. I agree the discrepancy is not huge. If you can tolerate a <span class="mat... | https://chemistry.stackexchange.com |
104,499 | [
"https://stats.stackexchange.com/questions/104499",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/48885/"
] | I'm using a service (Optimizely, as it happens), which show a confidence interval but doesn't explain what it is.
I'm assuming it is a Binomial Confidence Interval, but I have no idea what the % certainty of the range is, or the detailed meaning. Searching the Internet doesn't help - there is nobody with a clear expla... | Exactly? You want an <em>exact</em> answer to a statistical question? And you're willing to pay a service to provide it? Did you formerly work for AIG?
Seriously, we will need to guess here. My guess is that you are being offered an estimate of the 95% confidence interval using a Normal approximation around the point ... | Following DWin's answer, I used R to find some more binomial confidence intervals:
<pre><code>> library(binom)
> binom.confint(97, 939)
method x n mean lower upper
1 agresti-coull 97 939 0.1033014 0.08535696 0.1244784
2 asymptotic 97 939 0.1033014 0.08383472 0.1227681
3 b... | https://stats.stackexchange.com |
1,997,734 | [
"https://math.stackexchange.com/questions/1997734",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/353503/"
] | Please check my proof
$$\frac{2n}{n+2}-2< \epsilon $$
$$\frac{2n-2n-4}{n+2}< \epsilon $$
$$ 4\left |\frac{-1}{n+2} \right |< \epsilon $$
If we choose $N\leq n$
Because
$$4\left |\frac{-1}{n+2} \right |\leq \frac{4}{N+2}$$
and
$$ \frac{4}{N+2}<\frac{4}{N}<\epsilon $$
by transitivity property
... | Let $\epsilon>0$ given.
if we put $N=\lfloor \frac{4}{\epsilon} \rfloor+1$, then
$$\forall n \geq N \;\; n>\frac{4}{\epsilon}$$
$\implies$
$$\forall n\geq N \;\; \frac{4}{n}<\epsilon$$
$\implies$
$$\forall n\geq N\;\; |\frac{2n}{n+2}-2|<\frac{4}{n}<\epsilon.$$
$\implies$
$$\lim_{n\to+\infty}\fr... | Here is an alternative proof:
$\lim\limits_{n\to\infty}\left(\frac{2n}{n+2}\right)=$
$\lim\limits_{n\to\infty}\left(\frac{2n}{n+2}\cdot\frac{1/n}{1/n}\right)=$
$\lim\limits_{n\to\infty}\left(\frac{2}{1+2/n}\right)=$
$\frac{2}{1+\lim\limits_{n\to\infty}(2/n)}=$
$\frac{2}{1+0}=$
$2$
| https://math.stackexchange.com |
34,965 | [
"https://electronics.stackexchange.com/questions/34965",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/3757/"
] | In the circuit below the complex voltage \$u\$ should be calculated using Kirchoff's law. I know this law implies that sum of voltage should be zero. I'm trying to solve it like this but I am not sure if I aam doing correct steps:
<img src="https://i.stack.imgur.com/IUDvw.jpg" alt="enter image description here">
<str... | <ol>
<li>R30 limits the charging peak current to the capacitor somewhat, but at 1 ohm it still allows for 12 A, so of little use there. Also would limit the current through the zener if there are peaks above 20 V.</li>
<li>The larger capacitors work less well at higher frequencies, and that's where the smaller ones tak... | To expand on stevenvh's answer, for less-experienced designers:
<ol>
<li>R30 is there to limit inrush current so the fuse doesn't blow. Otherwise when you first plug the circuit in, the input capacitors can draw lots of current until they get charged up, which can be enough to blow the fuse. As stevenvh says it may ... | https://electronics.stackexchange.com |
25,491 | [
"https://ai.stackexchange.com/questions/25491",
"https://ai.stackexchange.com",
"https://ai.stackexchange.com/users/41615/"
] | I was pondering on the loss function of GAN, and the following thing turned out
<span class="math-container">\begin{aligned}
L(D, G)
& = \mathbb{E}_{x \sim p_{r}(x)} [\log D(x)] + \mathbb{E}_{x \sim p_g(x)} [\log(1 - D(x)] \\
& = \int_x \bigg( p_{r}(x) \log(D(x)) + p_g (x) \log(1 - D(x)) \bigg) dx \\
& =... | I guess the issue is you lost track of where the samples came from and since you requested a math explanation I'll try to go step by step using my notation and without checking other material to avoid being biased by how other authors present it
So we start from
<span class="math-container">$$ L(D,G) = E_{x \sim p_{r}(... | <span class="math-container">$\textbf{Remark.}$</span> I'd leave this as a comment if I could.
Regarding notation (which I believe may be the cause of your issue here), the loss function is better written as
<span class="math-container">\begin{align*}
\operatorname{Loss} &= \frac{1}{m}\sum_{i=1}^m \left(\log D\big(... | https://ai.stackexchange.com |
1,722,629 | [
"https://math.stackexchange.com/questions/1722629",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/320255/"
] | Are two similar matrices A and B unique? As in if A is similar to B, is it similar to B and itself only?
| Not even almost. Most matrices are even similar to infinitely many other matrices. An example is a matrix with the numbers $1,2,3,...,n$ on the diagonal, and zeros everywhere else. This matrix is similar to all of the other matrices with any permutation of $1,2,3,...,n$ on their diagonal.
The only matrices which are ... | No. If A is a square matrix that is not necessarily symmetric, then $A$ is similar both to $A$ and $A^{T}$.
| https://math.stackexchange.com |
3,088,446 | [
"https://math.stackexchange.com/questions/3088446",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/636610/"
] | Let <span class="math-container">$T_x, T_y$</span> be two random variables that describe the remaining lifetime of two persons aged <span class="math-container">$x$</span> and <span class="math-container">$y$</span>, respectively. The joint density of <span class="math-container">$T_x$</span> and <span class="math-cont... | The binomial theorem tells you
<span class="math-container">$$(5+2)^{98} = \binom{98}{0}\cdot 5^{98}+\binom{98}{1}\cdot 5^{91}2^1+ \binom{98}{2}\cdot 5^{90}2^2+\cdots + \binom{98}{97}\cdot 5\cdot 2^{97}+ \binom{98}{98}\cdot 2^{98} = 5k+2^{98}.$$</span>
So you need to find the remainder when <span class="math-container"... | When you expand <span class="math-container">${(5+2)}^{98}$</span>, you will get <span class="math-container">$5k+2^{98}$</span>, for some <span class="math-container">$k\in\Bbb N $</span>.
If you know Fermat's little theorem, you have <span class="math-container">$2^4\equiv 1\pmod 5$</span>. Hence <span class="ma... | https://math.stackexchange.com |
402,041 | [
"https://electronics.stackexchange.com/questions/402041",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/201701/"
] | I have a tiny PCB that I've been working on and off of for a while now. I received it with leaking batteries, and it had corroded some of the traces off the board. Most recently, I tried redrawing the burnt traces with conductive paint, but to no avail. I actually popped batteries in it last night and had activity for ... | You can use a thin single core wire (which you can easily form into any shape) and solder it directly to the smt resistor from where the trace begins. Strip the wire slightly and make a loop where you want the pad, and complete the trace by soldering it to the via on the right. Choose a thickness of the wire that you c... | I think that you can omit the missing pad - it looks like a test point used by the factory for testing.
I would use a short piece of 30 AWG wire-wrap wire to make the jumper. One end goes to the surface-mount resistor on the left, the other end goes right to the pad at the end of the trace on the right.
| https://electronics.stackexchange.com |
14,241 | [
"https://dsp.stackexchange.com/questions/14241",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/7794/"
] | How a mean filter is called as linear filter and a median filter is called as non linear filter? I understand how a mean and median filter operates, but I was not able to relate with the term linear and non-linear. Please explain me with an example.
| Nonlinear filters are those for which the linearity relationship breaks down.
Consider two signals $A$ and $B$, for linear filter such as mean filter $F_m$,you have
$F_m(A+\lambda B) = F_m(A) + \lambda F_m(B)$, but the equation is not satisfied for an nonlinear filter such as the median filter.
In application, the me... | <img src="https://i.stack.imgur.com/CHg4w.png" alt="linear vs non-linear">
To understand the linearity property more easily.Let us consider the above diagram,here we have 2 sequences namely <code>Xn</code> and <code>Yn</code>. when we add both the sequence we get <code>Xn+Yn</code> whose amplitude value are represente... | https://dsp.stackexchange.com |
8,581 | [
"https://mechanics.stackexchange.com/questions/8581",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/4361/"
] | I have 2011 Volkswagon Jetta 2.5 and have been having an intermittent issue where my lights seem to come on randomly in the middle of the night. This only seems to happen in the winter.
When I came out in the morning and I come outside, I notice that my lights are on. When I open the door, the signal dings like my lig... | The heater core is part of continuous loop of engine coolant propelled via the water pump, flowing through caverns in the engine, through pipes that run through the firewall into the heater core, back out the firewall, and finally through the radiator. If the thermostat dictates it (due to a temperature limit being rea... | The heater core works through heat transference. Hot radiator fluid goes into the core, the heater fan blows through it, heat is transferred to the cooler air, and heat is transferred into the cabin. The input is radiator fluid directly from the engine and output from the core goes directly back into the engine. It is ... | https://mechanics.stackexchange.com |
3,713 | [
"https://mechanics.stackexchange.com/questions/3713",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/876/"
] | For the past month or so my 2003 Honda Accord squeaks in the morning when I start driving to work. After a couple of minutes of driving the squeak stops. I'm pretty sure it's from the belt area. What can be causing this? How do I remedy it?
| Most likely cause for this is that you have some condensation built up overnight on either the belt or one of the pulley wheels, and until this has evaporated the belt will be able to slip a little, causing a squeak. Once the engine warms up, the condensation is gone.
With a new, well tensioned belt, this shouldn't m... | I had a similar problem with a Hyundai. In my case the drive belt had stretched after 2 years of constant use. A simple fix was to tighten the belt to ensure enough tension on the belt. This solved the problem for me.. Hope it helps!
| https://mechanics.stackexchange.com |
5,255 | [
"https://datascience.stackexchange.com/questions/5255",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/8478/"
] | Terms like 'data science' and 'data scientist' are increasingly used these days.
Many companies are hiring 'data scientist'. But I don't think it's a completely new job.
Data have existed from the past and someone had to deal with data.
I guess the term 'data scientist' becomes more popular because it sounds more fanc... | In reverse chronological order: data miner, statistician, (applied) mathematician.
| Terms that covered more or less the same topics that Data Science covers today:
<ul>
<li>Pattern Recognition</li>
<li>Machine Learning </li>
<li>Data Mining </li>
<li>Quantitative methods</li>
</ul>
| https://datascience.stackexchange.com |
3,733,465 | [
"https://math.stackexchange.com/questions/3733465",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/352368/"
] | <strong>If we have <span class="math-container">$A_{n \times n}$</span> and <span class="math-container">$AX-XA=A$</span>, how do we show that <span class="math-container">$\det(A)=0$</span>?</strong>
I've tried taking the trace on both sides to get
<span class="math-container">$$tr(AX)-tr(XA)=tr(A)$$</span>
<span clas... | Suppose <span class="math-container">$\det A \ne 0$</span>.
Left-multiplying <span class="math-container">$A^{-1}$</span> on both sides of <span class="math-container">$AX - XA = A$</span>, we have
<span class="math-container">$X - A^{-1}XA = I$</span>.
Right-multiplying <span class="math-container">$A^{-1}$</span> on ... | Rewrite the equation as <span class="math-container">$A (X-I) = XA$</span>. If <span class="math-container">$A$</span> is nonsingular, that implies <span class="math-container">$X - I = A^{-1} X A$</span>, i.e. <span class="math-container">$X$</span> is similar to <span class="math-container">$X-I$</span>.
That says ... | https://math.stackexchange.com |
583,056 | [
"https://physics.stackexchange.com/questions/583056",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/272815/"
] | I have been given the following function to model the behaviour of a simple physical pendulum:
<span class="math-container">$$\theta(t) = \theta_o e^{-\frac{t}{\tau}} \cos\left(2\pi \frac{t}{T} + \alpha\right)$$</span>
Where <span class="math-container">$\alpha$</span> is a constant, <span class="math-container">$T$</s... | This is similar to damped harmonic motion (where the damping is caused by air resistance and friction). The cosine term in your equation represents the oscillatory motion and the exponential part of the equation ("modulates") determines the decay of the amplitude over time. Assuming you have access to one, a ... | Just give the pendulum a push, track the angle vs time with a camera or some other time-stamped recording scheme, and then fit the data with your model. Your fit will give you estimates for the various parameters, as well as a confidence interval for each.
Is this the kind of detail you’re looking for?
| https://physics.stackexchange.com |
248,770 | [
"https://math.stackexchange.com/questions/248770",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/51396/"
] | I would like to know how the following expression was simplified?
From this
$$\frac{5+ \sqrt{20}}{6+ \sqrt{20}}$$
to this $$\frac{5+ \sqrt{5}}{8}$$
| You could simplify it a step prematurely for kicks and giggles. $\sqrt{20}=\sqrt{4\cdot 5}=2\sqrt{5}$. Now, $\frac{5+\sqrt{20}}{6+\sqrt{20}}=\frac{5+2\sqrt{5}}{6+2\sqrt{5}}$. Then, of course, multiply by the fraction by $\frac{6-2\sqrt{5}}{6-2\sqrt{5}}$.
Multiplying by the conjugate to remove the radical is a common p... | You can multiply by the "conjugate":
$$
\frac{(5 +\sqrt{20})}{(6+\sqrt{20})}\frac{(6 - \sqrt{20})}{(6 - \sqrt{20})} = \dots
$$
| https://math.stackexchange.com |
89,819 | [
"https://cs.stackexchange.com/questions/89819",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/86289/"
] | I am using a genetic algorithm to find the best way to pack circles inside a box without each touching the others and filling as much space as possible. My doubt is if an individual from a generation must be a circle or all the circles.
Can anyone help me on how would I go from here? Thank you
| In genetic algorithms, each individual should be a candidate solution to the problem. You're trying to find a packing of circles into the box, so each individual should be a complete packing that specifies the location of all the circles.
| A few people hear are pointing out that in a GA, individuals should be complete solutions to the problem. That's generally true, but there are evolutionary methods that do the opposite. Within the field of Learning Classifier Systems, the two approaches corresponding to your question are called the Michigan approach (e... | https://cs.stackexchange.com |
1,683,375 | [
"https://math.stackexchange.com/questions/1683375",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/291945/"
] | <blockquote>
If Salvatore has achieved the test and Carmela has achieved the
test,then Benedetto also has achieved the test.But Salvatore didn't
achieve the test.So:
A)Benedetto didn't achieve the test.
B)Benedetto could have achieved the test.
C)Benedetto or Carmela didn't achieve the test.
D... | <em>Ex falso sequitur quodlibet</em>. That is, if your assumption is false (and your assumption $S \land C $ is assumed false), than anything follows.
| The correct answer is B, not E. Your hypotheses are $(S \wedge C) \to B, \lnot S$ Having $S$ be false makes the antecedent of the implication false, so the implication is true regardless of the truth of $C$ or $B$.
| https://math.stackexchange.com |
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