qid int64 1 4.65M | metadata listlengths 3 3 | prompt stringlengths 31 25.8k | chosen stringlengths 17 28.2k | rejected stringlengths 19 40.5k | domain stringclasses 28
values |
|---|---|---|---|---|---|
784,897 | [
"https://math.stackexchange.com/questions/784897",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/126120/"
] | <blockquote>
$U$ is a subspace $ U= \{(x_1,x_2,x_3,x_4) \in \mathbb{R} \mid x_1+x_2+x_3+x_4=0\} $
Find a Basis of $U$.
</blockquote>
I know a basis of the whole $\mathbb{R}^4$ needs at least 4 independent vectors, since it's only a subspace, I don't know how many I need. Are 2 vectors, fullfilling the equation, su... | There is only one condition, which as a rule of thumb removes only one from the dimension.
Formally:
$U$ is the kernel of the surjective linear map $\mathbb R^4\to \mathbb R$, $(x_1,x_2,x_3,x_4)\mapsto x_1+x_2+x_3+x_4$. Therefore $\dim U + \dim \mathbb R=\dim \mathbb R^4$. In other words: you need three independant vec... | 2 vectors aren't enough. It is a 3D-plane in a 4D space. Its basis need to have 3, 4-dimensional vectors.
3 vectors: (1,0,0,-1), (0,1,0,-1), (0,0,1,-1).
| https://math.stackexchange.com |
374,350 | [
"https://mathoverflow.net/questions/374350",
"https://mathoverflow.net",
"https://mathoverflow.net/users/129428/"
] | Suppose we are given a group <span class="math-container">$G$</span> in terms of generators <span class="math-container">$t_1, ..., t_n$</span> which are order 2 in <span class="math-container">$S_m$</span> (however we don't assume anything other than that these elements generate <span class="math-container">$G$</span>... | There is a theoretical answer (as opposed to an algorithmic answer) found in Björner and Brenti's "Combinatorics of Coxeter groups", Section 1.5. (They seem to credit it to Matsumoto.) Their Theorem 1.5.1:
Suppose <span class="math-container">$W$</span> is a group generated by a subset <span class="math-con... | I don't think this is what the questioner means, so this is not really an answer. But it's worth mentioning and it's too long for a comment.
If we know that <span class="math-container">$t_1,\ldots,t_n$</span> are transpositions, then <span class="math-container">$G$</span> is a "reflection subgroup" of <spa... | https://mathoverflow.net |
1,498,442 | [
"https://math.stackexchange.com/questions/1498442",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/140308/"
] | Based on Williams' Probability w/ Martingales:
Let <span class="math-container">$(S, \Sigma, \mu)$</span> be a measure space.
<blockquote>
<strong>Scheffe's Lemma Part (ii)</strong>: Suppose <span class="math-container">$\{f_n\}_{n \in \mathbb{N}}, f \in \mathscr{L}^1 (S, \Sigma, \mu)$</span> and <span class="math-cont... | The question has been edited. The updated question now asks to prove:
<blockquote>
Suppose <span class="math-container">$\{f_n\}_{n \in \mathbb{N}}, f \in \mathscr{L}^1 (S, \Sigma, \mu)$</span> and <span class="math-container">$\lim_{n \to \infty} f_n(s) = f(s)$</span> a.e. in <span class="math-container">$S$</span>. T... | You're assuming that the $f_n$ are nonnegative, so $f_n^+ = f_n$ and you don't need to consider $f_n^-$.
First note that $$\left| \int_S f_n \, d\mu - \int_S f \, d\mu \right| \le \int_S |f_n - f| \, d\mu$$ so that one implication is easy and doesn't even require $f_n \to f$ a.e.
Next, since $|f_n - f| \le f_n + f$ y... | https://math.stackexchange.com |
115,163 | [
"https://softwareengineering.stackexchange.com/questions/115163",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/10326/"
] | I am very familiar with the concept of object pooling and I always try to use it as much as possible.
Additionally I always thought that object pooling is the standard norm as I have observed that Java itself as well as the other frameworks use pooling as much as possible.
Recently though I read something that wa... | It is deprecated as a general technique, because - as you noticed - creation and destruction of short lived objects per se (i.e. memory allocation and GC) is extremely cheap in modern JVMs. So using a hand-written object pool for your run-of-the-mill objects is most likely slower, more complicated and more error-prone ... | The answer to the concrete question: 'Is object pooling a deprecated technique?' is:
<strong>No. Object pooling is widely used in specific places - thread pooling, database connection pooling etc.</strong>
General object creation has never been a slow process. Pooling in itself consumes resources - memory and process... | https://softwareengineering.stackexchange.com |
146,481 | [
"https://dba.stackexchange.com/questions/146481",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/101583/"
] | I have a table with a field called <code>date_created</code> with <code>TIMESTAMP</code> format now I have a requirement to fetch records between <code>0-30</code>,<code>31-60</code>,<code>61-90</code> like this.
I able to fetch record between months like
<pre><code>SELECT received_quantity , delivery_qty , unit_cos... | Try this :
<pre><code>SELECT received_quantity , delivery_qty , unit_cost
FROM tbl_inward_list
WHERE datediff(current_date,date(date_created)) BETWEEN 31 AND 60;
</code></pre>
Similarly
<pre><code>SELECT received_quantity , delivery_qty , unit_cost
FROM tbl_inward_list
WHERE datediff(current_date,date(d... | <pre><code>WHERE date_created >= CURDATE() - INTERVAL 60 DAY
AND date_created < CURDATE() - INTERVAL 30 DAY
WHERE date_created >= CURDATE() - INTERVAL 90 DAY
AND date_created < CURDATE() - INTERVAL 60 DAY
</code></pre>
No, don't use 31 and 61; use the inequality.
<code>CURDATE()</code> will... | https://dba.stackexchange.com |
23,960 | [
"https://chemistry.stackexchange.com/questions/23960",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/2246/"
] | How can we determine how many delocalised electrons every atom of gold contributes to the 'sea of delocalised electrons'? More generally, how can we determine the number of electrons any metal contributes? Must we have thermodynamic data to determine this?
| Briefly: valence orbitals on adjacent metal atoms interfere to form "bands" of closely spaced molecular orbitals that spread throughout the metal.
If you have N metal atoms with occupied s orbitals, they interfere to form an "s band" with N molecular orbitals. The lowest energy orbital is fully bonding; the orbital o... | Valance electron of s and p orbital take part in delocalisation over the positively charged gold atoms
| https://chemistry.stackexchange.com |
31,338 | [
"https://datascience.stackexchange.com/questions/31338",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/51743/"
] | I'm sorry for being so weak in math. (I'm a student) For eg. this is a correlation matrix.
<pre><code> Q1 Q2 Q3
Q1 1.000000 0.707568 0.014746
Q2 0.707568 1.000000 -0.039130
Q3 0.014746 -0.039130 1.000000
</code></pre>
Why is it symmetric? Why is Q1:Q2, the same as Q2:Q1? Shouldn't they b... | The correlation matrix is a measure of linearity. It does not express how two variables are dependent on each other. If the relationship is approximately linear, the absolute value of correlation will be closer to 1. If there is no linear relationship, the value is zero.
Consider two sets of variables (x1,y1) and (x2... | Intuitively, the correlation matrix is symmetric because every variable pair has to have the same relationship (correlation) whether their correlation is in the upper right or lower left triangle. It doesn’t make sense to say the correlation between variables $X_1$ and $X_2$ is $\rho$, but the correlation between $X_2$... | https://datascience.stackexchange.com |
101,494 | [
"https://security.stackexchange.com/questions/101494",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/88011/"
] | As you know, SQLite have no security mechanism. My question is if i change the database file into some random name like '22OYIz23tmpyesTcMVFmw1tquFxzCIQD.sqlite' can someone exploit my database and how?
Assuming that i have my own server, not shared host, the database is only accessed throught my web app(PHP)
| SQLite is not like a traditional DBMS in that it does not serve the content of the database to a computer network. It is actually kinda like just storing everything in a file. SQLite is will then allow your application to load that file and query it.
As such there is no need or advantage to a user account system as on... | The database management system you are using have nothing to do with the fact that you could be compromised and your data stolen.
If someone can execute arbitrary commands through your PHP webservice, chances are that they can retrieve your website's sources and with it the name / IP / password of your database.
Bas... | https://security.stackexchange.com |
452,645 | [
"https://physics.stackexchange.com/questions/452645",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/75502/"
] | I have tried to hurl a stone with some added rotation and it performed slightly better but I have great difficulty replicating this feat for consistency, my question is should I add angular momentum to my throw or it was just a waste of energy?
| Try thinking of your stone as a spinning top. Its angular momentum will stabilize its orientation in the air, flat side down, which in turn will make it skip across the water more evenly. Without the angular momentum, it would topple over, just like a spinning top that has stopped spinning. That would make it hit the ... | There are two tricks to a good skipping stone.
Lydéric Bocquet published some meaningful work on this subject.
Firstly, the spin makes sure the stone maintains its orientation just as a spinning top does.
Secondly, the orientation you want to impart to the stone should cause its trailing edge to hit the water first s... | https://physics.stackexchange.com |
35,525 | [
"https://softwareengineering.stackexchange.com/questions/35525",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/12824/"
] | I am a relatively young programmer. I am 23 and I have been programming professionally for about 5 years.
As most programmers I started with C, learned some x86 assembly for fun and then I found C++ which turned out to be my greatest passion in the programming world. Programming with C and C++ forces you to learn plat... | Personally, I think "keeping track" of languages is a waste of time. It's always good to pick up new popular languages, but once you have a popular and well-established language like C++, Python, etc. under your belt, you shouldn't be worried. If you're a good programmer, the language is just a set of keywords.
The... | <blockquote>
I am worried that someday I won't be able to keep track of them all.
</blockquote>
After 30 years, let me say this.
Yep, you lose track.
So?
That's why they write reference manuals.
| https://softwareengineering.stackexchange.com |
24,507 | [
"https://dba.stackexchange.com/questions/24507",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/3460/"
] | I have a database that an application connects to using <code>db_owner</code> permissions.
How do I effectively determine the minimum set of requirements actually needed by this user (application) to run without causing service interruption? (ie. without trial and error)
| This should be available from the developer who wrote the software. However, on occasions this information is not available - for instance if the software is written by a third party vendor. Most vendors tend to say it needs <code>sa</code> or <code>db_owner</code> which is generally down to poor coding on their part i... | It all boils down to one simple thing: do you have access to the source code of the application?
If you do (and presumably buy-in to make these changes; also, technically, reverse-engineering falls into this category, but... you didn't hear that from me), it's simply a matter of searching, testing, and documentation. ... | https://dba.stackexchange.com |
2,604,337 | [
"https://math.stackexchange.com/questions/2604337",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/475505/"
] | What I can think of thus far is that $125^{100} = (\frac{1000}{8})^{100} = \frac{1000^{100}}{2^{300}}$
I know that $2^{10} = 1024$ so $\frac{1000^{100}}{1024^{30}}$.
That's all I can figure out this far.
I was thinking to divide the numerator and denominator of $\frac{1000^{100}}{1024^{30}}$ by $1000^{30}$ and I th... | $2^{10}\approx 10^3$, so approximately, $\frac {1000^{100}}{10^{90}} =\frac {100^{100}\cdot 10^{100}}{10^{90}}=100^{100}\cdot 10^{10}=10^{210}$... So about $211$.
| Calculate
$\log_{10}125^{100}$
$= 100\cdot \log_{10} (1000/8)$
$= 100\cdot(3-3\log_{10}2)$
$= 100\cdot(3-0.9030)$
$= 100\cdot(2.0970)$
$= 209.70$
Therefore number of digits $= [209.70]+1 = 210.$
| https://math.stackexchange.com |
44,463 | [
"https://chemistry.stackexchange.com/questions/44463",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/25206/"
] | I tried to burn a bunch of 8B pencil lead in a gas flame the other day. None of it actually caught fire, but being made of carbon (in the form of graphite) should cause it to burn, as graphite is essentially purified coal. What temperature is required to set it burning, and is extra oxygen required?
| You have two problems. <br>
1.) pencil lead graphite is actually a graphite-clay composite. <br>
2.) carbon does not sustain burning easily unless it is held at very high temperatures.
Pencil lead will not burn effectively for the first reason as clay is non-combustible and smothers any fire load. If you did have pur... | It’s all in the bonds. It doesn’t matter what you have, if it is in a tight covalent bond structure it is going to take a lot of input energy to trigger oxidation. What many people don’t understand is that anything can burn unless it has already been fully burned. Other than that, you can burn it, no questions ask. Eve... | https://chemistry.stackexchange.com |
2,510,759 | [
"https://math.stackexchange.com/questions/2510759",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/414802/"
] | <blockquote>
If $p \ge 5$ is a prime number, how to show that $p^2 + 2$ is always divisible by $3$?
</blockquote>
Please let me have a clear proof based on simple number theory.
| Since $p$ is a prime number which is <em>not</em> a multiple of $3$, $p$ can be written as $3k\pm1$, for some $k$. But then $p^2+2=9k^2\pm6k+1+2=9k^2\pm6k+3=3\times(3k^2\pm2k+1)$.
| You can also use Fermat's Little Theorem.
$a^{q-1} \equiv 1 \pmod q$ (where $q$ is a prime that does not divide $a$).
In particular, $a^2 \equiv 1 \pmod 3$.
Now substitute $a = p$. Since $p$ is a prime greater than $3$, it satisfies the non-divisibility criterion.
$p^2 \equiv 1 \pmod 3$
So $p^2 + 2 \equiv 3 \equiv... | https://math.stackexchange.com |
49,467 | [
"https://astronomy.stackexchange.com/questions/49467",
"https://astronomy.stackexchange.com",
"https://astronomy.stackexchange.com/users/41117/"
] | For context, at some point during the 20th century (and maybe earlier as well), the most popular planet formation theory and the one that was taught at (at least some) schools was the theory that the planets were somehow ‘spit out’ by the sun.
I am not very familiar with this theory, and I have only ever heard it from ... | Between 1965 and 1980
I recall a book from the "Ladybird" series of short non-fiction books for children which included two possibilities for the formation of the planets. One was "there was left-over gas in a nebula around the sun, this eventually formed planets" and the other was "A passing s... | That specific idea doesn't seem familiar to me, and I work in that field. Particularly this idea would only be called 'theory' when making testable predictions.
The only thing that comes to mind here is Laplace's extended solar atmosphere. Laplace (in the leate 1700's), as one of the first, invoked the centrifugal forc... | https://astronomy.stackexchange.com |
10,503 | [
"https://mathoverflow.net/questions/10503",
"https://mathoverflow.net",
"https://mathoverflow.net/users/943/"
] | Every surface can be triangulated in such a way that at most 7 trianlges meet at one vertex. Every surface can be decomposed in squares such that at every vertex at most 5 suqares meet. For surfaces of genus more than 1 this is the low bound.
What happen in higher dimensions, for example for 3 and 4-manifolds, ect...... | To change up the nature of the responses some, IMO a good theorem to think about is the Kan-Thurston theorem. It states that given any space $X$ you can find a $K(\pi, 1)$ space $Y$ and a map $f : Y \to X$ inducing isomorphisms $f_* : H_i Y \to H_i X$, $f^* : H^i X \to H^i Y$ for all coefficients (it can be souped-up t... | Here's one for number 2 that's pretty standard. Let $X$ be $S^2$ with two loops attached to it at points. Then the cohomology groups are $\mathbb{Z},\mathbb{Z}^2,\mathbb{Z}$, and we've seen that somewhere else: the torus $S^1\times S^1$. However, the cup product is different, because it's trivial on the sphere with ... | https://mathoverflow.net |
24,640 | [
"https://engineering.stackexchange.com/questions/24640",
"https://engineering.stackexchange.com",
"https://engineering.stackexchange.com/users/17943/"
] | In my country are sold gas with different octane number (RON: Research Octane Number): 88, 90, 92, 95, 97, 98, and the latest is 100. RON-88 is still sold since still many old cars, motor bike, and agriculture machines which are using low spec gas. RON-90 is intended for transition, as alternative in case customer want... | Imagine you have two boxes separated by a removable partition. Each box is filled with a different pure monatomic gas. When you remove the partition, the gases mix. Their equilibrium state tends toward an equitable mixture at every point in time. The likelihood of the gases spontaneously separating is essentially zero ... | Nature will always try to minimize the number of dislocations and grain-boundaries, whereas in the case of vacancies, there is an optimum number that will lead to the lowest Gibbs energy.
| https://engineering.stackexchange.com |
632,387 | [
"https://physics.stackexchange.com/questions/632387",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/297028/"
] | <blockquote>
Imagine a moving ice-block on an extremely smooth <em>(i.e. friction is zero)</em> and circular, rogue planet devoid of an atmosphere. Assuming that the planet in question is far away from any (massive) object and the only forces acting on the ice-block are the normal force and the gravitational force of t... | It's not about what causes the gravitational force to be greater. It's that the normal force will be less (these two statements are the same mathematically, but colloquially they change the focus).
Let's look at a simpler example. Take a box and put it on the floor. We can say the normal force acting on the box is equa... | To take an extreme example, keep increasing the speed of the block until it reaches orbital velocity. At that point the block is in orbit (even though it's right next to the surface, even touching the surface -- remember, you postulated no friction and no air resistance). For a body in orbit the normal force is 0. So t... | https://physics.stackexchange.com |
61,753 | [
"https://softwareengineering.stackexchange.com/questions/61753",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/20908/"
] | I have been struggling lately with the whole subject of documentation at my current position. I am at a point in my programming career in which I feel I have just been birthed into the whole world of proper and effective documentation. Currently there is very little to be found in any database or source code at the c... | I understand your pain when it comes to the lack of documentation. I constantly find myself on projects without clearly defined requirements because the requirements document exists in someone's head somewhere stemming from a conversation that took place four years ago.
I feel it depends to some degree on the p... | Documentantion standards should be policed by the company. If no such policy exists yet, management should gather with the developers and agree on a reasonable set of documentation that is obligatory for each and every project.
| https://softwareengineering.stackexchange.com |
72,166 | [
"https://cs.stackexchange.com/questions/72166",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/26733/"
] | Is there a term for programming languages that read like written sentences? I'm thinking of languages like Python, where you can almost read the code aloud as a sentence, as opposed to C++ which is really arcane.
For example, in Python <code>if 'pizza' not in animals</code> is very clear when read aloud.
Seems like m... | I know of no standard term for that aspect of PL. Maybe you can use "human-readable syntax" or "human-friendly syntax".
In PL theory, we (unapologetically) tend to disregard syntactic issues (e.g., look at LISP), and focus more on language features / semantics / types and more math-y stuff.
I mean: if you asked me ... | Natural language programming is what I believe you would call this
| https://cs.stackexchange.com |
1,600 | [
"https://mathoverflow.net/questions/1600",
"https://mathoverflow.net",
"https://mathoverflow.net/users/303/"
] | Let (X,x) be a pointed space. There is an action of π<sub>1</sub>(X,x) on π<sub>n</sub>(X,x) -- determined by considering π<sub>n</sub>(X,x)=π<sub>n-1</sub>(Ω<sub>x</sub>X,<b>x</b>), where Ω<sub>x</sub>X denotes the space of loops in X based at x, and <b>x</b> denotes the constant loop -- given... | For the last part of your question: given a group π<sub>1</sub> which acts on an abelian group π<sub>n</sub>, there is always as space X with these homotopy groups with this action, and you can manufacture one using Eilenberg-MacLane spaces. You can make the group π<sub>1</sub> act on the Eilenberg-MacLane sp... | If you are thinking about π<sub>1</sub>, the action is just that by conjugation in this group, so is trivial iff the fundamental group is abelian.
I think the easist place to see the nontriviality of this action in higher dimensions is for the space S<sup>1</sup> v S<sup>n</sup>. By considering its universal cover and... | https://mathoverflow.net |
1,012,157 | [
"https://math.stackexchange.com/questions/1012157",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/190384/"
] | My problem is defined to determining the max and min for the function:
$$f(x,y)=x^2+y^2-2x-4y $$ within the domain:
$$ x^2+y^2\leq9$$
And I've been able to locate the minimum which is found within the domain. I am having trouble locating the maximum on the rim (saddle?).
The way I've started is to substitute x, y... | Geometrically, this is easy. $f(x,y) = (x-1)^2+(y-2)^2-5 = d(x,y)^2 -5$, where $d(x,y)$ is the distance from $(x,y)$ to the point ($1,2)$.
So you only have to maximise the distance from $(1,2)$ to the circle $r^2=3$, which occurs at the point diametrically opposite $(1,2)$, i.e. $(-\frac{3}{\sqrt 5},-\frac{6}{\sqrt 5}... | If $\tan(t) = 2$, then $|\sin(t)| = \frac{2}{\sqrt{5}}$ and $|\cos(t)| = \frac{1}{\sqrt{5}}$.
| https://math.stackexchange.com |
444,691 | [
"https://electronics.stackexchange.com/questions/444691",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/223769/"
] | Does anyone know what is the internal structure of TLP250? Like how the connections are done inside the IC?
| The complementary emitter follower gives much more current gain using the secondary supply and inversely lower source impedance V/I=Zout from the opto current-source CTR ( equivalent to low hFE) suitable for the TLP250 to drive a MOSFET and IGBT rather than the open collector type Optos.
| Mosfet Driver TLP 250 . This image found by a quick search on the internet.<img src="https://i.stack.imgur.com/21MpQ.png" alt="enter image description here">
| https://electronics.stackexchange.com |
11,963 | [
"https://mechanics.stackexchange.com/questions/11963",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/6521/"
] | I just bought an used Toyota Yaris 2008 hatchback and I'm preparing to change oil for the first time.
There's a jack supplied with the car (along the with spare tire). However, the manual seems to suggest that I should use a floor jack to jack up my car instead.
Is this just a legal thing where they just recommend th... | I'm not sure what it specifically says, but it is indeed not safe to be under a car that is supported only by a jack, hydraulic or mechanical. To be safe, get a pair of jack stands, and use whatever jack you want to raise the car high enough to put the stands in place, then lower it so vehicle is resting on the stands... | The supplied car jack is for an emergency change of a wheel, pucture?, on firm ground, with the wheel nuts loosened before raising the vehicle, with the handbrake firmly applied. It is not intended for any other use. At no time put any part of yourself, or allow anybody else, under a vehicle raised on this type of jack... | https://mechanics.stackexchange.com |
282,228 | [
"https://mathoverflow.net/questions/282228",
"https://mathoverflow.net",
"https://mathoverflow.net/users/115131/"
] | Let $L^*$ be the total space of the line bundle $\mathcal{O}_{\mathbb{P}^n}(k)$ minus its zero section.
How can one compute the fundamental group of $L^*$?
For k = 0 the space $L^*$ is $\mathbb{P}^n \times \mathbb{C}^*$ hence $\pi_1(L^*) = \mathbb{Z}$.
For k=-1 the $L^*$ is $\mathbb{C}^{n+1} \setminus \{0\}$, theref... | The fibration $\mathbb{C}^\times\to L^\times\to\mathbb{P}^n$ can be "delooped" to a fibration $L^\ast \to\mathbb{P}^n\to{\rm B}\mathbb{C}^\times$ where the last map is the classifying map for the line bundle. Now we have ${\rm B}\mathbb{C}^\times\cong\mathbb{P}^\infty$, and we want to identify the map $\pi_2(\mathbb{P}... | Another way to see that the fundamental group is $\mathbb{Z}/k$ is using geometry of cyclic quotient singularities and it goes as follows.
We consider the negative twists first. In this case the total space $\mathcal{O}_{\mathbb{P}^n}(-k)$ is the resolution of singularities of the quotient space $\mathbb{C}^{n+1} / G... | https://mathoverflow.net |
54,261 | [
"https://dba.stackexchange.com/questions/54261",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/16080/"
] | I've got this query I'd like to get working. I'm trying to count all the instances of a foreign key after a certain date that isn't null, and then left join that to the distinct of the original table, so I can see the variables with no entries. Here's my query:
<pre><code>select fulllist.fk_fc_id
from
(
select di... | It looks like this is just a typo. Your subquery is returning <code>fk_fc_id</code> but your join is referencing <code>fc_fk_id</code>. It seems like you just need to alter the query:
<pre><code>select fulllist.fk_fc_id
from
(
select distinct fk_fc_id
from data_item
) as fulllist
left outer join
(
se... | Besides the typo (already found by @blueefet), your query can be simplified:
<pre><code>select fk_fc_id,
COUNT( CASE WHEN di_item_value not like 'Null'
and di_timestamp > '2013-11-01 00:00:00'
THEN 1 ELSE NULL END
) AS number
from data_item
group by f... | https://dba.stackexchange.com |
273,799 | [
"https://softwareengineering.stackexchange.com/questions/273799",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/22415/"
] | I'm wondering if the following use of typedefs is any good practice, or if there are any downsides to it.
Basically I have alot of "data"-structs, which are intended to be used in (globally unique) containers, and they hold indices to other (globally unique) containers, the reason is to keep memory access localized. R... | The heart of creating abstractions is to give things a good name, which makes clear what a specific data type represents. And that's what you are doing here, no less, no more.
Creating the right abstractions is what makes actually the difference between good code and bad code, so yes, assumed these abstractions serve... | In general, using a typedef is never really a bad thing, all it does is tell the compiler that when you use, say "Transform," you actually meant "Mat4." All typedef does is make it easier for the programmer to specify between separate types. There is usually no downside to using typedef.
Overall, it is neither a good ... | https://softwareengineering.stackexchange.com |
48,820 | [
"https://chemistry.stackexchange.com/questions/48820",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/28456/"
] | I figured out that KCl + NaOH may result in KOH but I think this reaction can not happen in an aqueous medium. So there I have a couple of questions.
<ol>
<li>Is there a way to make the reaction I stated above to happen?</li>
<li>Is there a way to turn KCl into KOH?</li>
</ol>
| <ol>
<li>I think the reaction does not works very well, because you get an equilibrium of KOH and NaOH.</li>
<li>If you have KCl in an aqueous solution you can use AgOH to get KOH. AgCl precipitate and the solution contains only KOH. If you like to get KOH it is better to buy them.</li>
</ol>
| On an industrial scale, the preferred method for the production of potassium hydroxide from potassium chloride is electrolysis (diaphragm, mercury, or membrane process).
$$\begin{align}
\ce{2Cl- &-> Cl2 + 2e-}\\
\ce{2H2O + 2e- &-> H2 + 2OH-}\\
\hline
\ce{2Cl- + 2H2O &-> Cl2 + H2 + 2OH-}
\end{align... | https://chemistry.stackexchange.com |
43,490 | [
"https://quant.stackexchange.com/questions/43490",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/38348/"
] | I'm looking at different option strategies and the ways that their payoffs differ (and therefore how they can differently be used).
I'm looking at the long seagull (buy a call spread and sell a put), and wondering if taking the opposite positions in these would provide an unlimited payoff with decreasing strike and a ... | Based on your example, at expiry, your gains on your put will be between 0 - 1.2 of your underlying; you will be flat between 1.2 - 1.3; you will lose between 1.3 - 1.4 due to your short call; and you will be flat > 1.4 of your underlying due to your long call offsetting your short call, ignoring premiums.
| Long put always has limited payoff (bounded by the strike)
| https://quant.stackexchange.com |
20,799 | [
"https://quant.stackexchange.com/questions/20799",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/6190/"
] | Suppose I have 2 strategies;
A) Buying A One Year Bond And Holding To Maturity (Buy & Hold To Maturity)
B) Buying A 3 Year Bond and Selling After One Year (Rolling Down The Yield Curve)
Assume that the 1 year treasury yield to be 0.24%, the two year 0.55%, and 3 year to be .80%. The cost of funding is assumed to... | Strategy (B) will always win. In most simple sense, you are achieving a yield of 0.80% for investment in the first year, and sell-buying back at 0.80% for investment in the second year (because as you state the yield curve has not moved). This is known as carry. There is an indirect gain through price-roll, too.
Stra... | Strategy A: You borrow at 0% and invest at 0.24% for one year, so you make 0.24% total return.
Strategy B: You borrow at 0% and invest for one year at 0.80%, making 0.80%. You then sell the 0.80% bond at a yield of 0.55% (it is now a 2yr bond, whose yield must be 0.55% if rates are unchanged). The price of this bo... | https://quant.stackexchange.com |
255,599 | [
"https://stats.stackexchange.com/questions/255599",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/132098/"
] | A small question
In the book of Rasmussen Page 115 last paragraph. When we have multiple databases you setup a gaussian for each database and the optimisation is said can be done by adding the likelihoods as a single one.
What about the gradients? do I add the gradients?
If I have a simple gaussian with three hype... | $4$ points? Did you mean $4$ different problems that you want to link together and learn the same set of hyperparameters? Then your new objective function would be the sum of $4$ (log marginal) likelihoods (Instead of optimizing the each of the $4$ likelihoods independently). To do so one can use gradient descent (wh... | Following the comment of Seeda
i can said that:
The likelihood is defined as:
$$\mathcal{L}\left( \Theta \right) = - \frac{1}{2}\log \left( {\left| K \right|} \right) - \frac{1}{2}{y^T}{K^{ - 1}}y - \frac{N}{2}\log \left( {2\pi } \right)
$$
Supposing that there are two set of data A and B.we can define two Likelih... | https://stats.stackexchange.com |
5,658 | [
"https://mathoverflow.net/questions/5658",
"https://mathoverflow.net",
"https://mathoverflow.net/users/1745/"
] | $l_1$ minimization / compressed sensing comes to mind. Does anyone have any concrete examples? Or is such a construct completely useless?
| One place where this is used is with Hilbert bundles. Taking $L^2$-functions on a space generally works very badly, so one instead takes $L^{2,1}$-functions - functions which are differentiable (almost everywhere) with square-integrable first derivative. However, the transition functions aren't isometries with respec... | suppe, have you heard of Banach spaces? They're a generalization of Hilbert spaces: they are complete normed vector spaces, but their norms need not arise from any inner product. $l\_1$ is just one example.
I'm sorry if you're already familiar with all that; your question has almost no context, so it's hard to answer... | https://mathoverflow.net |
3,828,003 | [
"https://math.stackexchange.com/questions/3828003",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/825615/"
] | <blockquote>
Show that <span class="math-container">$G=\{0,1,2,3\}$</span> over addition modulo 4 is isomorphic to <span class="math-container">$H=\{1,2,3,4\}$</span> over multiplication modulo 5
</blockquote>
My solution was to brute force check validity of <span class="math-container">$f(a+b)=f(a)f(b)$</span> for all... | Both are cyclic groups of order 4. This is enough to say they are isomorphic because all cyclic groups of order <span class="math-container">$n\in\mathbb{N}$</span> are isomorphic to <span class="math-container">$\mathbb{Z}_n$</span>, but the general idea is that you're mapping one cyclic generator to another.
Notice t... | In general, you need to prove the existence of a bijective homomorphism between the two groups.
In practice, there is only one cyclic group of each order, <span class="math-container">$\Bbb Z_n$</span>. Here can use that fact to establish the result.
To wit, <span class="math-container">$\Bbb Z_p^\times$</span> is kno... | https://math.stackexchange.com |
38,170 | [
"https://mechanics.stackexchange.com/questions/38170",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/23523/"
] | I brought my Peugeot 308 into a garage to have some panels resprayed.
Once the respray was complete, they told me that there was a problem with the car and it wasn't starting.
Now, after 5 weeks, they've told me that it's the <code>body control module</code> that needs replacing. The car was in perfect working orde... | <blockquote>
Do all cars that come factory fitted with alloys follow this rule?
</blockquote>
No, there are three basic options that I see all the time.
<ol>
<li>An Emergency wheel - Will get you to the next tire shop or gas station. Has a speed limit of 55/mph and is smaller than normal. Also has a limited range. ... | Because you rarely use it, reducing its size and weight is an effective method for cutting down costs. It also saves space and makes packaging easier for the engineers. The cost aspect is also the reason for the spare being being a steel one instead of the more expensive alloy.
| https://mechanics.stackexchange.com |
32,291 | [
"https://engineering.stackexchange.com/questions/32291",
"https://engineering.stackexchange.com",
"https://engineering.stackexchange.com/users/23840/"
] | Does a house heating system affect indoor air humidity?
Or to be more precise, does it affect your perception of how dry the air in the house feels (for instance dry is your mouth waking up)?
(I searched for this but could only find explanations from furnace company websites)
Every (lay)person I ask this seems to th... | Your last statement is basically correct:
<blockquote>
Another possible explanation is that the higher air temperature increases the air's water carrying capacity, which in the absence of an abundant water source, extracts water from your body faster. Therefore increasing perceived dryness.
</blockquote>
Heating ai... | To summarize the comments given here: During the winter, the cold air outside can hold only a small amount of water in solution, which means it takes very little moisture in cold air to get it to 100% relative humidity.
But then, when your furnace draws in some of that cold outside "makeup" air, it heats it up and it... | https://engineering.stackexchange.com |
143,297 | [
"https://cs.stackexchange.com/questions/143297",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/141586/"
] | I was asked to give an example of a graph that has edges with negative weight, but Dijkstra's algorithm will still give us the correct output. It was part of a prove/disprove question. The claim was.. <em>"If a graph has negative edges, Dijkstra's algorithm will return the wrong output"</em>.
| Suppose graph <span class="math-container">$G$</span> is directed, and acyclic such that just some of edges of source <span class="math-container">$s$</span> have negative weight, hence Dijkstra's algorithm does work in a such case.
| Take a look at the simplest possible example: Our graph has only two nodes: <span class="math-container">$s,t$</span>, and a single edge between them.
In this example, it won't matter what is the cost of this single edge (it can be negative), and Djikstra will always return the only single path between <span class="mat... | https://cs.stackexchange.com |
64,309 | [
"https://cs.stackexchange.com/questions/64309",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/-1/"
] | I am learning about preceptrons and my professor noted that z-scores are a commmon pre-processing step to normalizing input variables. Following this, I am having trouble understanding why z-scores are useful when training a preceptron?
My current understanding is that z-scores allows us to calculate the probability ... | You are correct!
According to the defenition Throughput (in case of scheduling in OS) is :
<blockquote>
Number of processes that can be completed in a unit of time.
</blockquote>
The process can use both the CPU or the I/O operations. So the I/O time and idle times and CPU time are calculated in throughput.
<stro... | But one more thing I want to add, let say first process is available at 2 units then time period from 0 to 2 will not be considered in CPU total time while computing throughput. As processes are not yet available.
<hr />
Throughput actually includes total time where processes are available and executing doing either co... | https://cs.stackexchange.com |
9,436 | [
"https://quantumcomputing.stackexchange.com/questions/9436",
"https://quantumcomputing.stackexchange.com",
"https://quantumcomputing.stackexchange.com/users/5008/"
] | A generalized measurement can be defined as follow:
It is a set of operators <span class="math-container">$\{M_m \}$</span> such that, given an initial quantum state <span class="math-container">$|\psi\rangle$</span>, we have the state after measurement being:
<span class="math-container">$$|\psi\rangle \rightarrow \... | It is not necessary to define the action of <span class="math-container">$U$</span> on other states. It is sufficient to know that the defined action is compatible with being a unitary (i.e. inner produces are preserved between all inputs) because there always exists an extension of the defined action such that it is u... | A matrix/operator is unitary <em>if and only if</em> it sends orthonormal bases into orthonormal bases.
Therefore, to check whether <span class="math-container">$U$</span> is unitary, it is enough to check that it sends a <em>subset</em> of a basis (i.e. a non-complete set of orthonormal vectors) into a set of orthono... | https://quantumcomputing.stackexchange.com |
238,842 | [
"https://security.stackexchange.com/questions/238842",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/231319/"
] | How safe are we when we use phone hardware from untrusted manufacturers and use end-to-end encrypted communication like Signal and Telegram? Are our conversations really safe from keyloggers or spyware? And what is the best option to communicate safely?
| No, the device can see anything you can see, so if it's compromised, using encryption wouldn't protect you against that specifically.
When you use encryption, it goes something like this:
<ol>
<li>You type or say something into the phone.</li>
<li>This goes through the phone's firmware / operating system to turn you to... | The short answer is that if the hardware is compromised, then anything you can read, it can read.
| https://security.stackexchange.com |
153,280 | [
"https://physics.stackexchange.com/questions/153280",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/5640/"
] | Say Alice could instantly encode another human being, Bob, into photons.
Say Alice then beams those photons to a receiver on another planet, Vogon, one light year away. Charlie is controlling the receiver on Vogon and when Bob's photons arrive, he immediately sends the photons back to Earth. When they arrive Bob is re... | This is the answer for the "continuum" bit of your question or what happens if Bob leaves for Vogon on a sublight spaceship.
I've assumed that Alice and Charlie are one light year away from each in the rest frame of either and that they are at rest with respect to each other (ie the separation will remain one light ye... | <strong>Alice:</strong> 2 years
<strong>Bob:</strong> No time has passed
<strong>Charlie:</strong> 2 years
There are a couple of things to point out:
<ol>
<li>First two planets aren't likely to be stationary with respect to each other, they will be rotating at different rates around different suns that are themselv... | https://physics.stackexchange.com |
398,950 | [
"https://softwareengineering.stackexchange.com/questions/398950",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/-1/"
] | Let's suppose I have the following objects in my domain:
<pre class="lang-cs prettyprint-override"><code>class Warehouse
{
int Id;
string Address;
Item[] Inventory;
}
class Item
{
int Id;
string Name;
int Price;
int WarehouseId;
}
</code></pre>
I have a collection of all the warehouses an... | I would separate the warehouse from the items:
<pre><code>GET api/warehouse
Gets a list of warehouses
PUT api/warehouse/{warehouseid}
Updates some meta information on that warehouse (address etc)
GET api/warehouse/{warehouseid}/inventory
Gets the inventory of the warehouse (you can supply some search parameters t... | If the Inventory is truly part of your Warehouse resource, then the PUT should override the state of the Inventory array. How you store the data in the back-end, e.g. an RDBMS or NoSQL DB, should not affect the semantics of your REST endpoints. The resources your consumers care about are the ones in your API, not the ... | https://softwareengineering.stackexchange.com |
86,844 | [
"https://softwareengineering.stackexchange.com/questions/86844",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/11785/"
] | Recently I was TDDing a factory method. The method was to create either a plain object, or an object wrapped in a decorator. The decorated object could be of one of several types all extending StrategyClass.
In my test I wanted to check, if the class of returned object is as expected. That's easy when plain object os ... | My thought is no, you shouldn't do anything <em>only</em> because it is needed for testability. A lot of decisions people make have benefit to testability and testability may even be the main benefit but it should be a good design decision on other merits. This does mean that some desired properties are not testable. A... | My take on this is - sometimes you have to rework your source code a little to make it more testable. It's not ideal and shouldn't be an excuse to clutter the interface with functions that are only supposed to be used for testing so moderation is generally the key here. You also don't want to be in the situation where ... | https://softwareengineering.stackexchange.com |
13,616 | [
"https://softwareengineering.stackexchange.com/questions/13616",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/1368/"
] | I happened to know some system admin and according to him, testing guys are not given preferences in an organization in comparison to developers. No doubt software releases are not possible without testers but I've never laid my hands on testing so not much aware of it. No offense intended.
| In my experience, unfortunately, they are often treated like second class employees and even worse a frivolous perk for programmers.
It stems from a number of things:
<ol>
<li>When the testers are doing their jobs correctly, it is easy for everyone but the programmers to forget they even exist. Much like a network ... | It depends on the company, but usually. They're often seen as second class citizens, and in many companies, testing is seen as an entry-level position from which you would then move on to becoming a real developer.
This is, of course, crap. Having worked with some good testers, I can say that they are both valuable ... | https://softwareengineering.stackexchange.com |
3,377,576 | [
"https://math.stackexchange.com/questions/3377576",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/634846/"
] | Would it be correct to say that <span class="math-container">$\emptyset \subseteq \emptyset$</span> or <span class="math-container">$\emptyset \subseteq \{\emptyset\}$</span>? To my understanding, the null set is just an empty set, so a null set is a subset of a set that contains the null set as an element, hence <span... | Every set is a subset of itself. And the empty set is a subset of every set. So both statements are true.
However, I suspect that this only adds a bit to the confusion.
Note that:
<ol>
<li><span class="math-container">$\emptyset\subseteq \emptyset$</span> (because any set is always a subset of itself), but <span cla... | They are both true but mean different things altogether.
<span class="math-container">$\emptyset \subset \emptyset$</span> is true.
It is true for any of the following reasons and maybe more.
1) Every set is a subset of itself.
2) The emptyset is a subset of any set
3) The emptyset has no elements so every element... | https://math.stackexchange.com |
148,230 | [
"https://electronics.stackexchange.com/questions/148230",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/63953/"
] | I recently bought a tubescreamer PCB from Tonelab and soldered it together yesterday. I decided to go for Ts-9 model which requires (Ra=470ohm, Rb = 100kohm, Ca = unused) I tried to match all parts as close as possible .. some of the capacitors and resistors are a tad off (eg: 47pf instead of 51pf) but I figure it won... | Based on measurements in the comments, your bias voltages are pretty much as expected, so there is no obvious problem in the DC portions of the circuit. Both transistors are properly biased and the op-amp appears to be functioning.
Look for problems like open capacitors (or connections to capacitors such as lifted pa... | I was thinking about this plowing my driveway tonight.
If it's still not working I might try replacing the
first transistor. (maybe you hooked something up backwards
and reversed biased it??)
What kind of multi-meter do you have?<br>
If the AC voltmeter has a decent max frequency,
you could use it to follow some... | https://electronics.stackexchange.com |
20,305 | [
"https://astronomy.stackexchange.com/questions/20305",
"https://astronomy.stackexchange.com",
"https://astronomy.stackexchange.com/users/15910/"
] | According to the big bang theory, the universe started from a small intial point and is essentially expanding. However, my question is that if the universe is expanding how is it possible for galactic collisons to occur? Are the galacies moving away from a relative position but not moving away from each other? If so, h... | Even more concise: <em>the universal expansion of space does not affect the space within gravitationally bound structures</em>.
A pair of colliding galaxies clearly fall into this category.
The general expansion of the universe is only apparent at the largest scales, where the universe can be treated as an isotropic... | The expansion of space is something that happens on the largest scales. At small scales, such as distances between nearby galaxies, other forces, such as gravity, dominate. Galaxy clusters are held together by the attractive force of gravity between these galaxies. Space in these regions is still expanding, but the ... | https://astronomy.stackexchange.com |
2,718,892 | [
"https://math.stackexchange.com/questions/2718892",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/420338/"
] | Let $G$ be an extension of$\mathbb{Q}$ such that $[G:\mathbb{Q}] =2$. Prove that $G = \mathbb{Q}(\sqrt{d})$ for some $d\in \mathbb{Z}$.
For one, I am having trouble picturing what $\mathbb{Q}(\sqrt{d})$ is so I'd like if someone could please explain that.
Now I have this theorem that says if G is an extension of $\m... | What $\mathbb{Q}(\sqrt{d})$ looks like: $$\mathbb{Q}(\sqrt{d})=\{p+q\sqrt{d}\mid p,q\in\mathbb{Q}\}$$
So you are just adding all rational multiples of $\sqrt{d}$.
$[G:\mathbb{Q}]=2$ means the $\mathbb{Q}$-basis of $G$ consists of only two elements, that is $$G=\left <1,\alpha\right >_{\mathbb{Q}}$$ for some $\al... | Since $G\supsetneq \Bbb Q$, you can pick $\alpha\in G\setminus \Bbb Q$. Then $G$ contains the three numbers $1,\alpha,\alpha^2$, and as $G$ is a $2$-dimansional $\Bbb Q$-vector space, they must be linearly dependent, say
$$ a\alpha^2+b\alpha+c=0$$
with $a,b,c\in\Bbb Q$ and not all of them $=0$. What can you conclude?
| https://math.stackexchange.com |
1,767,751 | [
"https://math.stackexchange.com/questions/1767751",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/258800/"
] | I want to prove that the distance between incentre and orthocentre is $$\sqrt{2r^2-4R^2\cos A\cos B\cos C} $$here $r$ is inradius and $R$ is circumradius.
I considered $\triangle API$ ($P$ is orthocentre and $I$ is incentre). I could find $AP=2R\cos A$, $AI=4R\sin\frac{B}{2}\sin\frac{C}{2} $ and $\angle PAI=\angle \fr... | <blockquote>
So applying cosine rule I got $$\small PI^2=4R^2+16R^2\sin^2\frac{B}{2}\sin^2\frac{C}{2} -16R^2\cos A\sin\frac{B}{2}\sin\frac{C}{2}\Bigg(\cos\frac{B}{2}\cos\frac{C}{2}+\sin\frac{B}{2}\sin\frac{C}{2}\Bigg)$$
</blockquote>
I think that you have a typo (the red part) :
$$\begin{align}&\small PI^2=4R^... | Use these facts
1. The Euler circle is tangent to the inscribed circle
2. The distance between the circumcenter and the incenter using the Euler formula.
3. The formula for the power of a point with respect to a circle
4. The properties of the Euler line
5. The fact that the reflection of the orthocenter with respect t... | https://math.stackexchange.com |
141,005 | [
"https://softwareengineering.stackexchange.com/questions/141005",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/48185/"
] | How would one know if the code one has created is easily readable, understandable, and maintainable? Of course from the author's point of view, the code is readable and maintainable, because the author wrote it and edited it, to begin with. However, there must be an objective and quantifiable standard by which our prof... | <strong>Your peer tells you after reviewing the code.</strong>
You cannot determine this yourself easily because as the author, you know more than the code says by itself. A computer cannot tell you, for the same reasons that it cannot tell if a painting is art or not. Hence, you need another human - capable of main... | Sometimes, the best way to know, is to come back to code you wrote six months ago and try and understand what it was written to do.
If you understand it quickly - it's readable.
| https://softwareengineering.stackexchange.com |
2,418 | [
"https://quant.stackexchange.com/questions/2418",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/1696/"
] | I am working on an intraday strategy using 5/10 minute bars. I am getting a decent return and sharpe on the strategy. But on close examination I see that I am making about 1 cent per trade (I haven't taken transaction cost into account).
How do you guys go about choosing a good intraday strategy ? I have always used ... | If you are making 1 cent per trade before slippage and commissions, I don't necessarily think this is a viable strategy - commissions alone might evaporate that edge. If you are adding liquidity (as defined by the exchanges) perhaps you can obtain rebates to offset commissions and slippage.
Slippage can vary depending... | Your PnL is a function of traded volume, transaction costs and mills p/share (mills meaning $0.001 p/share). You also have to include exchange rebates and fees, a significant adjustment given your backtested results.
1 cent net p/share is actually a good return for high-frequency or market making strategies that trade... | https://quant.stackexchange.com |
14,530 | [
"https://dsp.stackexchange.com/questions/14530",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/7973/"
] | I have a signal consisting of two sine waves of same frequency but have different phase (and also a lot of HF noise). Their amplitudes differ by about $120\textrm{ dB}$. How to I go about separating them? SNR is about $130\textrm{ db}$ but its frequency is far higher than my signal, i.e. $f(\textrm{noise}) \gg f(\text... | $E_1=E_{10}sin (\omega t)$
$E_2=E_{20}sin (\omega t + \delta)$
$E_{\theta} = E_1 + E_2 = E_{\theta0}sin (\omega t + \phi )$
This can be described as the figure below:
<img src="https://i.stack.imgur.com/ltjSq.png" alt="enter image description here">
Now given the $E_{\theta 0}$, you can rotate $E_1$ and $E_2$ arbi... | Two exact sine waves of the same frequency but different phases sum to another exact sine wave. That resultant sine wave can be decomposed into an infinite numbers of pairs (or any other greater number) of sine waves of the same frequency, not just the two original ones. Thus more information is required to reduce th... | https://dsp.stackexchange.com |
140,907 | [
"https://chemistry.stackexchange.com/questions/140907",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/98366/"
] | I found out that if we want to find the entropy change in an irreversible process let's say <span class="math-container">$\text{state A} \to \text{state B}$</span> , we find or make an <strong>reversible</strong> path for the same process and find the entropy change for that irreversible process. The thing I don't get ... | Quantities such as heat and work can vary depending on the path (how you get from an initial to a final state). In contrast a defining aspect of a state function is that it is independent of the path.
Entropy is a state function, as a corollary therefore a path-independent property. The experimental determination of en... | The answer above is perfectly fine. There is another way to see it. It is very important to realize what is a "system" and a "surrounding", what changes when we talk about Entropy and other state functions? when we talk about Entropy, we say "The total change in Entropy of Universe is never neg... | https://chemistry.stackexchange.com |
3,679,839 | [
"https://math.stackexchange.com/questions/3679839",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/732532/"
] | I have found that the Galois group <span class="math-container">$G$</span> of <span class="math-container">$f=x^p-a$</span> over <span class="math-container">$\mathbb{Q}$</span> is of order <span class="math-container">$p(p-1)$</span>. I need to show that if <span class="math-container">$P$</span> is a subgroup of <spa... | Using the fundamental theorem of Galois theory here sounds like a good idea to me. Let <span class="math-container">$K = \mathbb{Q}(b, \zeta)$</span> in your notation. Since <span class="math-container">$P$</span> is a normal subgroup of <span class="math-container">$G$</span>, <span class="math-container">$E := K^{P}$... | So <span class="math-container">$P$</span> is normal in <span class="math-container">$G$</span> and hence there exists a unique sub-field of index <span class="math-container">$K'|_\mathbb Q$</span> of <span class="math-container">$K$</span>, such that <span class="math-container">$[K':\mathbb Q]=p-1$</span> by the Gal... | https://math.stackexchange.com |
620,585 | [
"https://electronics.stackexchange.com/questions/620585",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/163516/"
] | I would like to mount some external flood-lights on a motorhome with a 12V DC system. I may need to install new positive wires to make sure they are sufficiently thick.
I wonder if I also need to install thicker negative wires, or if I can simply connect to the existing negative wires, without worrying that I overload ... | The proper wire gauge for a particular application is determined by a few factors:
<ol>
<li>The current (amperes) that the wire will carry. This is the most important consideration. Keep in mind that the positive and the negative wires carry the same current (the electric charge is a conserved property in physics, see ... | Whoever said the negative conductor can be thinner is either wrong or referring to some very specific edge-case.
Thermally, and most wiring is thermally limited, losses are proportional to P = I^2R, regardless of voltage so given same length and cross sectional area, the negative lead will dissipate as much heat as the... | https://electronics.stackexchange.com |
483,301 | [
"https://physics.stackexchange.com/questions/483301",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/-1/"
] | This may be silly question, but why does a helium ballon rise? I know it rises because helium is less dense than air. But what about the material of the ballon. It is made up of rubber/latex which is quite denser than air. An empty ballon with no air in it falls, so why does a helium filled balloon rise?
| The buoyant force* depends on the volume of the object (or at least the volume of the object submerged in the fluid) and the density of the fluid that object is in, not necessarily/directly on the density of the object. Indeed, you will usually see the buoyant force written as
<span class="math-container">$$F_B=\rho_{\... | The high-level explanation is "buoyancy". If you want to know the actual mechanism, it's that the pressure in a fluid increases with depth: the air pressure at the top of a balloon is slightly lower than the air pressure at the bottom of the balloon. If for each point on the surface of the balloon, you take the air pre... | https://physics.stackexchange.com |
174,385 | [
"https://math.stackexchange.com/questions/174385",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/28394/"
] | I was reading linear vector spaces. When doing some exercise to prove some statements based on the properties defined for linear vector spaces, i suddenly noticed, outside the things defined, i'm using a common notion without proof. This notion also surfaced when i was studying Group theory. After giving a thought i co... | The "sign basis" is defined as
$$\begin{aligned}
\lvert+\rangle &= \frac{1}{\sqrt{2}}\left(\lvert0\rangle+\lvert1\rangle\right)\\
\lvert-\rangle &= \frac{1}{\sqrt{2}}\left(\lvert0\rangle-\lvert1\rangle\right)
\end{aligned}$$
Those equations can be solved for $\lvert0\rangle$ and $\lvert1\rangle$. Then it's just... | Hint: All you need to know is that $|0>=\frac{1}{\sqrt{2}}(|+> +|->)$ and $|1>=\frac{1}{\sqrt{2}}(|+>-|->).$
| https://math.stackexchange.com |
111,946 | [
"https://electronics.stackexchange.com/questions/111946",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/43483/"
] | What would be the transfer function of the following circuit, assuming an ideal op-amp?
<img src="https://i.stack.imgur.com/uKkaa.jpg" alt="enter image description here">
| This looks like a homework problem, so I'm going to demonstrate the setup, and leave the algebra to you. As a note, you've got two resistors called \$R_1\$, so I'm going to denote them by \$R_{1L}\$ and \$R_{1R}\$ for left and right, respectively.
You have negative feedback, so (assuming an ideal op amp as stated) the... | This is a Phase Shifter, with transfer function given by:
$$\frac{V_o(s)}{V_i(s)} = H(s) = \frac{-R_2Cs+1}{R_2Cs+1}$$
<strong>Gain</strong>: \$1 V/V\$ (passes all signals without altering their amplitude).
<strong>Phase lag</strong>: \$0^0\$ to \$-180^0\$, with \$-90^0\$ at \$\omega = \omega_0=\frac{1}{R_2C}\$.
<im... | https://electronics.stackexchange.com |
581,655 | [
"https://electronics.stackexchange.com/questions/581655",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/290949/"
] | I'm wondering what happens to the current when the voltage is stepped down using a regulator. here is the situation: I have a load which draws about 500mA at 3.3 volts, and I would like to regulate power received from a 9 volt battery to the specified 3.3 volts. If the 9 volt battery has 500mAh, will it last for more t... | With a linear regulator, 500 mA out of the regulator means 500 mA goes in to the regulator, plus slightly more to power the regulator itself.
If a 9V battery has 500 mAh capacity, it most likely is not rated 500 mAh at 500 mA current, so it won't last even an hour.
With a switch mode regulator, 3.3V at 500mA will take ... | Energy unit conversion: 1 Watt-hour equals 3600 joules.
Energy stored in 0.500 Ah 9 volt battery is 4.5 Watt-hour x 3600 joules per Watt-hour equals 16,200 {J}.
Watt {W} is also joule per second {J/s}.
If the load draws 1.65 {J/s} then divide 16,200 {J} by 1.65 {J/s} which is 9,818 seconds {s}. Divide by 3600 to get ho... | https://electronics.stackexchange.com |
662,466 | [
"https://physics.stackexchange.com/questions/662466",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/312704/"
] | In this question,
<blockquote>
A circus acrobat of mass <span class="math-container">$M$</span> leaps straight up with initial velocity <span class="math-container">$v_0$</span> from a trampoline. As he rises up, he takes a trained monkey of mass <span class="math-container">$m$</span> off a perch at a height <span cla... | The conservation of momentum is used to know the velocity of the pair monkey + acrobat just after they join. The momentum immediatly after the pair is formed must be equal to the momentum just before the acrobat take the monkey.
The rest of the problem is solved by uniformly accelerated movement.
| It is true that the momentum of the acrobat and monkey are not conserved, owing to the effect of gravity. However, you can still apply the principle of conservation of momentum in the presence of a force. What you have to do is to account for the effect of the force on the momentum; you can then say that apart from th... | https://physics.stackexchange.com |
543 | [
"https://robotics.stackexchange.com/questions/543",
"https://robotics.stackexchange.com",
"https://robotics.stackexchange.com/users/457/"
] | I've noticed that almost all research being done with helicopter robots is done using quadcopters (four propellers). Why is there so little work done using tricopters in comparison? Or a different number of propellers? What about four propellers has made quadcopters the most popular choice?
| At least in part quadrotors offer a nice balance between the complexity of the dynamics and power requirements. With traditional single rotor helicopters, control is a function of the orientation of the rotor which means you must change its orientation to change direction of the craft. This makes for very complex mecha... | You need 4 degrees of freedom to control yaw, pitch, roll and thrust.
Four props is therefore the minimum number of actuators required. Tricoptors require a servo to tilt one or more rotors which is more mechanically complicated.
There is no restriction to only 4 props, hexa+ coptors are also very common.
Generally... | https://robotics.stackexchange.com |
698,456 | [
"https://physics.stackexchange.com/questions/698456",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/156987/"
] | Suppose the Hamiltonian is <span class="math-container">$H+H_1(t)$</span>. In the Schrodinger picture, the evolution is:
<span class="math-container">$$|\psi(t)\rangle=e^{i(Ht+\int_0^tH_1(t)dt)}|\psi(0)\rangle$$</span>.
The interaction picture introduces a change of basis <span class="math-container">$e^{-iHt}$</span> ... | In the following I set <span class="math-container">$\hbar=1$</span>. Let's assume that in the Schrodinger picture, we have
<span class="math-container">$$i \psi_S'(t) = \big[H_0 + V_S(t)\big]\psi_S(t)$$</span>
where we assume <span class="math-container">$H_0$</span> does not explicitly depend on time. If we define <s... | I think the first expression might not be correct. I should have changed the basis of the Schrodinger equations instead:
<span class="math-container">$$\frac{d}{dt}e^{iHt}|\psi (t)\rangle$$</span>
<span class="math-container">$$=e^{iHt}\frac{d}{dt} |\psi (t)\rangle +\frac{d}{dt} e^{iHt}|\psi (t) \rangle $$</span>
<span... | https://physics.stackexchange.com |
10,082 | [
"https://biology.stackexchange.com/questions/10082",
"https://biology.stackexchange.com",
"https://biology.stackexchange.com/users/4323/"
] | Can the colour of skin change with age? Can a dark skinned person become fair or vice versa? Can Melanocyte production vary?
| <blockquote>
Doesn't Saliva contain anti-bacterial properties?
</blockquote>
Yes, in particular some IgA antibodies and lysozyme, but there are also plenty, and I mean <strong>plenty</strong> of bacteria in your mouth at any given point; we do, after all, still have to brush our teeth! In fact, animal (including hu... | Whether infant formula or food for adult consumption - should be discarded if there is salivary contamination since there is universal presence of microorganisms in saliva. These act as culture media and the microorgnisms rapidly degrade the food, not to mention the selective overgrowth of pathogenic microorganisms.
| https://biology.stackexchange.com |
270,530 | [
"https://mathoverflow.net/questions/270530",
"https://mathoverflow.net",
"https://mathoverflow.net/users/99863/"
] | I am trying to prove or disprove
$$\sum_{k=1}^{\infty}e^{-\lambda_{k}t}c_{k} \xrightarrow{t\to 0} \sum_{k=1}^{\infty}c_{k} ,$$
where $\sum c_{k}<\infty, \sum c_{k}^{2}<\infty\text{ and }\frac{\lambda_{k}}{k}\to c$(Weyl's law). The $c_{k},\lambda_{k},t\in \mathbb{R}$ and
<strong>Update</strong>: the $\lambda_{... | The $\lambda_k$ correspond to the Laplacian eigenvalues of a domain in $\mathbb{R}^2$ implies that
\begin{equation}
0<\lambda_1\le \lambda_2\le\cdot\cdot\cdot\le \lambda_n\le\cdot\cdot\cdot
\end{equation}
and $\lambda_n\rightarrow\infty $ as $n\rightarrow\infty$. Hence for any $t>0$
\begin{align}
\sum_{n=1}^{\inf... | Let me try one more time, inspired by Pietro's idea.
Let's for now focus on $t=1/N$ and an interval $I_N$ of length $N$ located near $k\simeq (N/3)\log N$. Let's take $\lambda_k=(1\pm\delta)k$ on this interval, with $\delta=1/\log N$, and the sign is the same as that of $c_k$. Then the terms of
$$
\sum_{k\in I_N} c_k(... | https://mathoverflow.net |
4,095,541 | [
"https://math.stackexchange.com/questions/4095541",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/912623/"
] | I'm stuck on this quadratic problem:
<span class="math-container">$$\sin^4 x + 2(1 + k)\sin^2 x- k - 2 = 0$$</span>
<span class="math-container">$$k = ?$$</span>
I substituted <span class="math-container">$t = \sin^2(x)$</span> this gave me <span class="math-container">$t^2 + 2(1 + k)t - k - 2 = 0$</span> and than I fi... | Consider the simpler problem of HH vs HT in 3 coin tosses.
<span class="math-container">\begin{array}
3Flips & HH & HT \\ \hline
HHH & 1 & 0 \\ \hline
HHT & 1 & 1 \\ \hline
HTH & 0 & 1 \\ \hline
HTT & 0 & 1 \\ \hline
THH & 1 & 0 \\ \hline
THT &... | The casual, yet intuitive answer is that whenver <span class="math-container">$\textsf{HHHH}$</span> occurs, you are basically guaranteed to get <span class="math-container">$\textsf{HHHT}$</span>, as the sequence of <span class="math-container">$\textsf{H}$</span> must stop with a <span class="math-container">$\textsf... | https://math.stackexchange.com |
357,548 | [
"https://stats.stackexchange.com/questions/357548",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/214847/"
] | I applied a bootstrap-process to calculate confidence intervalls for the paramters of a multiple lineare regression.
In R it's pretty simple to implement (functions: 'boot' and 'boot.ci') but I still have two comprehension problems:
<ol>
<li>Why does it make sense to perform a bootstrap procedure before calculating t... | <blockquote>
Why does it make sense to perform a bootstrap procedure before calculating the confidence intervals? Will they be more precise? And if so, can anyone explain why?
</blockquote>
You can calculate bootstrap confidence intervals for complex situations, i.e. properties ("statistics") that are not easily ac... | Let's take the simplest case of using just the percentiles to compute the confidence interval. In that case you repeatedly sample with replacement from your data, compute your statistic in each of these samples and store those estimates. The 2.5th percentile of those stored estimates represents the lower bound and the ... | https://stats.stackexchange.com |
240 | [
"https://astronomy.stackexchange.com/questions/240",
"https://astronomy.stackexchange.com",
"https://astronomy.stackexchange.com/users/83/"
] | In my tenacious attempts to observe moonrise (surprisingly difficult in a poor weather heavily urbanized hilly area with frequent bad smog, with busy work schedule and a bicycle) I was frequently trying to find good observation spots that would provide view to open distant horizon at specific azimuth.
While generally ... | The monthly variations are caused by the change in declination of the Moon as it travels (roughly) along the ecliptic. This is caused by the tilt of the Earth's rotation axis, which results in an angle of about 23° between the equator (declination = 0°) and the ecliptic. Around the constellations of Taurus an... | Moonrise azimuth... The Moon travels around the Ecliptic over a months time, so its azimuth angle at rising will be "comparable" to the Sun's rising azimuth angle pn each day - equal to the Sun's at times but other times with a sizable "offset" in angle as large as +/- 30 degrees.
| https://astronomy.stackexchange.com |
3,062,086 | [
"https://math.stackexchange.com/questions/3062086",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/631749/"
] | Prove by induction:
<span class="math-container">$$\sum_{i=0}^n 3^i =\frac {1}{2} (3^{n+1}-1)$$</span>
Now, i know how to do the first step and i understand it but then i have a problem with the second step which is showing that its true for n+1.
My question is:
Is this notation corect:
<span class="math-container... | Neither of those is correct, but the first one is closer. What you want to say is
<span class="math-container">$$\sum_{i=0}^{n+1}3^{i}=\sum_{i=0}^{n}3^{i}+3^{n+1}$$</span>
since <span class="math-container">$3^{n+1}$</span> is the extra term missing from the sum on the right hand side. To complete the proof, the induct... | Notation looks good.
Your base case is:
<span class="math-container">$$\sum_{i=0}^0{3^i}=3^0=1$$</span>
<span class="math-container">$$\frac12(3^1-1)=\frac12(2)=1$$</span>
so true for <span class="math-container">$n=0$</span>
Inductive Step:
Assume the statement true for <span class="math-container">$n=k$</span>, t... | https://math.stackexchange.com |
24,642 | [
"https://astronomy.stackexchange.com/questions/24642",
"https://astronomy.stackexchange.com",
"https://astronomy.stackexchange.com/users/21145/"
] | If I had a directional photon-emitting source and placed it inside a black hole pointing upward and out towards the visible universe, I assume the photons traveling at the speed of light would slow and reverse direction back into the center.
So if I took the same source and placed it outside of the black hole pointing... | It doesn't work like that. An observer at the light source (and indeed any observers anywhere else) will always see light travelling (in vacuum) at the speed of light locally.
There is also a major problem with your thought experiment. It is <em>not possible</em> for you to have a stationary light source within the ev... | You cannot exceed the speed of light "locally". But you can -see- imagine<sup>*</sup> <strong><em>distances increase quicker than the speed of light</em></strong>.
If by traveling you mean "moving compared to local space time", then light cannot <strong><em>travel</em></strong> quicker than the speed of light. In your... | https://astronomy.stackexchange.com |
109,535 | [
"https://physics.stackexchange.com/questions/109535",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/45031/"
] | I know this isn't the right place for asking this question, but in other places the answers are so awfull.. I'm studying eletricity, so, I start seeing things like "charges", "electrons has negative charges",etc. But I didn't quite understand what charge is. I search a little bit on the internet and found it related to... | I would say that charge is a theoretical prescription describing a way of how a particle interacts with electromagnetic field. Since we are talking about a theory that should describe and predict various phenomena, we need to start with definition of fundamental object.
If we are talking about Newtonian mechanics we f... | If you're looking for a definition of charge, then I can't give you any more than this.
<blockquote>
Charge is the property of matter to interact with electromagnetic field, analogous to masses in gravitational field (but, much more stronger!).
</blockquote>
If you're looking for a way to visualize, then there's <s... | https://physics.stackexchange.com |
11,956 | [
"https://dba.stackexchange.com/questions/11956",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/6311/"
] | So we have a customer site that is complaining about some seriously slow performance. I took one look and it's obvious that the problem is because <strong>Somebody Else</strong> (grrrr) designed a table holding some 20 million-plus records without a clustered index.
Now I want to create a clustered index on that tabl... | <ul>
<li>If your sql server is Enterprise+ edition and table has no any BLOB fields - <code>CREATE CLUSTERED INDEX ... WITH(ONLINE=ON)</code></li>
<li>If not - there is no any way except to create the table with the same schema on side and wisely transfer data to it, including all INSERT/UPDATE/DELETE operations (using... | Not sure what version of SQL server is your customer using. In enterprise you can build the index with (ONLINE = ON), so the table is available till the index is created.
| https://dba.stackexchange.com |
1,468,208 | [
"https://math.stackexchange.com/questions/1468208",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/277751/"
] | I'm having a lot of problems with this one linear recurrence problem ...
First, verify that:
$x^3 − 3x − 2 = (x^2 + 2x + 1)(x − 2). $
<pre><code>Then, solve the linear recurrence
f(0) = 0, f(1) = 1, f(2) = 7,
f(n) = 3f(n − 2) + 2f(n − 3).
</code></pre>
I'm able to get this far but I don't know how t... | you wrote the answer: "verify that: $x^3 − 3x − 2 = (x^2 + 2x + 1)(x − 2)$".
solve that polynomials separately:
$ x-2=0$ and $x^2 + 2x + 1=0$.
| For the recurrence relation $$f(n) = 3f(n − 2) + 2f(n − 3)$$ the characteristic equation is $$r^3=3r^2+2$$ which is exactly the equation you had to solve at the beginning; using D.A.'s answer, the solutions are then $r=2$, $r=-1$, $r=-1$. So, because of the double root, the solution will be $$f(n)=c_1(2)^n+c_2(-1)^n+c_... | https://math.stackexchange.com |
330,767 | [
"https://mathoverflow.net/questions/330767",
"https://mathoverflow.net",
"https://mathoverflow.net/users/90940/"
] | I have been trapped in solving the following ODE for a long time. I wonder if it has unique analytical solution
<span class="math-container">\begin{equation}
[b+c_B(\bar{\beta}^H-\bar{\beta}^L)]\frac{dF(x)}{dx}+c_BF(x)-c_BF(x+\bar{\beta}^H-\bar{\beta}^L-b/c_B)-c_B=0.
\end{equation}</span>
I could try to assume <span c... | To continue Liviu Nicolaescu's simplification: put <span class="math-container">$F(x):=f(x/B)$</span> so the equation writes
<span class="math-container">$$f'(x)+f(x)-f(x+1)+1=0.$$</span>
A particular solution of it is simply <span class="math-container">$f(x):=x^2$</span>, so we are left with the homogeneous equation ... | This is not an answer but a suggestion. There is a lot of "noise" in your equation. You can simplify it a bit. Set
<span class="math-container">$$
A:= b+c_B(\bar{\beta}^H-\bar{\beta}^L),\;\;c:=c_B.
$$</span>
Then your equation reads
<span class="math-container">$$
AF'(x)+cF(x)-cF(x+A/c)-c=0,
$$</span>
or
<span c... | https://mathoverflow.net |
33,551 | [
"https://dba.stackexchange.com/questions/33551",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/3928/"
] | We have a user who is leaving and I need to know every database object that he owns. Is there a query that will provide this information?
| This should get you what you're looking for:
<pre><code>;with objects_cte as
(
select
o.name,
o.type_desc,
case
when o.principal_id is null then s.principal_id
else o.principal_id
end as principal_id
from sys.objects o
inner join sys.schemas s
on ... | To show all non-sa database owners:
<pre><code>SELECT suser_sname( owner_sid ) OwnerID , * FROM sys.databases where suser_sname( owner_sid ) <> 'sa'
</code></pre>
If you need SQL system Job owners:
<pre><code>select s.name,l.name
from msdb..sysjobs s
left join master.sys.syslogins l on s.owner_sid = l.sid ... | https://dba.stackexchange.com |
108,969 | [
"https://electronics.stackexchange.com/questions/108969",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/39265/"
] | I am driving an OLED display via two AAA batteries plus a charge pump to step up the voltage to 9V. I was wondering if there is likely to be any significant difference in efficiency between (a) using the two batteries in parallel and using a charge pump to step up from 1.5V to 9V, or (b) connecting the batteries in ser... | You can calculate the efficiency by using \$n = 1 - \dfrac{p_{in}}{p_{out}}\$ and get the power \$p_{in}\$ by measuring the current from the batteries times the batteries voltage and the power \$p_{out}\$ by multiplying the current to the LED times the voltage across the LED(s) (at the output of the converter).
This a... | As a rule of thumb, the closer your input and output voltages of smps's the more efficient it will be. With a Dickson, that rule is the same as you've got diode (or mosfet) voltage drops to overcome. And you'll need more stages to get to higher voltage with a 1.5V input. With 3V you have less stages and therefore less ... | https://electronics.stackexchange.com |
345,299 | [
"https://mathoverflow.net/questions/345299",
"https://mathoverflow.net",
"https://mathoverflow.net/users/126606/"
] | In the correspondence between projective and Kaehler geometry an ample line bundle corresponds to a positive line bundle, where the latter requires that the curvature of the Chern connection is a positive <span class="math-container">$(1,1)$</span>-form. A very ample is a strengthening of ample (no need to take powers)... | J. P. Demailly, Multiplier ideal sheaves and analytic methods, p. 26, (3.15) A holomorphic line bundle <span class="math-container">$F$</span> over a compact complex manifold <span class="math-container">$X$</span> is
a) <em>very ample</em> if the map <span class="math-container">$\Phi_{|F|}\colon X \to \mathbb{P}^{N... | As Ben says, the complex-geometric version of an ample line bundle is a positive line bundle: A line bundle is ample if and only if it admits a positively curved Hermitian metric.
However, I'm not aware of a complex-geometric version of the notion of a <em>very ample</em> line bundle. That is, I don't know of a conditi... | https://mathoverflow.net |
82,249 | [
"https://security.stackexchange.com/questions/82249",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/68834/"
] | What I'm about to say is probably going to make me look like an idiot but it's better to be wrong and learn than have unanswered questions.
Ever since I started dealing with passwords I wondered what the big fuss about hashing passwords is anyway, since if somebody got a hold of your database they could find out any p... | Your first assumption, that an attacker who gained access to the database can find all the passwords, is invalid. Even a single invocation of MD5 with no salt and no iterations will keep the strongest passwords secured.
But only a very small percentage of users use a password, which is strong enough to remain secure u... | If someone gets access to the database with passwords in, they might well have enough access on the system to read the code implementing the hashing as well. If they do, then they know the structure of the hashed passwords.
It is also common for the attacker to know at least one password on the system (their own), so ... | https://security.stackexchange.com |
266,238 | [
"https://stats.stackexchange.com/questions/266238",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/152280/"
] | Suppose that $Y_1,\dots,Y_{n+1}$ is a random sample from a continuous distribution function $F$. Let$X\sim\mathrm{Uniform}\{1,\dots,n\}$ be independent of the $Y_i$'s. How can I compute $\mathrm{E}\!\left[\sum _{i=1}^X I_{\{Y_i\leq Y_{n+1}\}}\right]$?
| Here is an alternative answer to @Lucas' using the law of iterated expectations:
$$ \begin{align}
E\left[\sum_{i=1}^X1_{(Y_i \leq Y_{n+1})}\right] & = E\left[E\left[\sum_{i=1}^X1_{(Y_i \leq Y_{n+1})}|X\right]\right]
\\
& = E\left[\sum_{i=1}^XE[1_{(Y_i \leq Y_{n+1})}|X]\right]
\\
& = E\left[\sum_{i=1}^XE[... | By distributional symmetry, $\Pr\{Y_i\leq Y_{n+1}\}=\Pr\{Y_{n+1}\leq Y_i\}$, for each $i=1,\dots,n$. Since $F$ is continuous, we have
$$
\Pr\{Y_i\leq Y_{n+1}\} = 1-\Pr\{Y_{n+1}< Y_i\}=1-\Pr\{Y_{n+1}\leq Y_i\}.
$$
Therefore, $\mathrm{E}\left[I_{\{Y_i\leq Y_{n+1}\}}\right]=\Pr\{Y_i\leq Y_{n+1}\}=1/2$. Now, we have
$... | https://stats.stackexchange.com |
337,204 | [
"https://electronics.stackexchange.com/questions/337204",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/166051/"
] | If I have a 24V source, and then step down the voltage to 5V. Then at this 5V level I draw 1 Amp, what current draw will this be on the 24V source. My gut tells me it's 5V/V24 * 1 Amp mathematically but is this true in real life?
Edit:
DC/DC converter
| It depends on what you use to step down the voltage. If you use a switching regulator, you are mostly correct, less inefficiencies in the regulator.
On the other hand, if you use a linear regulator, you’ll see a 1 amp (plus inefficiencies) draw on the 24 volts.
In either case, the losses will show up mostly as heat. ... | You can't break conservation of energy, and you have losses, so basically:
Power In = Power Out + losses
So, if power out = 5V * 1A = 5W, then:
Power In = 5W + losses
If a regular DC/DC converter has ~80% efficiency, then losses = 5W * ~0.2 = ~1.25W
So, Power in = ~6.25W
~6.25W = 24V * I → I = ~6.25W/24V = ~260mA... | https://electronics.stackexchange.com |
71,591 | [
"https://security.stackexchange.com/questions/71591",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/59480/"
] | I've started browsing the deep web recently and have seen a lot of illegal content. Now while I have no interest in any of the illegal content, I don't want to be caught on one of these sights because of a misclick, clicking on a misleading link, or clicking on an unknown link and get in trouble. Plus I guess I would f... | The communication link between the application and the DB is the issue. If the application is what you consider your CDE then the database will be in scope because it is connected to the application. Depending on your access controls and other security controls for segmentation you might be able to remove the managemen... | IANAQSA
No, I don't believe you can achieve the separation you're looking for. The PCI v3 quote you've provided describes using <em>adequate network segmentation</em> to isolate systems. What you're really asking, though, is can you isolate Shared DB (card details) from Shared DB (configuration)? And the answer is ... | https://security.stackexchange.com |
323,356 | [
"https://softwareengineering.stackexchange.com/questions/323356",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/-1/"
] | The PHP codebase I'm working on has an utility class that returns a database connection. The implementation looks like this:
<pre><code>class Database {
private static $conn;
private static $init = false;
public static function connect() {
if (!self::$init) {
$cnf = new Configuration();
self::$c... | No.
Unless you are on a really crappy O/S (or, to be precise, your ODBC library and/or database access drivers/library, which may or may not be part of the O/S proper), your code is not actually creating/using/closing database connections. It is creating/using/closing database connection <em>objects</em>, which are a... | Yes.
Other answers are missing the fact that PHP is single-threaded and blocking. You generally process one HTTP request per process at a time. You <em>can</em> use more than one connection at a time if you really need to, but usually you won't. So it's perfectly fine to have a single connection per process.
Now if t... | https://softwareengineering.stackexchange.com |
333,120 | [
"https://mathoverflow.net/questions/333120",
"https://mathoverflow.net",
"https://mathoverflow.net/users/105900/"
] | Consider the heat equation
<span class="math-container">$$u_t = u_{xx}$$</span>
for <span class="math-container">$t \ge 0$</span>, <span class="math-container">$0 \le x \le L$</span>, given boundary conditions
<span class="math-container">$$u(0,t) = u(L,t) = f(t)$$</span>
and an initial condition
<span class="math-c... | Imposing some conditions on <span class="math-container">$f$</span> and <span class="math-container">$g$</span> the solution can be represented via Green's function G:
<span class="math-container">$$
u(x,t)=\int_0^L G(x,y,t)g(y)\,dy+\int_0^t\partial_y G(x,0,t-\tau)f(\tau)\,d\tau-
$$</span>
<span class="math-container">... | In general, there isn't a solution at all, let alone an explicit one. For example, take <span class="math-container">$f(t)=0$</span> and <span class="math-container">$g(x)=(x-L/2)^2 -L^2/4$</span>. Then, at <span class="math-container">$t=0$</span>, we have <span class="math-container">$u_t = u_{xx} =2$</span> at all <... | https://mathoverflow.net |
298,526 | [
"https://electronics.stackexchange.com/questions/298526",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/139567/"
] | I've got a LCM1602C 16x2 liquid crystal display which I'm experimenting on using switches (without a micrcontroller) to understand how it works (I know that it is much more easier with arduino and using the liquidcrystal library).
I was able to send different instructions like display clear, setting address, changing ... | You might find it easier to forget you ever heard the word "Thevenin." You're trying to find a single voltage source and single resistor that would have the same open circuit output voltage and short circuit output current as the circuit in the problem (definition of Thevenin and Norton equivalents). There's no special... | Yes to 5k. No it is not more complicated than that
| https://electronics.stackexchange.com |
199,836 | [
"https://stats.stackexchange.com/questions/199836",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/-1/"
] | I need a reference request for bayesian inference for self-study, I'm look for something containing:
1-Fundamentals about prior and posterior distribution, conflict between prior and posterior
2- General principles about Bayesian inference: likelihood, sufficiency, alacrity, not identifiability.
3- Estimation: utili... | Gelman's Bayesian Data Analysis satisfies this list. There's a solution guide for some problems online.
| I'll second the recommendation of <em>Bayesian Data Analysis</em>. It's thorough and written by some of the top people in the field.
| https://stats.stackexchange.com |
334,145 | [
"https://physics.stackexchange.com/questions/334145",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/156593/"
] | I recognize that Snell's Law relates indices of refraction and the angles of incidence and refraction, but why does the equation use sine to describe those angles and not simply angle values? It would seem that angle values would make the most sense since it directly refers to the size of the angle entering or exiting ... | An operator is, by definition, a linear map from some <em>given</em> linear space $V$ to itself. And the definition of a linear map cannot, in general, be independent of the linear structure if the space it acts upon.
There are, however, algebraic objects that can be defined abstractly, and that they are always repres... | What about the operator for the energy of a particle in a box? It's easy to see that it is bounded below. It corresponds to discrete sinusoid wave functions with half a wavelength. Different half-wavelength functions differ a little in energy (the ones with the highest energy are the ones with are perpendicular to two ... | https://physics.stackexchange.com |
617,203 | [
"https://math.stackexchange.com/questions/617203",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/103063/"
] | The theorem mentioned in the question is listed in Artin's Algebra (page 106, $2^{nd}$ edition). Artin indicates that it can be proved by using row and column operations. I am wondering whether anyone could provide more hints, please? Thank you!
The theorem is quoted below.
"Given an $m\times n$ matrix $A$, there are ... | Let's work on $G_1$ and assume $c=e_{G_1}$. So $$ac=a, bc=b,cc=c$$ But what about $ab$. It is either $a$, $b$ or $c$. If $ab=a=ac$, since $G_1$ is a group so we can cancel $a$ to have $b=c$ which is wrong because $b\neq c$. The same short story is when we assume $ab=b$, so we just have $ab=c$. This means that $$a=b^{-1... | Let $p$ be a prime and $G$ a groups of order $p$. If $a\in G$ is not the neutral element show that the homomorphism $\mathbb Z\to G$ given by $1\mapsto a$ induces an isomorphims $\mathbb Z/p\mathbb Z\to G$. Conclude that any two groups of order $p$ are isomorphic.
| https://math.stackexchange.com |
75,537 | [
"https://math.stackexchange.com/questions/75537",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/15616/"
] | Why is $1 + 1 = 0$ in $(\{0, 1\}, +, x) \subset \mathbb{R}$, which is the smallest field in $\mathbb{R}$.
| What you seem to ask is why $\{0,1\}$ with addition in which $1+1=0$ is a field.
Well, to verify that an object is a field we only need to verify a few axioms. First let us write the <em>complete</em> table of addition and multiplication (this can be skipped if we use an alternative definition, such as $\mathbb Z/2\ma... | The smallest field doesn't necessarily have to have elements with the names $0$ and $1$. Any field of two elements under an operation $\oplus$, say $(\{a,b\}, \oplus)$ will have a unique identity $e$ of $\oplus$ so that $a \oplus e = a$ and $b \oplus e = b$. Now $e$ must be either $a$ or $b$; suppose it's $a$.
Then $b... | https://math.stackexchange.com |
760,757 | [
"https://math.stackexchange.com/questions/760757",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/124475/"
] | As a sort of exercise, I tried to derive the formula for arc length in polar coordinates, using the following logic:
$$dS = r(\theta)d\theta\\
\implies S=\int r(\theta)d\theta$$
However, it turns out the formula is
$$S = \int \sqrt {r^2+\left(\frac {dr}{d\theta}\right)^2}d\theta$$
I could follow the derivation for ... | Imagine a segment of the curve along a radius from the origin of your polar co-ordinates. That increases the arc length without changing $\theta$ at all and $rd\theta=0$ for this segment. So you need to take into account the radial component.
| Your first formula is incorrect. For instance, consider the line $y=x$ and you want to get the length of the line segment from $x=0$ to $x=1$. The length is $\sqrt2$, and the equation in polar coordinates is $\theta=\dfrac{\pi}4$. If we use the first formula, we get the length to be $0$.
In terms of $x$, $y$, we have
... | https://math.stackexchange.com |
4,112,269 | [
"https://math.stackexchange.com/questions/4112269",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/768502/"
] | I saw some posts about it, but with a lot of other assumptions and, honestly, they did not help. The problem is:
<blockquote>
<span class="math-container">$S \subset \mathbb{R}^{d\times d}$</span>, with <span class="math-container">$d \in \mathbb{N} \backslash\{0\}$</span>. Show that <span class="math-container">$Eig(S... | Suppose <span class="math-container">$A_nx_n=\lambda_n x_n$</span>with <span class="math-container">$\|x_n\|=1$</span> for all <span class="math-container">$n$</span>. Suppose <span class="math-container">$\lambda_n \to \lambda$</span>. Then there is subsequence <span class="math-container">$(A_{n_k})$</span> of <span... | You already have it’s bounded.
You can consider <span class="math-container">$f(S,x)=|S-xI|$</span> as a continuous function from <span class="math-container">$R^{d\times d} \times R$</span> to <span class="math-container">$R$</span>. Let <span class="math-container">$p(S,c)=c$</span> be the projection from the product... | https://math.stackexchange.com |
96,512 | [
"https://softwareengineering.stackexchange.com/questions/96512",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/29925/"
] | I'm pretty familiar with a B Tree, mainly having to keep the databases well fed with electricity, air conditioning, and harddrive space. I associate with a double (doubl[ie,ey]?) linked list.
Today, one of the developers at lunch mentioned a R tree.
I hopped on Wikipedia and started reading. It sounded an awful like... | An R Tree can be thought of as generalization of a b-tree. Where a b-tree provides O(log n) access over a "bounded range" of the keys it contains, an R Tree provides O(log n) access over a "K dimensional region" of the keys it contains.
If you wanted to map zip codes to county names, You could use a B-Tree, since y... | Most tree structures can be reduced to some form of linked list, as long as you ignore how the list is constructed (specifically, how elements are added and removed, and how the nodes are rebalanced, if applicable). It's essentially the insertion/deletion/retrieval algorithm that distinguishes one data structure from a... | https://softwareengineering.stackexchange.com |
165,497 | [
"https://softwareengineering.stackexchange.com/questions/165497",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/65680/"
] | I am currently working on a small project in which I work with some "static data", which is extracted from one source. The data might change from time to time (up to once a week).
I don't need all of the information the data includes in my project so to allow easier and a more logical access to it I wrote some small t... | Generally speaking, duplication is a greater evil than bad performance. The answer will typically depend on whether the performance is <em>good enough</em> with the on-the-fly conversion. You will have to figure that part out yourself. Stick to the current implementation until you have proof that it's too slow; do n... | <strong>I would write a Data Conversion Process that Runs on a timer</strong>. (Once a Day or once a week.) Once the data it conversion is Complete, <strong>generate a JSON file from it</strong> drop it on you Web server, overwriting your previous JSON file .
| https://softwareengineering.stackexchange.com |
139,135 | [
"https://dba.stackexchange.com/questions/139135",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/83636/"
] | I recently joined a company, and I just noticed that many of the stored procedures have the same section of code repeated throughout. I noticed because I was tasked with changing a small section of that code in every SP that it occurred :)
It is a fairly hefty chunk of code, about 30 lines. The code is part of an inse... | This should work for you:
<pre><code>CREATE VIEW MyView AS
SELECT <colList>
FROM <TableOne>
INNER JOIN <TableTwo> ON ...
AND .....
AND .....
LEFT JOIN <TableThree> ON ...
AND .....
AND .....
WHERE .....
AND .....
AND .....
</code></pre>
Then replace in Procs with:
<pre><code>...
FROM MyView
W... | This solution would replace the need for having 100 + procs doing the same thing. you have a proc and a function. The function splits all your medical codes from a string to a table which can be use in a CROSS APPLY in the new proc. This way you only have to call the one proc. Of course you'd have to update all the... | https://dba.stackexchange.com |
9,276 | [
"https://dsp.stackexchange.com/questions/9276",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/4638/"
] | What are the salient differences between Lumped and Distributed systems? In what contexts are distributed systems the appropriate model and in what are lumped systems the appropriate model?
Also, Lumped systems are said to be described by ordinary differential equations while the latter is said to be described by part... | The elements building a lumped system are thought of being <strong>concentrated at singular points</strong> in space. The classical example is an electrical circuit with passive elements like resistor, inductance and capacitor. The physical quantities current and voltage are functions of <strong>time (only)</strong>. E... | It is important to realize that the terms <em>lumped</em> or <em>distributed</em> are not properties of the system itself. These properties are related to the size of the system <em>compared to</em> the wavelength of the voltages and currents passing through it. So a resistor is or isn't a lumped element (even though i... | https://dsp.stackexchange.com |
551,646 | [
"https://electronics.stackexchange.com/questions/551646",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/278498/"
] | I'm trying to understand modern American housing circuitry, which have 15 and 20 amp breakers and work on 120v circuits. They're described using the simple formula Watts = Volts*Amps.
I'm using physics logic I've read online to understand this.
I understand volts as electrons in a high state of energy ready to move to ... | You have to make a distinction between two different things that you are blurring together. First, there is a rating for a circuit. 120V at 20 Amps. That is how much it can deliver if it is connected to the right load. 2400 Watts is the maximum capacity of the circuit. Then there is the details of real loads. How do th... | Your standard mains sockets always have AC voltage of 120V, within some tolerance. A single breaker can provide multiple wall sockets, so a 10A breaker allows powering a single 10A load (e.g. 1200W heater) on single socket, or up to ten loads of 1A each (e.g. 120W lamps). If you only turn on 1 lamp, only 1A flows, so ... | https://electronics.stackexchange.com |
221,568 | [
"https://dba.stackexchange.com/questions/221568",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/164134/"
] | I'm building a relational database that holds 21 tables, which is connected to my website application through PHP.
The website has search functionality which allows users to search this database. Also, users can enter new data, through a special form. Because these entries must pass rigorous control by administrators... | I'd build a UI specifically for the admin and his approval. It would display the things that are awaiting approval. He would click [Approve] or [Reject] (etc?) by each one. That would go off to the backend, perhaps via AJAX, for processing.
In the database would be two sets of tables. They would not necessarily be... | You basically have two options:
<ol>
<li>Extend each table with a 'Status' column, with possible values 'New', 'Approved' and 'Rejected'. Maybe another column for remarks by the administrator, and one to identify the owner of the record (which might already exist). All modules except for the import module should alway... | https://dba.stackexchange.com |
98,403 | [
"https://mathoverflow.net/questions/98403",
"https://mathoverflow.net",
"https://mathoverflow.net/users/14517/"
] | I'm currently facing the problem of computing chern classes for Varieties. More precisely the product of such varieties.
Let $C_i$ be a variety in $\mathbb{CP}^2$ given by the Weierstraß $\wp$-map.
I want to construct a product of 3 such varieties. Nothing fancy, just $C_1\times C_2 \times C_3$ and calculate it's cher... | It appears that you are assuming that your varieties $C_{i}$ are smooth (you seem to assume that since you are talking about the tangent <em>bundle</em>). In this case each $C_{i}$ is an elliptic curve (I guess, this is what you meant by "toric") and so $C_{1}\times C_{2}\times C_{3}$ is a three dimensional abelian var... | For the product of three varieties in $\mathbb {CP}^2$, the Segre map gives an embedding into $\mathbb {CP}^{26}$. This may be unsatisfying. Since the variety in question has dimension $3$, an argument similar to the one that shows all curves embed in $\mathbb {CP}^3$ should give an embedding into $\mathbb {CP}^7$.
Bu... | https://mathoverflow.net |
195,568 | [
"https://softwareengineering.stackexchange.com/questions/195568",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/26621/"
] | Been brushing up on my C++ lately, on Ubuntu QQ. I love the Qt framework for everything, especially building GUI's. I became quite familiar with it when using PyQt over the last few years.
When using PyQt, I had some issues that are now more pronounced when using C++ with Qt: <strong>Qt has many extensions to C++ tha... | I wouldn't refrain from using Qt just for those reasons. You are not required to use all of Qt's utility classes; for the ones that replace the STL, you'll at most be forced to use QString and, possibly, QStringList. Also, there's usually much more to a program than the GUI. You can always use exclusively generic C++ f... | I agree with most of high praise of Qt, but the question was <em>What's the best GUI framework to use that allows/requires the most use of generic C++ and the STL?</em> In this respect Qt is a little schizophrenic: it duplicates STL containers and algorithms with it's own twists. It also provides containers, which are ... | https://softwareengineering.stackexchange.com |
150,344 | [
"https://mathoverflow.net/questions/150344",
"https://mathoverflow.net",
"https://mathoverflow.net/users/36688/"
] | It is well known that $T\mathbb{S}^{n-1}$ is diffeomorphic to $M= f^{-1}(1)$ where
$f:\mathbb{C}^n\rightarrow \mathbb{C}$ is $f(z):=\sum_{i=1}^{n} z_{i}^{2}$.
Two questions:
1) Is $M$ a symplectic submanifold of $\mathbb{C}^n\sim \mathbb{R}^{2n}$ (with the standard symplectic structure)?
If the answer is aff... | Here is a formula for an explicit symplectomorphism $F$ from $T^*S^{n-1}$ to the affine quadric $\{\sum z_{j}^{2}=1\}$ in $\mathbb{C}^n$:
$$
F(p,q) = \left(\frac{1+\sqrt{1+4|p|^2}}{2}\right)^{1/2} q - i\left(\frac{1+\sqrt{1+4|p|^2}}{2}\right)^{-1/2}p
$$
Here I view $T^*S^{n-1}$ as consisting of pairs $(p,q)\in \ma... | $
\def\C{{\mathbf C}}
\def\d{\delta}
\def\e{{\mathbf e}}
\def\r{{\mathbf r}}
\def\u{{\mathbf u}}
\def\x{{\mathbf x}}
\def\y{{\mathbf y}}
\def\z{{\mathbf z}}
\def\<{\langle}
\def\>{\rangle}
$
<del>If I understand</del> Now that I understand your questions correctly, the answers are 1) Yes 2) <del>No</del> Yes.
1)... | https://mathoverflow.net |
248,273 | [
"https://stats.stackexchange.com/questions/248273",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/139673/"
] | I am trying to determine the sample size using power analysis for ANOVA. That requires as inputs the number of groups, effect size, significance level and power. By default, the significance level is 0.05, the power is 0.8 and I use 10 groups. The only parameter left is the effect size. I have no idea how much influenc... | I think effect size is an issue separate from the sample size. It should be chosen as a difference worth detecting. This is something that should come out of an understanding of the problem. Then sample size is determined to be large enough to provide high power.
| I would recommend that OP looks into the area/system in which the experiment would be taking place. If you are looking for differences that are significantly large than you do not need too much of an effect size. However, if you're experiment delves into differences that are very hard to tell then a stronger effect siz... | https://stats.stackexchange.com |
431,975 | [
"https://physics.stackexchange.com/questions/431975",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/172985/"
] | Some misconceptions over here, For <span class="math-container">$x=$</span>position and <span class="math-container">$p=$</span> momentum,
I know <span class="math-container">$[x,p]=i\hbar$</span> but does <span class="math-container">$[x(t),p(t)]$</span> still have the same relation where <span class="math-container"... | <span class="math-container">$$
[\hat{x}(t),\hat{p}(t)] = [\hat{x}_0,\hat{p}_0] = i\hbar
$$</span>
This is easily proven as follows. We have the unitary time evolution operator given by <span class="math-container">$\hat{T}(t)$</span>. Unitary means <span class="math-container">$\hat{T}(t)\hat{T}^{\dagger}(t) = \hat{T... | As always <span class="math-container">$\vec p = - i\hbar \vec \nabla$</span> this is always true in Schrödinger quantum mechanics.
| https://physics.stackexchange.com |
366,053 | [
"https://softwareengineering.stackexchange.com/questions/366053",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/260913/"
] | I've lately been involved with WPF and looked into MVVM. I understand that view model shouldn't be aware of view.
However, sometimes I come across situation where my initial instinct is to write property in view model something like <code>public bool ShowDialog { get; private set; }</code> or <code>public bool ShowPop... | Sometimes this can just be a naming issue.
For example, calling something <code>ShowDialog</code> in the ViewModel when what you mean is <code>AskTheUserForAFilePath</code>, which is independent of how the View handles that request. Similarly, <code>ShowPopup</code> when you mean <code>InformTheUserOfThisEvent</code>.... | Maybe.
If you were doing MVC I would say no. the purpose of the view model is to provide a model of the view which you can manipulate, query and test in code. It doesn't contain any business logic. But can contain view logic. ie when this is hidden show that.
You may well have a one to one mapping of View to ViewMode... | https://softwareengineering.stackexchange.com |
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