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
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|---|---|---|---|---|---|
198,805 | [
"https://softwareengineering.stackexchange.com/questions/198805",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/46350/"
] | It is little depressive, but it seems to me that there is much more support for manipulating ugly proprietary closed format like xls in rails (and ruby) in general that to documented open Open Document format. Why it is so? (I know it is kind of philosiophical, but I just do not get the fact, that the opensource is not... | Your question is tied to "Why do corporations/people use commercial suites/applications when equally good open source versions are available?"
Answer that and you'll have a good idea why there is less ODS support than XLS. For now, here's a hint: People do what their management asks.
| Supply and demand.
Manipulating Excel spreadsheets is a task which is extremely useful to many people, hence there is considerable demand for code that does this, hence that code exists.
Manipulating ODS spreadsheets is useful only to a very tiny niche, hence there is minimal demand for code that does this, hence nob... | https://softwareengineering.stackexchange.com |
95,637 | [
"https://softwareengineering.stackexchange.com/questions/95637",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/10981/"
] | While most interview questions are focused on current knowledge of a candidate or check his/her skill to solve algorithmic problems I would like to hire a developer who is passionate about programming.
What if instead of asking questions like
<blockquote>
What do you know about technology "X"?
</blockquote>
I wil... | All you have to do is ask him to tell you about one of the projects he's worked on that he most enjoyed. You'll find out more about his enthusiasm in the following 60 seconds than you ever could showing him photographs of deceased notables.
| <blockquote>
For example if I look for a Java developer I can ask who are the most influential people in Java world or show a basic Scala snippet and ask a candidate to interpret the code. I even considered to show photo of Alan Turing and let the interviewee guess who is on the photo.
</blockquote>
<strong>That is ... | https://softwareengineering.stackexchange.com |
111,950 | [
"https://electronics.stackexchange.com/questions/111950",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/43484/"
] | I am designing a modular system that will take 9 or 16 photodiode inputs, depending on its setup, and put them through an op amp. The diodes and the op amps are on different PCBs, connected by pin headers and sockets.
The schematic looks like this: <img src="https://i.stack.imgur.com/aTCct.png" alt="Photodetection sc... | There is nothing more you need to do. Your circuit without the diode is stable all by itself. R2 provides DC feedback so that the output will be driven to the same as the positive input, which is ground.
Think of it this way. Your circuit is basically a transimpedance amplifier. By disconnecting the diode, you are... | You can leave the opamp with it's feedback components and with no diode connected. This won't be a problem at all. The ideas suggested by the data sheet are just typical suggestions.
| https://electronics.stackexchange.com |
287,232 | [
"https://electronics.stackexchange.com/questions/287232",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/139467/"
] | I just purchased an RGB LED mechanical keyboard, where each key has its own color.
I was playing with the settings, and found one I like: each LED under each key rapidly cycles between colors.
It got me thinking - does rapidly changing an LED's color reduce lifespan, as opposed to keeping it at a constant color?
| It would likely decrease reliability a little bit. Temperature is a much larger factor in LED reliability. LEDs are much more complicated than they appear at first glance. There are many factors with the pulsing that would need to be known. (e.g. Pulse duration, current amplitude, current ripple, frequency,and duty cyc... | It will increase the life. Any amount of off vs on time will change how long it lasts. Most of those leds will out last your keyboard even if fully on. Each individual diode inside will see some measurable life increase. Obviously colors that use multiple diodes in a RGB led will not benefit. All on White is worst cas... | https://electronics.stackexchange.com |
85,012 | [
"https://softwareengineering.stackexchange.com/questions/85012",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/27130/"
] | I am developing an application in Flex. There are 20+ 'pages', each with its own form. I need to collect all the entered data and pass it to an API method. What I am wondering is when I should begin to pass the data and what the best way to collect it might be. I was thinking of instantiating a class
<pre><code>public... | I would opt for an item-by-item feed to the server. This not only protects against the poor user having to re-input all of that stuff, it also gives you the chance to do on-the-fly modifications of the form.
| I like your idea of saving everything in a "collector class" for two reasons. One, it makes sense to perform everything at once at the end of the process, so you may as well have easy access to all of the information collected along the way. And two, having everything that you need stored away allows you to create stat... | https://softwareengineering.stackexchange.com |
185,819 | [
"https://dba.stackexchange.com/questions/185819",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/5705/"
] | If I fetch only one row, the query takes much longer: 1433 ms vs 23 ms
Is there a work around?
Slow:
<pre><code>EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM "modwork_ticket" WHERE
"modwork_ticket"."email_sender_id" = 'foo@example.com'
ORDER BY "modwork_ticket"."date_created" ASC limit 1;
---------------------------... | The problem is that rows for 'foo@example.com' are deficient in values with an early date_created, but the planner doesn't know that. It thinks they are evenly scattered, so it will find one early on when walking <code>modwork_ticket_date_created</code> and will be able to stop then. But instead it has to walk throug... | <ul>
<li>In the fast one the query scans the <code>modwork_ticket_email_sender_id_like</code> index</li>
<li>In the other it scans the <code>modwork_ticket_date_created</code> index</li>
</ul>
So it thinks there is less work to start from the oldest row on the ordered index until it finds a row that has <code>((email_... | https://dba.stackexchange.com |
390,260 | [
"https://electronics.stackexchange.com/questions/390260",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/194891/"
] | <strong><em>Observation:</em></strong> Recently I worked with a piezoelectric sensor and read that the sensor itself has a high impedance. Because of this, an op amp of high impedance was required to amplify the signal.
<strong><em>Question:</em></strong> How is it possible for a signal to have high impedance? How ca... | In electronics a high impedance signal would refer to a signal from a high impedance source (and low power).
I've only observed this expression applied to low voltage applications. It would not be applied to a high voltage source which is generally also high impedance.
In your observations you indicate that the sourc... | <blockquote>
How is it possible for a signal to have high impedance? How can the signal itself have resistance
</blockquote>
You are technically correct that the signal itself doesn't have an impedance but the source does.
In the case of a piezoelectric sensor the transducer is pretty much a pair of capacitor plate... | https://electronics.stackexchange.com |
60,491 | [
"https://math.stackexchange.com/questions/60491",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/15195/"
] | Consider 2009 cards which are lying in sequence on a table. Initially, all cards have their top face white and bottom face black. The cards are enumerated from 1 to 2009. Two players, Amir and Ercole, make alternating moves, with Amir starting. Each move consists of a player choosing a card with the number k such that ... | There is a solution on page 780 of Dusan Djukic, Vladimir Jankovic, Ivan Matic, Nikola Petrovic, The IMO Compendium; A Collection of Problems Suggested for The International Mathematical Olympiads, 1959-2009, which I found on Google Books.
| It would be worth telling us what you have tried.
<strong>Some hints:</strong>
<ul>
<li>Are there are a finite number of possible positions (and if so what
is an upper bound)?</li>
<li>Can there be a cycle of positions (consider the card with the
smallest number turned over)?</li>
<li>Will the starting player have a ... | https://math.stackexchange.com |
112,481 | [
"https://mathoverflow.net/questions/112481",
"https://mathoverflow.net",
"https://mathoverflow.net/users/4971/"
] | Hi to all!
Let $M$ be a compact smooth manifold without boundary and let $$d:M\times M\rightarrow [0,+\infty)$$ a distance on $M$ compatible with its topology. Suppose there exist $\varepsilon\in (0,+\infty)$ s.t. on the open set $\Delta_{\varepsilon}\subset M\times M$
$$\Delta_{\varepsilon}:=d^{-1}\left([0,\varepsil... | This is not even true on the circle. In that case, given any metric that comes from a Riemannian metric there is a constant $c>0$ such that there is a diffeomorphism of the circle with itself that carries the metric to $c$ times the standard metric:
$$
d\bigl((x_1,y_1),(x_2,y_2)\bigr) = \cos^{-1}(x_1x_2+y_1y_2).
$$... | The Riemannian distance has the following property: it is the so called length space. I recall (one of) the definitions of the length space. Having a distance function $d$, we can define the length of any curve $c(t)$, $t\in [0,1]$ ,
as the supremum of the sum $\sum_{i}d(c(t_i), c(t_{i+1}))$ over all $0=t_0\le t_1... | https://mathoverflow.net |
193,941 | [
"https://stats.stackexchange.com/questions/193941",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/95978/"
] | Take a simple linear regression $y = ax + b$. Here, $y$ is predicted. In my analysis I need to compare the predicted $y$s to the actual $y$s from my data set upon which the regression was built. What diacritical mark should I use to denote the predicted $y$? ŷ, ý, or maybe $y'$? Is there a standard mathematical symbol ... | The predicted outcome is generally expressed as $\large \hat y$ - or "why - hat". You can think of the <em>hat matrix</em>, which "puts a hat on the $\large y$" as in $\large \hat y = H y$, the linear algebra equation that relates the observed values of the response variable, $\large y$, to the predicted values, $\larg... | Yes, there is a standard symbol for predicted value. It is $\hat{y}$. You can see that in, e. g., Introduction to Statistical Learning
| https://stats.stackexchange.com |
30,771 | [
"https://cs.stackexchange.com/questions/30771",
"https://cs.stackexchange.com",
"https://cs.stackexchange.com/users/4911/"
] | In the literature, the terms of "process calculus" and "process algebra" are often interchangeable. Meanwhile, it confused me.
<strong>My questions are:</strong>
<blockquote>
<ol>
<li>Are there formal, standard, and widely-accepted definitions of "calculus" and "algebra"?</li>
<li>What are the significant dist... | To my knowledge the two terms do not have a formal established definition and are often used interchangeably.
There are different characteristics associated to each however, as @Dave already pointed out. I'll try to elaborate a bit further.
In both cases you start by introducing the syntax of a language for expressin... | The distinction is not so clear. Nevertheless, here is an attempt to provide an alternative answer to your question.
A calculus involves calculation. Thus a process calculus will involve a notion of reduction. Examples include the $\pi$-calculus and CCS (and others from the Milner School).
Algebra involves equations.... | https://cs.stackexchange.com |
113,013 | [
"https://math.stackexchange.com/questions/113013",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/24930/"
] | Have the following let $P_2(R)$ denote a vector space of the real polynomial functions of degree less than or equal to two and let $B:=[P_0,P_1,P_2]$ denote the natural ordered basis for $P_2(R)$ (so $P_i(x)=x^{i})$ . Define $f\in P_2(R)$ by $f(x)=5x^{2}-2x+3$ . Write f as a linear combination of the elements of B. Com... | Note that $$6\int_0^{2x-1}(at^2+bt+c)dt=16ax^3+(12b-24a)x^2+(12a-12b+12c)x-2a+3b-6c$$
so as a linear map from $P_2(R)$ to $P_3(R)$, $F$ maps
$$aP_2+bP_1+cP_0\mapsto 16aP_3+(-24a+12b)P_2+(12a-12b+12c)P_1+(-2a+3b-6c)P_0$$
which should suggest both a basis for the image and how to write the matrix $M_B^A(F)$. Given a basi... | Assuming you know how to determine the basis of the kernel of a linear transformation given by a matrix, the first problem can be solved by finding the matrix of the transformation $F$. That again is a matter of figuring out what $F$ does to basis vectors; consider a linear transformation $F: V \to W$ and let $\{u_i\}_... | https://math.stackexchange.com |
53,593 | [
"https://electronics.stackexchange.com/questions/53593",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/12152/"
] | How to use different formulas of power ( <span class="math-container">\$\frac{V^2}{R}\$</span>, <span class="math-container">\$V \cdot I\$</span>, <span class="math-container">\$I^2 \cdot R\$</span> ) in different situations?
For example, I have a heater of 1000W which converts 220 V to power (heat) in some infrared re... | There's little need to calculate the power of your heater, since you have already said it's a 1000W heater.
However, to address your confusion, your choice of which to use will depend on what values you know. They will all give the same result, and they are related by Ohm's law. Here's how:
You know Ohm's law, voltag... | Given Ohm's law V = I × R, and the definition of power P = I × V, you can derive a couple of other equations:
P = I<sup>2</sup> × R and P = V<sup>2</sup> / R
You can solve each of the latter two for R:
R = P / I<sup>2</sup> and R = V<sup>2</sup> / P
Since these equations are all derived from the s... | https://electronics.stackexchange.com |
44,129 | [
"https://chemistry.stackexchange.com/questions/44129",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/11412/"
] | So in my coursebook it says <em>enthalpy</em> is a property that accounts for the increase in internal energy and the accompanying work done by the system and that its equal to the sum of the <em>internal energy</em> $U$ and product of <em>pressure</em> $P$ and <em>volume</em> $V$. Also on the net, mostly on the intern... | I got this from Wikipedia:
<blockquote>
The $U$ term can be interpreted as the energy required to create the system, and the $pV$ term as the energy that would be required to "make room" for the system if the pressure of the environment remained constant. When a system, for example, $n$ moles of a gas of volume $V$ ... | In my judgment, it's not worth spending much of your valuable time trying to assign physical significance to the physical property we call Enthalpy. Enthalpy is just a convenient parameter to work with when we are doing certain types of thermodynamic analysis. One situation which it is particularly useful for is in a... | https://chemistry.stackexchange.com |
215,700 | [
"https://security.stackexchange.com/questions/215700",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/215071/"
] | I understand that a signed certificate is a server public key signed by the private key of a certificate authority. As a result, a client allegedly rest assured that the signed public key is for the desired server. Really? How does the client get the public key of the certificate authority? Can't the attacker just sign... | <blockquote>
How does the client get the public key of the certificate authority?
</blockquote>
The certificate store is created at (OS/Application) install time, and updated as part of normal secured (OS/Application) updates. Firefox is notable as an application that uses it's own cert store. Because root certifi... | An attacker with enough privileges to put their own root CA in your browser could just steal your information directly, so that "attack" in particular isn't worth worrying about. But yes, if they managed to do that, then your browser would trust their certificates.
| https://security.stackexchange.com |
25,609 | [
"https://security.stackexchange.com/questions/25609",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/17323/"
] | I understand that sending data from the user to the server, unencrypted, is a bad practice. That is logical.
So how do I encrypt 'userside' and send the data to the server in encrypted form?
Can anyone suggest a good practice example?
What are the advantages and disadvantages of encrypting in Javascript? (What if ... | The standard solution is to use SSL (HTTPS).
Please spend some time with the search tool on this site. You will find these questions have all been answered on other questions, so you can find lots of good information. For instance, a search on "Javascript encryption" will find a lot of information about the pitfalls... | other than confidentiality, https solves a lot of other problems. There is no safe way to send passwords in the clear or encoded or obfuscated somehow. Https really is the best solution.
| https://security.stackexchange.com |
214,158 | [
"https://softwareengineering.stackexchange.com/questions/214158",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/104659/"
] | There is the ongoing argument of free trial versus a freemium model (that is, a free-for-life version of their software with restricted and/or stripped down features) for allowing potential customers and users to test run their product. Upon my research, I can conclude that the free trial is the way to go on both for t... | There are two issues here - one is a programming problem, and one is a business problem. For the second one, asking programmers about business analysis is about as good advice as you can get from your local bus driver; which is to say it may be good, or terrible, but you aren't asking experts so assign no inherent weig... | You're looking for something that's essentially impossible. Some of the biggest, best-funded companies in the software industry have spent years, and millions upon millions of dollars, looking for a way to accomplish what you're trying to do, and it's never once actually produced successful results.
First off, you ca... | https://softwareengineering.stackexchange.com |
2,172,836 | [
"https://math.stackexchange.com/questions/2172836",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/281052/"
] | I'm writing a small java program which calculates all possible knight's tours with the knight starting on a random field on a 5x5 board.
It works well, however, the program doesn't calculate any closed knight's tours which makes me wonder. Is there an error in the code, or are there simply no closed knight's tours on ... | No closed knight's tour is possible on a board with an odd number of squares, because each move changes the colour of the knight's square. So after an odd number of moves, you can't be back at the starting square, because it's the wrong colour.
| The definition of a 'closed' knights tour on a $m \times n$board, is a sequence of steps from a starting square $a_1$ to another square $a_{mn}$ , such that every square is visited exactly once, and the last sqaure is only one knight step away from $a_1$. Having said that, it is obvious, that for $mn $(mod2) $= 1$, the... | https://math.stackexchange.com |
51,824 | [
"https://electronics.stackexchange.com/questions/51824",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/17117/"
] | I would like to make what I can best describe as a reverse USB switch, where one USB 2.0 device (a dongle) is physically connected to 2 computers (located side by side). The dongle is powered through USB.
I.e. what I'd like to make is a Y-split, with one female going to two males.
Could this be done simply by wiring ... | You will not be able to make a Y cable as you describe and get it to work. USB is a point to point connectivity protocol --- that is a single host end connects to single target end. The connection that you describe would be a multi-host single target type bussed connection and USB is not designed to support that.
As ... | The USB protocol is not designed for this. As already suggested, you can use a switch, or you might want to try an USB over IP solution to share the USB dongle over the network.
| https://electronics.stackexchange.com |
66,781 | [
"https://mathoverflow.net/questions/66781",
"https://mathoverflow.net",
"https://mathoverflow.net/users/4685/"
] | The usual modular parametrization says that if one takes a modular form $f$ which is a newform for $\Gamma_0(N)$ (i.e. a form in the new subspace which is a normalized eigenfunction for the Hecke operators), then one can associate to $f$ an abelian variety $A_f$ of dimension $d$, where $d$ is the degree of $K$ over $\m... | No, the abelian variety $A_f$ is typically absolutely simple, and hence does not factor
(even up to isogeny, and even over $\overline{\mathbb Q}$ or $\mathbb C$) as a product of elliptic curves. What will happen is that if you integrate $f(z)dz$ over the homology of $X_0(N)$, you will get a finitely generated, but <em... | This is an answer to the question A. Pacetti asked in his comment to Emerton's answer.
The modular variety $A_f$ does not have to be geometrically simple. William Stein and I computed many examples of this years ago: we were looking for modular abelian surfaces with <em>Quaternionic Multiplication</em> and it turns o... | https://mathoverflow.net |
267,986 | [
"https://dba.stackexchange.com/questions/267986",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/209672/"
] | I am trying to enhance the search performance of a table based on whether certain columns contain values.
I have the following columns :
id |
link |
text |
date |
lat |
long |
type
If a record has a value in lat & long it can be shown on a map. It’s considered a “dot”
If a record has a value in date & ... | Use binary (or some integer as binary) or SET generated encoding column. For example:
<pre class="lang-sql prettyprint-override"><code>CREATE TABLE tablename
( id SERIAL PRIMARY KEY,
linkfield VARCHAR(255),
textfield TEXT,
datefield DATE,
lat DECIMAL(10,8),
long DECIMAL(10,8),
type ENUM('value1', 'value2... | First, I would consider normalising this table into several, since it contains different kinds of facts. You can create a union view if you need the "API" to stay the same.
If not possible for one reason or another you can use a generated column:
<pre><code>create table T
( txt text
, dt date
, lat int
, lng... | https://dba.stackexchange.com |
16,690 | [
"https://security.stackexchange.com/questions/16690",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/10923/"
] | I guess this questions is directed at Product Managers, but I would be very interested to learn what people in the know think.
What are the most common factors that drives FIPS140-2 compliance for hardware modules? Why do you want/need to implement FIPS140 compliant security in your products?
Here are a few reasons ... | When you are protecting more than your twitter account, it would be nice to know that whatever algorithm that's being used to protect credentials, or key exchanges, or whatever other crypto that's being used, is protected in a nice professional manner. There are A LOT of vendors that could not care less how well somet... | It is driven entirely by US Government demand.
FIPS stands for Federal Information Processing Standards which are US Government standards.
No one else cares about it except them. Others claim to care about it only because it is widely adopted by the "big boys". Look around at FIPS 140-2 requirements in other nati... | https://security.stackexchange.com |
30,928 | [
"https://electronics.stackexchange.com/questions/30928",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/9499/"
] | I wish to understand how a battery charger may best be designed in order to maximise the life of Lithium Ion batteries. I have specified Lithium Ion batteries as they are what is used in most laptops and in many other pieces of modern equipment.
There is much advice and opinion available but as some is contradictory i... | <strong>Preamble:</strong>
I'll assume a standard Lithium Ion (LiIon ) battery type as used in the very large majority of laptops and similar equipment. Lithium Polymer (LiPo) is for practical purposes the same.<br />
Lithium Ferro Phosphate (LiFePO4) is of the same family as LiIon but has some fundamental differences ... | <blockquote>
Force charging-Charging even after reaching 100% in battery backup.
</blockquote>
I guess you mean tickle-charging: Keeping the the charger attached, so the battery keeps being held at 100%. This is indeed not the best you can do to a Li-Ion Battery, but the manufacturer actually took care of this: 100%... | https://electronics.stackexchange.com |
263,399 | [
"https://physics.stackexchange.com/questions/263399",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/121428/"
] | I read a response asking why flights of equal distance east and west take roughly the same time (disregarding wind actions). I have trouble visualizing part of the answer; "the speed of the rotation of the Earth is already imparted to the aircraft, and the Earth matches that speed during the entire flight. (Of course, ... | What may be confusing you is that the passage you quote about the Space Shuttle is talking about speed relative to a fixed frame of reference: one fixed relative to the distant stars. On the other hand, when you think about aircraft flying through the air (or people walking along the ground), you think about a co-rotat... | There is a slight effect on the lift requirement for an aircraft. A "stationary" object on the equator is actually traveling in a circle, at one earth-circumference per sidereal day velocity. That lessens its apparent weight because it is accelerating toward the center of the earth (centripetal acceleration).<br>
The... | https://physics.stackexchange.com |
69,661 | [
"https://electronics.stackexchange.com/questions/69661",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/10423/"
] | Silver has a lower resistivity than gold and is cheaper, so why are high end audio components gold plated?
| Gold is highly resistant to corrosion or oxidation, so prevents poor connections from those sources.It is also fairly soft, so the mating surfaces deform slightly, increasing contact area to reduce resistance. The gold plating is very thin, so the added resistance from the gold is easily overcome by its other propertie... | Mainly for marketing purposes: Gold is always better than silver, right?
But also, silver oxidizes (tarnishes) readily while gold does not.
| https://electronics.stackexchange.com |
210,813 | [
"https://dba.stackexchange.com/questions/210813",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/89051/"
] | I have two versions of a multiple join which produce identical results. The five tables are:
<pre><code>customers ← sales ← saleitems → paintings → artists
</code></pre>
The arrows (hopefully) show the relationship between the tables.
Each table has a primary key called <code>id</code>, and the inner tables have a fore... | You can't randomize them, the order is important. Maybe you can see it clearer if we add a few parenthesis between the joins.
<pre><code>SELECT
c.id
FROM
customers c
JOIN (sales s JOIN saleitems si ON s.id = si.saleid)
ON c.id = s.customerid
</code></pre>
What this query is doing is joining first ... | The JOIN syntax in most cases requires both JOIN and ON to be present (unless some specific situations like CROSS JOIN for which ON would not make any difference).
Consider this to be similar to <code>BEGIN..END</code> statements - they have to be the <code>END</code> statement relates to the nearest previous <code>BEG... | https://dba.stackexchange.com |
56,085 | [
"https://softwareengineering.stackexchange.com/questions/56085",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/7981/"
] | We have recently moved to a scrum process and are working on tasks and user stories inside of sprints. We would like to do code reviews frequently to make them less daunting. We are thinking that doing them on a user story level but are unsure how to branch our code to account for this.
We are using VS and TFS 2010 an... | It depends on the nature of the user stories.
It can be effective to create a branch for each user story, progress on different stories are visible, they can be passed around if needs be, if stories aren't completed in the sprint then the progress can stay in the branch for the next sprint. Final reviews can then be p... | The most effective way to review code is to stand up, find someone, and ask them to come over and discuss the code you have just developed.
Don't use a tool unless you cannot find someone to review your code locally.
You can avoid code reviews altogether by pairing.
| https://softwareengineering.stackexchange.com |
153,772 | [
"https://stats.stackexchange.com/questions/153772",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/9162/"
] | I am testing this model in SPSS AMOS.
<img src="https://i.stack.imgur.com/qrY5A.png" alt="AMOS picture">
The value of .23 above the top right corner of <em>timedrs</em> is the squared multiple correlation for that variable.
I also ran the same analysis as two multi-step regressions. The results came out like this:
... | Although as far as I can tell the 3rd edition of Schumacker & Lomax doesn't answer my question, the 4th edition (from 2015) does! Quoting p84 of that text (but changing the figure to match my data), the answer to the question is:
"The $R^2m$ for the path model would suggest that [32%] of the variance in [timedrs] ... | You're making a serious mistake by using estimation shown. You're pointing path model on graph, which can not be done in the SPSS, and SPSS output instead of AMOS output. The SPSS for which you specify showed tables can not execute the path-model as You showed, a simple regression or even multiple regressions outpu can... | https://stats.stackexchange.com |
330,488 | [
"https://stats.stackexchange.com/questions/330488",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/151233/"
] | I have measured a large data sample from an underlying Gaussian distribution and want to estimate the variance and its error. However, the measured values are noisy with some Gaussian noise with a standard deviation that is approximately known. How can I estimate the error of the sample variance in this case? First I c... | I would answer this question through the use of bayesian estimation with a "non-informative" prior.
<strong>notation and setup</strong>
Data model... $(y_i|\mu,\sigma^2)\sim N(\mu,\sigma^2+\sigma_e^2) $ for $i=1,\dots,n $
Prior... $p (\mu,\sigma^2)\propto(\sigma^2+\sigma_e^2)^{-2} $
The prior is not favouring one s... | If (using the notation of the answer from @probabilityislogic) $\sigma_e^2$ is known, then the maximum likelihood estimator of $\sigma^2$ is
$$\hat{\sigma}^2={{n-1}\over{n}}s^2-\sigma_e^2$$
(or rather the maximum of this and zero). The estimate of the asymptotic variance of $\hat{\sigma}^2$ is given by
$$\frac{2 (n... | https://stats.stackexchange.com |
128,853 | [
"https://mathoverflow.net/questions/128853",
"https://mathoverflow.net",
"https://mathoverflow.net/users/36285/"
] | Hi,
Is there a simple example of an (affine) algebraic variety $X$ over $\mathbb C$ where
the $H^*_{dR}(X/\mathbb C) = H^*(\Omega^\bullet_{A/\mathbb C})$ differs from the singular cohomology $H^*_{sing}(X(\mathbb C)^{an},\mathbb C)$?
Such an example has to be singular (by a theorem of Grothendieck), but I am having a... | A likely candidate for this would be a non-Du Bois singularity.
Du Bois, following Deligne's ideas, constructed a filtered complex of sheaves with coherent cohomology sheaves that gives a resolution of the constant sheaf $\mathbb C$ for any reduced finite type scheme over $\mathbb C$. This complex agrees with the de ... | A very explicit example is given by the cusp $X=\operatorname {Spec}(A)$ where $A=\frac {\mathbb C[\xi,\eta]}{(3\eta^2-2\xi^3)}=\mathbb C[x,y]$.<br>
Since the set of closed points $X(\mathbb C)$ in its classical topology is homeomorphic to $\mathbb C$ we have $H^1_{sing}(X(\mathbb C),\mathbb C)=0$, but we will now show... | https://mathoverflow.net |
301,349 | [
"https://mathoverflow.net/questions/301349",
"https://mathoverflow.net",
"https://mathoverflow.net/users/46290/"
] | Let <span class="math-container">$(M,g)$</span> be a smooth <span class="math-container">$d$</span>-dimensional Riemannian manifold, <span class="math-container">$d$</span> even. Are there obstructions (I guess in terms of curvature) for <span class="math-container">$g$</span> to have the following property:
<blockquot... | <strong>Update (1 June 2018):</strong> I have now figured out the 'little linear algebra lemma' in all dimensions $d = 2n$ and can give a complete answer to the OP's question: A metric $(M^{2n},g)$ possesses coordinate charts of the kind that the OP desires if and only if it is 'locally conformally unimodular Hessian... | I don't have an answer to the question about obstructions. However, I can suggest some simplifications to the condition you are trying to satisfy.
Let $J_a^i = \nabla_a x^i$, where $x^i$ are your sought after coordinates. Let us restrict our attention to the case when $J_a^i$ is invertible. In the sequel, the Latin le... | https://mathoverflow.net |
69,090 | [
"https://chemistry.stackexchange.com/questions/69090",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/41676/"
] | I am an AP Chemistry student and there is a small detail concerning polarity of molecules that I am confused about.
My review book (Princeton review) says that ethanol has stronger intermolecular forces than methanol because it has a large molecular mass and is therefore more polarizable and more soluble in water tha... | If your book says exactly that, then it says nonsense. Sure enough, the addition of CH2's makes a molecule <strong>less</strong> polar. As for the solubility in water, methanol is miscible - i.e., <strong>infinitely well</strong> soluble in it. So is ethanol. How would you compare the two <em>infinities</em>?
That bei... | <blockquote>
My review book (Princeton review) says that ethanol has stronger
intermolecular forces than methanol because it has a large molecular
mass and is therefore more polarizable
</blockquote>
True
Ethanol <em>does</em> on a molecule by molecule basis have stronger intermolecular forces between itself an... | https://chemistry.stackexchange.com |
172,859 | [
"https://stats.stackexchange.com/questions/172859",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/89664/"
] | True or false: In a survey of your neighbors (asking for family size, the kind of pets they have, the grade of the youngest child in the family, the family's annual income in dollars, what the dad does for a living, whether mom works, and their phone number), the only discrete, numerical data you're collecting about yo... | Your problem seems to come from your definitions, which are not orthodox. For example, a question in a survey that asks family size may collect counts, which are by definition discrete but are also clearly numeric, and it represents something (i.e., the size of the family).
I suspect that you will find this area a lot... | I think discrete numerical data is number data that can be counted (number of kids, cars, dollars??). COntinuous numerical data is number data that can be measured e.g. time, length, weight, and can have any numerical value. Categorical data is like kind of pets they have. I'm unsure whether dollars is counted or me... | https://stats.stackexchange.com |
215,719 | [
"https://dba.stackexchange.com/questions/215719",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/159178/"
] | I am kind of new to Postgres and am using recent version on Mac. I'm writing a <code>.sql</code> script that will create a database and tables and load data. (I need to be able to automate this, so I will be running it in a <code>cron</code> job, I will NOT be running it from within <code>psql</code> shell.) Every t... | <blockquote>
How could the relation still exist if the DROP of its database was successful? Do I need to be using fully qualified object names like my_db.public.progress?
</blockquote>
You can't <code>DROP</code> the database you're connected to.
<pre><code>psql -U postgres -d some_db -a -f db_etl.sql
</code></pre>... | You can't drop the database you are currently connected to. You are connected to <code>some_db</code>, not <code>mydb</code>. You drop <code>mydb</code>, then create <code>mydb</code> agsin, but you don't change your connection from <code>some_db</code> to <code>mydb</code>. Creating a database does not automatically... | https://dba.stackexchange.com |
159,160 | [
"https://softwareengineering.stackexchange.com/questions/159160",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/60663/"
] | I'm looking for some pointers on class design for a global application.
Let's say I have to make a class structure to manage products, and the products are sold in different countries. Some of the fields for the product will have the same value across all countries (eg. product code, ERP Description) I will call the... | I assume you have a database of some sort. I'd create a factory class that creates the objects (e.g. <code>Product</code>) from the data in the database.
Let's say your designing the <code>Product</code> class. It has some of your international fields:
<pre class="lang-cs prettyprint-override"><code>public class Prod... | Define a table for products which contains
<br><code>ProductCode, ERP Description</code> and other "constant" fields.
Define a table for properties which contains only 4 columns:
<br><code>PropertyId(long),</code> <br><code>PropertyOwnerId(long),</code><br><code>PropertyType(int, predefined in C# code, enumeration f... | https://softwareengineering.stackexchange.com |
60,251 | [
"https://mechanics.stackexchange.com/questions/60251",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/13901/"
] | So I was pulled up at the traffic lights yesterday in my 1980 Nissan Urvan (E23) diesel after just coming off a 30 minute commute on the freeway when all of a sudden the engine cut out and a few seconds later the battery exploded/ruptured spraying acid all over the battery compartment. After pushing it of the road I tr... | The most likely cause is that the cable that runs down to the starter motor has been rubbing against the body or the engine and has worn through the insulation.
| Sounds like a dead short, one large enough like that should be easily visible, either through burn marks on a metal edge or bracket or a hole in the insulation.
Check and follow all the main or thick wires.
| https://mechanics.stackexchange.com |
693,345 | [
"https://physics.stackexchange.com/questions/693345",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/290438/"
] | When I was first introduced to the quantum operators, I thought the terminology was just a convention, like the momentum operator is just a convention and it's not really momentum per se, because momentum is defined for classical particles in motion. But then I found the expectation values of those operators behaving j... | Not quite sure where your confusion lies, since you seem to have grasped the concept. There is a linear operator corresponding to each observable of a quantum system. The eigenvalues of the operator tell you what values the observable can take (and the physical requirement that these eigenvalues be real means that each... | In all of physics there is a difference between the mathematical concepts and the physical goings-on. For example, in classical physics a velocity in a physical sense is the rate of change of the physical position of something, and in a mathematical abstract sense it can be represented as a mathematical vector <span cl... | https://physics.stackexchange.com |
265,871 | [
"https://stats.stackexchange.com/questions/265871",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/40447/"
] | If my one sided t-test result is significant but the sample size is small (e.g. below 20 or so), can I still trust this result? If not, how should I deal and/or interpret this result?
| In theory if all the assumptions of the t-test are true then there's no problem with a small sample size.
In practice there are some not-quite-true assumptions which we can get away with for large sample sizes but they can cause problems for small sample sizes. Do you know if the underlying distribution is normally di... | You should rarely trust any single significant result. You didn't say why you were using a one-tailed instead of a two-tailed test, so hopefully you have a good reason for doing so other than struggling to be able to claim a statistically significant outcome!
Setting that aside, consider the following from p. 261 of ... | https://stats.stackexchange.com |
55,821 | [
"https://datascience.stackexchange.com/questions/55821",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/77889/"
] | I am starting off with machine learning so could someone tell if there is some site where one can find the current best performing trained models for any specific problem like sentiment analysis or objection detection or any machine learning problem of that sort?
| I think this does what you want.
<pre><code>for column in data_set:
If isinstace(data_set[column].dtypes,str)
data_set[column]=data_set[column].astype(int)
</code></pre>
| I put together the following code but I don't claim it to be the best option:
<pre><code>for col in data_set.columns:
for element in data_set[col]:
if element.isdigit():
data_set[col] = data_set[col].astype(int)
</code></pre>
Please note, that this code will try to convert the whole column to ... | https://datascience.stackexchange.com |
125,861 | [
"https://mathoverflow.net/questions/125861",
"https://mathoverflow.net",
"https://mathoverflow.net/users/20491/"
] | Let $f$ be a real function with domain R.
If $f^2$ and $f^3$ are both infinitely differentiable on R,
how to prove $f$ is infinitely differentiable on R?
I have been thinking about this problem for a long period, but I
I can not find an accurate proof. So if somebody can help me,
I will appreciative this very much.
| $\displaystyle \prod_{n=1}^\infty \left(1- \frac{s}{a+ (-1)^n i n} \right)=\frac{\Gamma \left(\frac{1}{2}+\frac{i a}{2}\right) \Gamma \left(1-\frac{i a}{2}\right)}{\Gamma \left(\frac{1}{2}-\frac{1}{2} i (s-a)\right) \Gamma \left(1+\frac{1}{2} i (s-a)\right)}$
$\displaystyle \prod_{n=1}^\infty \left(1- \frac{s}{a+ (-1)... | I did explore the second question a bit further and now have a follow up question.
Assuming the RH, the following product:
$$\displaystyle \displaystyle \prod_{n=1}^\infty \left(1- \frac{s}{a+ (-1)^n i \Im(\rho_n)} \right) \left(1- \frac{s}{a+ (-1)^{n+1} i \Im(\rho_n)} \right) = \frac{\xi_{rie}(\frac12 - a+s)}{\xi_{... | https://mathoverflow.net |
94,423 | [
"https://softwareengineering.stackexchange.com/questions/94423",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/31418/"
] | Put this image inside <code>images</code> folder. All JavaScript files should go inside a folder called <code>js</code>. Put templates inside a folder called site-templates and for each template, have three folders called <code>layouts</code>, <code>looks</code>, and <code>pages</code>.
We're all familiar with these ... | Sure we can, we even must sometimes. I've seen several projects which subdivided their file storages into several folders solely for performance purposes — this is quite the architecture.
| If the file system layout is known to all parts of the application then it is not decoupled enough, and you should reconsider how to centralize this knowledge.
| https://softwareengineering.stackexchange.com |
3,900,476 | [
"https://math.stackexchange.com/questions/3900476",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/420264/"
] | Consider the linear transformation: <span class="math-container">$$ f(x)\longmapsto \int_{-x}^x f(t)dt,$$</span> in the linear space (over <span class="math-container">$\mathbb{R}$</span>) that is span by <span class="math-container">$ \langle 1,\cos(x),sin(x),\dots,\cos(mx),\sin(mx)\rangle$</span> for <span class="mat... | First, suppose <span class="math-container">$f \in \mathrm{span}\left(1,\cos,\sin,\ldots,x\mapsto\cos(nx),x\mapsto \sin(nx)\ldots \right)$</span> and write <span class="math-container">$u(f) : x \mapsto \int_{-x}^x f$</span>. Then <span class="math-container">$u(f)$</span> is an odd function because:
<span class="math-... | Let <span class="math-container">$T$</span> denote the transformation. Note that the eigenspace associated with the eigenvalue <span class="math-container">$0$</span> consists of the set of all odd functions within the domain. Now, show that <span class="math-container">$Tf$</span> is itself an odd function for all <sp... | https://math.stackexchange.com |
2,542 | [
"https://mechanics.stackexchange.com/questions/2542",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/1331/"
] | I have a VW Golf mk4, 1.9 TDI from 2001 with 230.000+ km on board and for some time now I have blue smoke coming out of the exhaust.
The problem aggravates when I'm stuck in traffic or when the engine is kept at low revs for a long time. After that, when I rev it up, tons of smoke come out, then it goes back to just ... | Have a look at your owners manual if it states a maximum oil consumption. I don't think that 750ml over a whole oil change interval is a lot.
My guess would be that it's more likely an issue with the turbo and investigating if it is shouldn't be that expensive. Essentially you or your mechanic would need to pull the p... | Golf TDis (and I'd imagine other VWs of that era) are notorious for the Turbos failing. It can be a very expensive fix once it goes properly, so I'd get it investigated asap.
From your description, I'd surmise that oil is seeping through one of the seals into the turbo, then when you rev the engine and the turbo spool... | https://mechanics.stackexchange.com |
1,326 | [
"https://dba.stackexchange.com/questions/1326",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/892/"
] | I've been waiting now for 36 hours for a 12 GB .sql file to be imported with a simple <code>type site.sql | mysql</code> command. I can see the <code>ibdata1</code> is growing still, currently nearly 40 GB.
Considering the triggers and stored procedures are at the end of the .sql, I only think MySQL should be adding d... | Try this:
<pre><code>$ ps -ef|grep [m]ysql
</code></pre>
Identify the process id then
<pre><code>$ strace -cp <pid>
</code></pre>
Leave it 10 seconds or a minute then <code>^C</code>. That will tell you where the process is spending its time, e.g. it could just be waiting for the disk if you seen <code>read</... | Are you sure that the tables where you are reading in are without triggers and indexes and constraints?
What hardware and OS are you running on?
How is your storage configured?
I am more familiar with oracle but 12G importing on tables without triggers, indexes and constraints should easily go with 200GB/h. One single... | https://dba.stackexchange.com |
106,893 | [
"https://mathoverflow.net/questions/106893",
"https://mathoverflow.net",
"https://mathoverflow.net/users/6951/"
] | Let $C$ be a curve of genus 3 and suppose that it admits a branched cover $\varphi:C\rightarrow E$ with $E$ elliptic and such that $\varphi$ does not factor through any \' etale cover. Then the degree of the ramification divisor $R$ is $4$ and the branch divisor $B\subseteq E$ is supported in a set of cardinality betw... | According to Sofia's and my comments to the original question, we look for instance for elements $a,b\in S_n$ which generate a primitive subgroup of $S_n$ and where the commutator $[a,b]=a^{-1}b^{-1}ab$ is a $5$-cycle.
This is possible for each $n\ge5$: Set $a=(1,2\,3,\cdots,n)$, $b=(1,2,\cdots,n-4,n-3,n-1,n-2)$. Then... | It is often easier to construct <em>Galois</em> covers of degree $n$.
In the case where $E$ is an elliptic curve and we have $t$ branch points, by Riemann existence theorem a Galois cover with Galois group $G$ is defined by the following data:
$\bullet$ a finite group $G$ of order $n$;
$\bullet$ elements $a, b, g_... | https://mathoverflow.net |
102,461 | [
"https://dba.stackexchange.com/questions/102461",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/67197/"
] | <img src="https://i.stack.imgur.com/YrqAT.png" alt="enter image description here">
I have a RDS instance that has workload at specific time window, however, cpu utilisation is a constant 20% even outside this window.
There is no replication configured.
Can't find a solution for this case.
Spec:
The database has mo... | Quoting AWS support which I think was really disappointing:
"Thanks so much for your patience. I'm one of the RDS Support Engineers looking at your instance issue.
I've escalated this CPU utilisation issue of your instance to our RDS team and they are aware of an issue that affects a very small number of db.t2.medium... | This is normal behavior from a t2.medium instance when it has burned off all its CPU credits. You'll notice on your graph that when CPU dies down a bit, the instance is suddenly capable of actually going to 100% for a limited period of time.
t2.medium instances can push 20% CPU per core as a baseline when all credits... | https://dba.stackexchange.com |
256,594 | [
"https://electronics.stackexchange.com/questions/256594",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/123036/"
] | In other words, does a 1000-watt inverter, draw the same as a 500-watt inverter if they are charging only a laptop? The laptop will draw the same amount and the inverter capacity is really just that, capacity?
i.e. 1000-watt inverter CAN draw UP TO 1000-watts, and the same for 500-watt inverter.
My issue is that I pl... | If two 100% efficient inverters, one 500W max throughput, one 1000W, are used to drive the same load, they will pull the same power from the battery.
Of course neither will have 100% efficiency. Even at no output load, they will draw some power. The chances are high that the 1000W inverter would draw more idle power t... | My thoughts were if sizing a AC sine wave inverter, go with larger than needed for the current solar panels installed but not push it to the limits, but still have the option under peak demand. But of course still wondering the efficiency of doing so or more importantly how to size the battery array feeding the inverte... | https://electronics.stackexchange.com |
78,676 | [
"https://electronics.stackexchange.com/questions/78676",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/26864/"
] | So in an application like this:
<img src="https://i.stack.imgur.com/pyZgQ.png" alt="enter image description here">
The coil of the motor will, by Lenz's Law, induce a very large negative voltage across its terminals (without the diode in place). Would this voltage be very negative at the left terminal of the inductor... | In your first example, before the switch opens, there is a current flowing through L. When the switch opens the current wants to continue flowing in the same direction so this <strong>has to</strong> create a positive voltage on the right-hand side of L relative to the left hand side (which is fixed at +Vs).
This posi... | Flyback diodes provide a path for current from the motor's inductivity to continue rather than cause voltages that can kill circuits and solenoids (the latter by arcing). For mere circuit/coil protection, a straightforward flyback diode is fine and in an idealised circuit, the result would be "idling": a mechanically ... | https://electronics.stackexchange.com |
380,815 | [
"https://softwareengineering.stackexchange.com/questions/380815",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/302396/"
] | Is there anyone who can explain the relationship between customer, users, and the company ?
As far as I understand, the product is used by the users, and before that product is designed, the users list their requests for that product. This is called <code>user requirements</code>. The customer is responsible for const... | A <strong>user</strong> is a person that uses the product.
A <strong>customer</strong> is the person or the company that buys the product:
<ul>
<li>If your customer is a single person, the user and the customer may be the same (typical case if your product is a game).</li>
<li>If your customer is a company, the cust... | There are 2 types of users: customer and consumer.
1. Customer: Buy the product
2. Consumer: Use the product
Customer can buy the product but not using it. Or he can buy and use it. Consumer may not be the one buy the product but using it. This will depend on what kinds of product: Enterprise, or Consumer product.
Ex... | https://softwareengineering.stackexchange.com |
122,295 | [
"https://softwareengineering.stackexchange.com/questions/122295",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/133/"
] | Is this good or bad design to add, by default, ID field in every table in a database, even when you don't see currently to usage of this ID (For instance in a MxN table) ?
| In many cases, having an artificial ID on every table is very conveniant; in some cases, it's just a pain in the a**.
Generally, I do that on every project I'm working on.
Advantages:
<ul>
<li>It's generally easier to write or generate access code when each and every table can be accessed by a key field called <code... | synthetic keys vs natural keys and single keys vs compound keys are both hotly debated topics that have good positives and negatives on both sides. They are like the tabs vs spaces and curly brackets on their own line debates.
The most important thing is to pick a side and stick with it for consistency throughout the... | https://softwareengineering.stackexchange.com |
9,834 | [
"https://dsp.stackexchange.com/questions/9834",
"https://dsp.stackexchange.com",
"https://dsp.stackexchange.com/users/4967/"
] | Okay, this might be a very basic question, but I'm completely new to the field.
I have a $40$ Hz sine wave ($1000$ samples, $fs = 1$ kHz), which I have amplitude modulated to a $4$ Hz sine wave ($1000$ samples, $fs = 1$ kHz). In the time-domain, the result looks like I expected. However, when I calculate the FFT, ther... | What you did is multiply two sinusoids with each other:
$$z(n)=\sin(n\theta_1)\cdot\sin(n\theta_2)$$
where $\theta_1=2\pi\cdot 40/1000$ and $\theta_2=2\pi\cdot 4/1000$.
The result $z(n)$ can be written as
$$z(n)=\frac{1}{2}\left[\cos(n(\theta_1-\theta_2))-\cos(n(\theta_1+\theta_2))\right]$$
So you get two sinusoids... | From the code, it appears that you are multiplying both signals, i.e. $c(t)\cdot s(t)$. However, regular amplitude modulation with 100% depth is $c(t)\cdot (0.5+0.5\cdot s(t))$.
| https://dsp.stackexchange.com |
84,742 | [
"https://mathoverflow.net/questions/84742",
"https://mathoverflow.net",
"https://mathoverflow.net/users/586/"
] | Let $f$ and $g$ be complex rational functions (of degree $\geq 2$ if that helps). What can be said about the relationship between $J(fg)$ and $J(gf)$, the Julia sets of the composite functions $f \circ g$ and $g \circ f$?
If I'm not mistaken, $f$ restricts to a map $J(gf) \to J(fg)$, and $g$ restricts to a map $J(fg)... | I'm not sure if this is helpful, but here is an example. The following picture shows the filled Julia set for $z^6 - 1$.
<img src="https://i.imgur.com/4vacH.png" alt="alt text">
and the following picture shows the filled Julia set for $(z^2-1)^3$:
<img src="https://i.imgur.com/P61Jx.png" alt="alt text">
This is th... | Answer. $J(fg)$ is the full $g$-preimage of $J(gf)$. (And vice versa with interchange of
$f$ and $g$).
Proof. Let $A=fg$ and $B=gf$. Then we have a semi-conjugacy $gA=Bg$.
Now it is a general fact, that whenever you have such a semi-conjugacy (of rational functions)
the Julia set of $A$ is the $g$-preimage of the Jul... | https://mathoverflow.net |
114,558 | [
"https://mathoverflow.net/questions/114558",
"https://mathoverflow.net",
"https://mathoverflow.net/users/5661/"
] | Given an nonisotrivial elliptic fibration $f:X\rightarrow P^1$, where $X$ is smooth and $P^1$ is a projective line. Could anybody provide some information on the restriction of the cotangent bundle $\Omega^1_{X}$ to the smooth fibers of $f$, e.g. does it split to line bundles, (semi)stable...?
| It's semi-stable, but not stable on every fiber.
Assuming that the characteristic is zero this sheaf does not split.
This may be true in positive characteristic, but as <em>Damian Rössler</em> points out the proof below requires characteristic zero.
<blockquote>
<strong>Claim 1.</strong>
Let $V\subseteq \mathbb... | We have the exact sequence $0 \to f^*\Omega^1_{\mathbb P^1} \to \Omega^1_X \to \Omega^1_{X/\mathbb P^1} \to 0$
Because the map is smooth, this is an exact sequence of bundles, i.e. $f^* \Omega^1_{\mathbb P^1}$ and $\Omega^1_{X/\mathbb P^1}$ are both line bundles.
Since you've removed at least one fiber, and both lin... | https://mathoverflow.net |
2,116,499 | [
"https://math.stackexchange.com/questions/2116499",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/381589/"
] | What should I substitute to get from :
$\int \frac{1-x^2}{x \cdot \sqrt[]{1-x^2} }dx$
integral which will be:
$\int \frac{a_nx^n+...+a_1x+a_0}{\sqrt[]{1-x^2} }dx$
| Note that
$$S_n=\frac1n \sum_{k=1}^n \frac{1}{1+k/n+(k/n)^2}$$
is the outer Riemann sum for the integral $\int_0^1 \frac{1}{1+x+x^2}\,dx$, while
$$T_n=\frac1n \sum_{k=0}^{n-1} \frac{1}{1+k/n+(k/n)^2}$$
is the inner Riemann sum for the same integral $\int_0^1 \frac{1}{1+x+x^2}$.
Since the summand and correspond... | We have $$S_n < \lim_{n \to \infty} S_{\infty} = \lim_{n \to \infty} \sum_{k=1}^{n} \frac {1}{n} \frac {1}{1+ \frac {k}{n} +(\frac {k}{n})^2 }$$ $$ = \int_{0}^{1} \frac {\mathrm {d}x}{1+x+x^2} = \frac {\pi}{3\sqrt {3}} $$
Now $$T_n > \frac{\pi}{3\sqrt {3}} \text { as } h\sum_{k=0}^{n-1} f (kh) > \int_{0}^{1... | https://math.stackexchange.com |
43,293 | [
"https://dba.stackexchange.com/questions/43293",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/21546/"
] | <ol>
<li>I have a table where I have a column named <code>state(int(2))</code> and <code>modify_time</code> (<code>time_stamp</code> on update current timestamp).</li>
<li>I have an update trigger which changes the value of state column to <code>0</code> based on some condition.</li>
<li>I want to set the value of <cod... | <pre><code>SET GLOBAL event_scheduler = ON; -- enable event scheduler.
SELECT @@event_scheduler; -- check whether event scheduler is ON/OFF
CREATE EVENT e_store_ts -- create your event
ON SCHEDULE
EVERY 24 HOURS -- run every 24 hours
DO
UPDATE myschema.youtable set mycolumn='1'
</code></pre>
| Your <code>UPDATE</code> query will need some work, because as-written, it will update all of the <code>state</code> values to 0 every minute, but if it's not working, you should verify that the event scheduler is actually running.
<pre><code>mysql> show variables like 'event_scheduler';
+-----------------+-------+... | https://dba.stackexchange.com |
37,082 | [
"https://mechanics.stackexchange.com/questions/37082",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/19042/"
] | Are there any benefits of running independent throttle bodies with a super/turbocharged engine, as opposed to running a single throttle body on the same engine?
If so, what are the benefits (and how extreme are they)?
<hr>
<strong>Static characteristics of engine</strong>
Hypothetically, the engine is as follows:
... | "Mineral water", for this purpose, is equivalent to using tap water. It won't immediately destroy the battery, but you are disturbing the chemistry by adding some kind which don't belong there. You may have shortened the battery's life a bit.
But there isn't much you can do about it now short of draining and replacin... | The acid in the battery has attacked the minerals in the water, essentially neutralizing a proportionate amount of acid and leaving some salts in the bottom of the battery. It is probably a negligible amount, and won't affect capacity or lifetime <strong>if you do it once</strong>. Making a habit of it will shorten ... | https://mechanics.stackexchange.com |
185,064 | [
"https://dba.stackexchange.com/questions/185064",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/133954/"
] | These are the tables
<strong>Orders</strong>
<ul>
<li>id</li>
<li>status</li>
</ul>
<strong>Order_lines</strong>
<ul>
<li>id</li>
<li>order_id</li>
</ul>
<strong>Invoice_lines</strong>
<ul>
<li>id</li>
<li>invoice_id</li>
<li>order_line_id</li>
</ul>
<strong>Invoices</strong>
<ul>
<li>id</li>
<li>status</li>
</... | There are a couple issues I'll have to gloss over at this point due to lack of info:
<ul>
<li>no DDL; no sample data; no list of valid <code>invoices.status</code> values</li>
<li>"invoices are only in approved <strong>and</strong> closed status" implies an invoice needs at least 2 records ... one with <code>status='a... | I had a look at this - the design of the system seems strange - why you have to go from <code>orders-> order_lines -> invoice_lines -> invoices</code> strikes me as strange.
You should have a correspondance between invoices and orders. An extra level of complexity could be added by deliveries - with many ord... | https://dba.stackexchange.com |
235,255 | [
"https://dba.stackexchange.com/questions/235255",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/166932/"
] | I have database with mdf and ldf file residing on external drive. There was a long powercut and the database restarted. Now the database has gone for recovery pending. The database is around 600 gb.
I have tried the following commands
<pre><code>ALTER DATABASE ‘DATBASE NAME’ SET OFFLINE WITH ROLLBACK IMMEDIATE
ALTE... | thanks for all your support, I figured out the issue. Our Admin skipped one user. domain\Users was missing from the folders permission. I know it is a silly mistake. But because of this I got to learn more.
I added permission to the folder by adding the domain\Users, and the error disappeared.
Thanks again.
| As previously mentioned, make sure the SQL service account has permissions on the files.
Here is a quick snippet to help you do that:
<pre><code>-- From SQL:
-- get full path for master
SELECT SUBSTRING(physical_name, 1,
CHARINDEX(N'master.mdf',
LOWER(physical_name)) - 1) DataFileLocation
FROM master.sys.master_files
... | https://dba.stackexchange.com |
1,613,973 | [
"https://math.stackexchange.com/questions/1613973",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/219052/"
] | Other than ∞, is there another case where a supremum (or an infimum for that matter) doesn't exist?
| <blockquote>
<strong>Supremum axiom</strong>: Any nonempty subset $A\subset\mathbb{R}$ which is bounded above has a supremum $s\in\mathbb{R}$.
</blockquote>
This is only valid in $\mathbb{R}$. For example, the set $\{x\in\mathbb{Q}:x\geq 0, \ x^{2}<2\}$ does not admit supremum, since $\sqrt{2}\notin\mathbb{Q}$.
| Yes, depends on the partial ordering you define. A partial ordering for which a bounded set have sup and inf is called complete.
An example of an incomplete ordering would be on set $\lbrace a, b, c \rbrace$, $a < b$, $a < c$, but $b$ and $c$ are not comparable. Then the subset $\lbrace a, b, c \rbrace$ do not ... | https://math.stackexchange.com |
66 | [
"https://quant.stackexchange.com/questions/66",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/74/"
] | Explain pair trading to a layman. What is it, why would you want to do it, and what are the risks? Provide a real life example.
| Pair trading is a market neutral bet. Instead of saying the market in general is going higher, you say one investment under/overvalued relative to another, typically similar, investment. The bet is that the spread between the two will widen or narrow depending on how you set it up.
For instance, say I feel GM is goi... | Quantitative pair trading (as we are on the quantitative finance forum) is based on cointegration.
Two stocks are said to be cointegrated if they move together, which means that they share the same long term trend.
Precisely:
It exists a linear relationship between the price of the 2 stocks so that is mean reverting.... | https://quant.stackexchange.com |
130,988 | [
"https://dba.stackexchange.com/questions/130988",
"https://dba.stackexchange.com",
"https://dba.stackexchange.com/users/31372/"
] | I have a number of tables that have triggers on them that insert records in queueing and logging tables. The problem is that those inserts are then part of the same transaction. That means that any IUD on the former tables, can be locked by other IUD that trigger inserts into the same queue, and that IUD that fails wil... | If you have access to the Aria engine or another non-transactional engine, then yes.
Very short proof of concept, tested on MariaDB 10.5.4:
<pre><code>CREATE TABLE test_aria_engine ( a int, b varchar(1000) )
ENGINE=Aria DEFAULT CHARSET=utf8 PAGE_CHECKSUM=1 TRANSACTIONAL=1 TABLE_CHECKSUM=1
COMMENT='test-aria_engine with... | If possible, listen to the mantra: "Don't queue it, just do it."
Otherwise, keep queuing and normalization code <em>outside</em> of the main transactions.
As for logging, I don't think there is a solution other than writing to a MyISAM table for your log.
Back to your question...
There is no way in MySQL to have n... | https://dba.stackexchange.com |
38,132 | [
"https://quant.stackexchange.com/questions/38132",
"https://quant.stackexchange.com",
"https://quant.stackexchange.com/users/31759/"
] | February 2/8/2018 - context in case the question is still around beyond today: the stock market has been falling for almost a couple of weeks in the midst of fears of overheating of the economy (increased salaries with full employment), concern about Fed rate hikes accelerating; and a huge blow to the fiscal deficit re... | <blockquote>
If the rates go up as a result of fiscal deficit concerns
</blockquote>
The impact of fiscal deficit is likely to be a very small factor (if at all) behind the recent selloff in bonds. If deficit is a major driver behind yield levels, we should've been rapidly rising yields around the world for many yea... | Short answer: The FED controls the short-term interest rates while inflation controls the long-term interest rates. If the 10Y or 30Y move higher, it means inflation is picking up either because of good economic conditions or depreciation of the dollar. In this case the FED will hike further in order to slow down the e... | https://quant.stackexchange.com |
879,832 | [
"https://math.stackexchange.com/questions/879832",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/66096/"
] | <blockquote>
Let <span class="math-container">$f,g$</span> be two continuous <strong>positive</strong> functions over <span class="math-container">$[a,b]$</span>
Let <span class="math-container">$m_1$</span> and <span class="math-container">$M_1$</span> be the minimum and maximum of <span class="math-container">$f$</sp... | We use continuity of $f$ and $g$, to prove that the integral inequality: $$\displaystyle \int_a^b f^2\int_a^bg^2 \le \dfrac{1}{4}\left(\sqrt{\dfrac{M_1M_2}{m_1m_2}}+\sqrt{\dfrac{m_1m_2}{M_1M_2}}\right)^2\left(\int_a^bfg\right)^2$$
follows from proving the discrete case of the inequality, taking $a_k = f(a+\frac{b-a}{n... | By scaling, which does not change the ratio of maximum to minimum, we can assume
$$
\int_a^bf(x)^2\,\mathrm{d}x=\int_a^bg(x)^2\,\mathrm{d}x=1\tag{1}
$$
Trivially, we have
$$
\int_a^bf(x)g(x)\frac{f(x)}{g(x)}\,\mathrm{d}x=\int_a^bf(x)^2\,\mathrm{d}x=1\tag{2}
$$
and
$$
\int_a^bf(x)g(x)\frac{g(x)}{f(x)}\,\mathrm{d}x=\int_... | https://math.stackexchange.com |
4,424,848 | [
"https://math.stackexchange.com/questions/4424848",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/2941/"
] | Any finite sheeted connected cover of the circle is again homeomorphic to a circle. On the group level, this is consistent with the fact that subgroups of <span class="math-container">$\mathbb Z$</span> are isomorphic to <span class="math-container">$\mathbb Z$</span>. This leads to the question of whether one could fi... | Let <span class="math-container">$G\subset F_2=\langle a,b\rangle$</span> be the subgroup generated by <span class="math-container">$a^2$</span> and <span class="math-container">$b^2$</span>. Let <span class="math-container">$X_0=S^1\vee S^1$</span> and recursively define <span class="math-container">$X_{n+1}$</span> ... | So I think the answer is simple. Start with <span class="math-container">$S^1\vee S^1$</span> and take any cover for which <span class="math-container">$\pi_1$</span> of that cover is a free group of rank 2. Now thicken these examples slightly by replacing each vertex by an open disk and each edge by an open strip. So ... | https://math.stackexchange.com |
34,314 | [
"https://mathoverflow.net/questions/34314",
"https://mathoverflow.net",
"https://mathoverflow.net/users/8008/"
] | Let C be an nxn matrix, then the polynomials in C (with appropriate coefficients) form an algebra of commuting matrices. I feel that I should know if the converse is true but I do not. So my first question (adding several conditions) is: Suppose A and B are symmetric integer matrices with AB=BA, must there be a matrix... | Thanks for the answers. Just to wrap up a bit, here are a few examples.
<ol>
<li>Sometimes an ACM (algebra of commuting matrices) is sure to be generated by one of its members
</li>
<li>Other times it has dimension too large to possibly be (embedded in) an ACM with a single generator.
</li>
<li>An ACM might be generate... | Consider the subalgebra of $M_{2n}(k)$ spanned by the matrices of the form $\left(\begin{smallmatrix}0&A\\0&0\end{smallmatrix}\right)$ (all blocks are $n\times n$) together with the identity, which is commutative. Its dimension is larger than $2n$ when $n$ is sufficiently large, so it is not generated by a sing... | https://mathoverflow.net |
23,504 | [
"https://engineering.stackexchange.com/questions/23504",
"https://engineering.stackexchange.com",
"https://engineering.stackexchange.com/users/16376/"
] | I need to thermally insulate two metal (say aluminum) parts, the first of which is at 200-400 °C (473-673 K), and the second one is at room temperature. It should be possible to screw the parts together or something, to form one solid assembly. The typical dimensions are like 10-40 mm. The insulator should be able to b... | have you considered firebrick, a product designed to be used in high temperature kilns, for example? It's light weight and easily cut with ordinary hand tools. It may not tolerate severe compression, but should bond well with aluminum and high-temperature adhesive.
| The aluminum can barely support itself at 400 C."Lightweight castable" refractory ( can be handled like concrete) should do the job; but you would need to find a supplier .It is what a petrochemical or power-plant might use . Portland cement/concrete type material will not tolerate temperatures of 400 C. Johns-Manville... | https://engineering.stackexchange.com |
2,174,818 | [
"https://math.stackexchange.com/questions/2174818",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/395613/"
] | if $$\sum_0^\infty a_n x^n = (\sum_0^\infty x^n )(\sum_0^\infty x^{2n})$$
what is $a_n$?
Here is my approach
let $b_n= 1$ and $c_n= x^n$
Then by forming / relating to cauchy product we can conlude that the product is equal to :
$$\sum_0^\infty a_n x^n$$
where $a_n = \sum_{k=0}^{n} b_k c_{n-k} = \sum x^{n-k}= x^... | Let us write the first sum on the RHS as a geometrical series:
$$\sum_{n=0}^\infty a_n x^n =\dfrac{1}{1-x}\sum_{n=0}^\infty x^{2n} \ \ \iff$$
$$(1-x)\sum_{k=0}^\infty a_k x^k =\sum_{n=0}^\infty x^{2n} \ \ \iff$$
$$\tag{1}\sum_{k=0}^\infty a_{k}x^k - \sum_{k=0}^\infty a_{k}x^{k+1} = \sum_{n=0}^\infty x^{2n}.$$
Setti... | Using the geometric summation formula $\displaystyle\sum x^n=\dfrac1{1-x}$,
$$A=\frac1{1-x}\frac1{1-x^2}=\frac1{4(1+x)}+\frac1{4(1-x)}+\frac1{2(1-x)^2}\\
=\frac1{4(1+x)}+\frac1{4(1-x)}+\frac12\frac d{dx}\frac x{1-x}$$ so that
$$a_n=\frac14(-1)^n+\frac14+\frac12(n+1).$$
This is a <em>generating function</em> approach... | https://math.stackexchange.com |
14,241 | [
"https://chemistry.stackexchange.com/questions/14241",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/1677/"
] | I particulary enjoy performing chemical experiments without buying any chemicals for them.
I was always sad about the fact that one glowstick lights out, you just throw it into trash can (I like all means of recycling). So I was wondering if I could (pick one or more):
<ol>
<li>Recycle the chemicals in it to make it ... | Here is a generalized version of what's going on chemically. The key steps are dioxetane formation (the highly strained 4-membered ring) and its subsequent decomposition and energy transfer to a fluorescent dye, which then decays back to its ground state by emitting light. By using different fluorescers, you can chan... | IIRC the reaction is an oxidation. Perhaps it could be reduced. I believe that hair permanent may be a reducing agent. It would be interesting to see if permanent would work but only done in a safe ventilated place.
| https://chemistry.stackexchange.com |
374,959 | [
"https://physics.stackexchange.com/questions/374959",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/165756/"
] | For an electric dipole $p$ in a uniform field $E$ we write the work done by an external agent in rotating dipole as $\text{d}W = pE\sin\theta \, \text{d}\theta$.
I'm having trouble understanding where this equation comes from and why
$W = \int_{\theta_i}^{\theta_f} pE\sin\theta \,\text{d}\theta$ is true, as the only ... | The general expression for the work done by a variable force along a path between the initial position <span class="math-container">$(x_i, y_i, z_i)$</span> and the final position <span class="math-container">$(x_f, y_f, z_f)$</span> is
<span class="math-container">$$
W = \int_{(x_i, y_i, z_i)}^{(x_f, y_f, z_f)}\vec{\... | Analogous to the work $$W=F\Delta s$$ done by a force $F$ over a distance $\Delta s$ , there is also a work done by a torque $T$ for a rotational movement $$W=T\Delta \theta$$ where $\Delta \theta$ is the angle of rotation. The torque experienced by an electric dipole with dipole moment $\vec p$ in an electric field ... | https://physics.stackexchange.com |
59,257 | [
"https://mathoverflow.net/questions/59257",
"https://mathoverflow.net",
"https://mathoverflow.net/users/4832/"
] | I am reading about Lyapunov functions for Markov processes, and I am having trouble thinking of examples to keep in mind as I read. If $X_t$ is a continuous-time Markov process with generator $L$, a Lyapunov function is supposed to be a function $V$, in the domain of $L$, with $V \ge 1$ such that $LV \le -aV + b 1_C$,... | This is called a "drift condition" in the applied probability literature -- this is used quite often when dealing with MCMC simulations, for example.
In continuous time, what about the good old Ornstein-Uhlenbeck process $dz = -zdt + \sqrt{2}dW$ and generator $L \phi(x) = -x \phi'(x) + \phi^{''}(x)$: the Lyapunov func... | But why Lyapunov function belongs to the domain of the generator. The Lyapunov function is not bounded.
| https://mathoverflow.net |
42,520 | [
"https://engineering.stackexchange.com/questions/42520",
"https://engineering.stackexchange.com",
"https://engineering.stackexchange.com/users/30224/"
] | I am working out the temperature for an Ideal diesel cycle:
Based on the isentropic properties for an ideal gas.
<span class="math-container">$$T_f = T_i * r^{k-1}$$</span>
Where <span class="math-container">$T_i = 50 ºF = 510ºR$</span>
If I use ºF, then <span class="math-container">$T_f = 172.9ºF = 632.9 ºR$</span>
If... | We cannot obtain negative temperatures in thermodynamics calculations. Only the absolute scales prevent this. The two absolute scales are Kelvin and degrees Rankin.
Here are the two calculations to show that both give the same result. I take that <span class="math-container">$r^{k-1} = 3.45$</span> based on using your ... | To my knowledge when you are doing thermodynamic calculations you should always use K (Kelvin), unless stated otherwise in the textbook you are using.
Otherwise the results from those relationships (with powers) will always be all over the place.
Another way to put it (probably more correct) is, "Always use the u... | https://engineering.stackexchange.com |
55,536 | [
"https://physics.stackexchange.com/questions/55536",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/21512/"
] | What effects would occur if the earth's core goes cold? Would the planet stay liveable after this happens?
| Once the core has solidifed it can no longer generate a magnetic field. There may be some frozen in field, but I would guess that the strength of the magnetic field with decrease as more and more of the core freezes, so any residual field is likely to be small. The core of Mars is thought to be frozen and it's magnetic... | I am not educated in this field but I suggest that the moon acts upon the liquid earth outer core much like it does upon our oceans, acting as a mixer therefore constantly stirring the molten pot if you will. The liquid is thus ever moving within the mantle causing a more consistent and uniform magnetic field.
| https://physics.stackexchange.com |
2,925,014 | [
"https://math.stackexchange.com/questions/2925014",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/424975/"
] | <blockquote>
Is function
<span class="math-container">$$f(x) = \frac 1 x - \frac 1 {x+1} + \frac{1}{x+2} - \frac 1{x+3} + \cdots$$</span>
continuous on <span class="math-container">$(0, \infty)$</span> ?
</blockquote>
I think I should extend the continuity of each term to <span class="math-container">$f$</span... | <span class="math-container">$$\left( \frac 1 x - \frac 1 {x+1} \right) + \left( \frac 1 {x+2} - \frac 1 {x+3} \right) + \cdots + \left( \frac 1 {x+2n} -\frac 1 {x+2n-1} \right)$$</span> <span class="math-container">$$=\frac 1 {x(x+1)}+\frac 1 {(x+2)(x+3)}+\cdots +\frac 1 {(x+2n) (x+2n+1)}$$</span> <span class="math-co... | How to prove this function is continuous:
<ol>
<li>Show the sequence of partial sums is uniformly Cauchy on <span class="math-container">$(0,\infty)$</span>.</li>
<li>Conclude the sequence of partial sums is uniformly convergent on <span class="math-container">$(0,\infty)$</span>.</li>
<li>Conclude the sequence of par... | https://math.stackexchange.com |
87,200 | [
"https://softwareengineering.stackexchange.com/questions/87200",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/6331/"
] | I'm just looking for some tips on getting away from using PHP for web development. I'm self-employed but it seems like all of the work I find deals with PHP. I'm not complaining about the work -- just the poor choice of a language that is incredibly popular.
I'd love to do my web development in Python, Perl, C#, or ev... | I understand that the great bulk of the work out there is being done in PHP. But does that mean it <em>needs</em> to be done in PHP? I imagine that employers aren't going to say "we've looked into all the available tools for this type of job, and we've decided PHP is the best".
Well, actually, they might, if their... | Unless you have the financial cushion, you're going to need to learn Python or whatever else you want to learn <em>on your own time</em> while still taking on PHP jobs for income. Once you get proficient enough at the new language of your choice you can start taking on projects in it, and slowly transition away from P... | https://softwareengineering.stackexchange.com |
386,247 | [
"https://stats.stackexchange.com/questions/386247",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/231899/"
] | I'm facing a regression task with many categorical and few numeric features. I encoded them into dummies and removed the first dummy column for each feature.
I am not getting very good R2 at all. I am wondering if, aside from creating dummies, there are any special strategies in these situations related to having so m... | <span class="math-container">$R^2$</span> isn't a good measure of model quality. It is a value devoid of context. Imagine you had an <span class="math-container">$R^2$</span> of 0.11. Now let us assume that you are modeling human behavior. Think about how many behaviors you do in a day. Consider how many distinct ... | Regression is not the best tool if you want good out-of-sample predictive performance on stable data. I would try using a random forest on your problem. This method will handle automatic interaction/non-linearity detection for you and is not too difficult to use off the shelf.
If you want to stick with regression, I w... | https://stats.stackexchange.com |
38,967 | [
"https://stats.stackexchange.com/questions/38967",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/14464/"
] | I've read that the <em>t</em>-test is "reasonably robust" when the distributions of the samples depart from normality. Of course, it's the sampling distribution of the differences that are important. I have data for two groups. One of the groups is highly skewed on the dependent variable. The sample size is quite small... | Questions about robustness are very hard to answer well - because the assumptions may be violated in so many ways, and in each way to different degrees. Simulation work can only sample a very small portion of the possible violations.
Given the state of computing, I think it is often worth the time to run <em>both</em>... | @PeterFlom hit the nail dead on with his first sentence.
I'll try to give a rough summary of what studies I have seen (if you want links it could be a while):
Overall, the two sample t-test is reasonably power-robust to symmetric non-normality (the true type-I-error-rate is affected somewhat by kurtosis, the power is i... | https://stats.stackexchange.com |
25,274 | [
"https://security.stackexchange.com/questions/25274",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/15187/"
] | Can I protect against keylogging by using the mouse? For example, to enter the password <code>hunter2</code>, I might do the following:
<ol>
<li>type <code>ttter2</code></li>
<li>select the first <code>tt</code> and press <kbd>← Backspace</kbd></li>
<li>type <code>hun</code></li>
</ol>
To a keylogger, it would like l... | A lot of keyloggers are smart enough to catch this. Not only do they log the keys being physically pressed, but they also log the contents of password boxes whenever window messages are processed, or when browser forms are posted. They also often capture small blocks of the screen around the mouse cursor, to catch case... | It really just depends on how advanced the said keylogger is. Some keyloggers collect data from password boxes when the form is submitted, and some monitor when the keys are pressed. Some keyloggers can be really advanced and even record mouse movements, and your screen. If you do have an unwanted keylogger, I'd take t... | https://security.stackexchange.com |
277,185 | [
"https://electronics.stackexchange.com/questions/277185",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/134326/"
] | I am designing an AC to DC converter. My power source is a single phase synchronous alternator that has a voltage range of 170~260 VAC and a frequency of 40~60 Hz. Can i use a transformer designed on 50 Hz and 220 VAC to work on my alternator specs mentioned above?
| <blockquote>
Can a 50 Hz, 220 VAC transformer work on 40 Hz, 180VAC?
</blockquote>
Yes it probably can - the initial worry is saturation problems due to operating at the lower frequency but, with the voltage dropping to 180 V at 40 Hz, this produces virtually the same magnetization current as 220 V at 50 Hz.
Strict... | Be on the safe side by using a transformer designed for heavier load. So the iron loss due to lower frequency can occupy the margin originally designed for heavier load (copper+iron) loss.
| https://electronics.stackexchange.com |
736 | [
"https://datascience.stackexchange.com/questions/736",
"https://datascience.stackexchange.com",
"https://datascience.stackexchange.com/users/802/"
] | I have a dataset which contains ~100,000 samples of 50 classes. I have been using SVM with an RBF kernel to train and predict new data. The problem though is the dataset is skewed towards different classes.
For example, Class 1 - 30 (~3% each), Class 31 - 45 (~0.6% each), Class 46 - 50 (~0.2% each)
I see that the mo... | I would suggest you to use libsvm, which already has adjustable class weights implemented in it. Rather than replicating the training samples, one modifies the C parameter for different classes in the SVM optimization. For example if your data has 2 classes, and the first class is only 10% of the data, you would choose... | I am not an export in using SVMs, but usually (if you are using a machine learning library like Python's <code>scikit-learn</code> or R's <code>libsvm</code>, there is the <code>class_weight</code> parameter, or <code>class.weights</code>, respectively.
Or if you'd use a Bayes classifier, you would take this "skew" i... | https://datascience.stackexchange.com |
518,152 | [
"https://physics.stackexchange.com/questions/518152",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/228535/"
] | In dimension <span class="math-container">$3$</span>, we have the angular momentum <span class="math-container">$\omega = q \times v$</span>, and since
<span class="math-container">$$\frac{d}{dt} \omega = v \times v + q \times (F/m)$$</span>
from the fact the force is central (so <span class="math-container">$F$</span>... | In general, the angular momentum is defined as <span class="math-container">$\mathbf{L}=\mathbf{r}\wedge\mathbf{p}$</span>. In our problem, switching to index notation, we have, in the CM frame, <span class="math-container">$$L_{\mu\nu}=r_\mu p_\nu-r_\nu p_\mu=\mu(r_\mu\dot{r}_\nu-r_\nu\dot{r}_\mu)$$</span>
Now, since ... | Tesseract's answer is correct. More conceptually, in general dimensions the angular momentum is what is known as a "two-form", which you can think of a plane with a magnitude and orientation of in-plane rotation. This works in any dimension, and in any dimension the conservation of this plane means that motion under a ... | https://physics.stackexchange.com |
2,930 | [
"https://mechanics.stackexchange.com/questions/2930",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/1558/"
] | I'm considering the purchase of a 2001 VW Golf TDI "mechanic's special", so named because it apparently has a failed head gasket (white smoke). I'm not a mechanic but I'm fairly handy with tools and figure I could probably fix it.
My concern is that head gaskets are not the kinds of things that just fail for no reaso... | By far the most common cause of failure of the gasket is overheating.
So why would it overheat? Check for coolant leaks in the following
<ul>
<li>engine gaskets</li>
<li>water pump</li>
<li>radiator</li>
<li>pipes </li>
</ul>
Also check for oil leaks or low oil - these can have knock on overheating effects.
Check y... | Be aware that a diesel head gasket is not necessarily a job for an amateur. It is generally not as straight forward as a gasoline engine head gasket.
You may need special tools that many amateurs lack -- a torque wrench, for sure.
Be sure to consult a repair manual for your particular engine. Some VW diesel engines w... | https://mechanics.stackexchange.com |
32,691 | [
"https://cstheory.stackexchange.com/questions/32691",
"https://cstheory.stackexchange.com",
"https://cstheory.stackexchange.com/users/-1/"
] | I am not an expert in this field, but I have read that the existence of one-way functions implies $P \neq NP$. Since there seem to be so many different definitions of one-way-functions and I have not seen a proof I wanted to ask if someone knows where to read this, or knows the proof.
Please if you give a proof, also... | $P \ne NP$ if and only if worst-case one-way functions exist.
Reference:
Alan L. Selman. A survey of one-way functions in complexity theory. Mathematical systems theory, 25(3):203–221, 1992.
| Let $f:\{0,1\}^* \rightarrow \{0,1\}^*$ be a OWF and define $L_f = \{(x,y)\in \{0,1\}^* \times \{0,1\}^*:$ there exists $z$ such that $f(xz)=y\}$.
I construct the following non-deterministic Turing machine:
$A((x,y)):$"Non-deterministically guess $z$. return ACCEPT if $f(xz)=y$ and REJECT otherwise"
$A$ makes a single ... | https://cstheory.stackexchange.com |
992,534 | [
"https://math.stackexchange.com/questions/992534",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/181401/"
] | Let $X$, $Y$ two random variables such that $P(X \le a)=P(Y\le a) \quad\forall a \in \mathbb{R}$ (in other words, $X$ and $Y$ have the same distribution). Suppose $X \le Y$ almost surely. Can I conclude $X=Y$ almost surely?
| If $X$ and $Y$ have finite expectation, you can, simply because
$$
\Vert Y - X\Vert_{L^1} = \int Y - X \, dP = E(Y)-E(X)=0.
$$
Otherwise, apply the following <strong>"truncation trick"</strong>: Note that
$$
X_n = \min \{\max \{-n, X\} ,n \}
$$
has finite expectation for each $n$ (because it is bounded).
How doe... | Yes, you can.
Let $Z=Y-X$, then by assumption $P(Z<0)=0.$ By linearity of the expectation, $$E(Z)=E(Y)-E(X)=0,$$and since $Z$ is non-negative almost surely, it follows that $P(Z>0)=0$.
| https://math.stackexchange.com |
4,062,272 | [
"https://math.stackexchange.com/questions/4062272",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/838056/"
] | <blockquote>
So I am not sure if this is the correct way and answer to this problem but here is my solution
</blockquote>
<blockquote>
The problem:
<span class="math-container">\begin{equation*}
{\bigg( x,y,z \bigg)| x+y=0 \space and \space x-2z=0}
\end{equation*}</span>
Solution:
<span class="math-container">$$
x \ ... | Indeed your work is correct. Another way of looking at it would be geometrically. The set of points in question you're looking at is <span class="math-container">$$ \{(x,y,z): x+y=0 \text{ and } x-2z=0\}$$</span> can be thought of as the intersection of two planes. In this case since the planes are not parallel, the in... | Your solution is Correct!
In terms of matrix language, we have to solve <span class="math-container">$$AX=0$$</span> where <span class="math-container">$A=\begin{pmatrix} 1 &1&0\\1&0&-2 \end{pmatrix}$</span> , <span class="math-container">$X=(x,y,z)^T$</span>. So Treat <span class="math-container">$A$</... | https://math.stackexchange.com |
25,606 | [
"https://electronics.stackexchange.com/questions/25606",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/6007/"
] | Does anybody know the impedance of the Tektronix A622 current probe? I don't find it neither on the datasheet, nor in the user manual...
<strong>EDIT</strong>: As requested, here is some extra info: I need the <em>output</em> impedance of the probe, for proper impedance matching with the scope. And no, it's <em>not</e... | Like you say, the required output imedance doesn't seem to be mentioned in the manual (or both of us can't read), but they say you can hook it up to a multimeter with a BNC-to-Banana adapter. I am not aware of any multimeter with a 50 Ohm input impedance, so I guess the porbe's output is a voltage buffer designed ... | Most probes will list on the side of the probe or on the connector somewhere the impedance of it. I think common impedance is 1M or 10M, with around 15pF of capacitance.
| https://electronics.stackexchange.com |
27,807 | [
"https://electronics.stackexchange.com/questions/27807",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/5035/"
] | I am trying to design an ON-OFF Keying decoder for an RFID reader project I've been working on. Below is the code that runs on a PIC16F887 @20MHZ. The compiler is Mikroelektronika mikroC PRO for PIC.
What I was trying to do is to toggle <code>PORTC.1</code> every 200uS. The code runs as wished, but only for about 5.8 ... | If the compiler is ANSI C compliant then the two unsigned chars will be promoted to integers based on the Usual Arithmetic Conversions specified in the ANSI C standard. If data_time is greater than fiftymicros due to fiftymicros overflowing, then the result of fiftymicros - data_time will be a negative number.
With th... | I think it all boils down to:
<pre><code> if(fiftymicros-data_time>3)
{
data_time=fiftymicros;
PORTC.F1=~PORTC.F1;
}
</code></pre>
I don't know how that compiler handles the <code>>3</code>, but it may be performing a signed comparison. So, here's the scenario:
Assume <code>fiftymicros</code> just incr... | https://electronics.stackexchange.com |
50,548 | [
"https://mathoverflow.net/questions/50548",
"https://mathoverflow.net",
"https://mathoverflow.net/users/5229/"
] | An integer-valued polynomial is a polynomial with real coefficients mapping integers to integers. It is well known that all such polynomials $h(x)$ are generated as an additive group by the binomial coefficients $\binom{x}{n}$. My question concerns the problem of approximating an arbitrary polynomial $f$ with real coef... | <blockquote>
I would be grateful if you could spell out some details in an answer.
</blockquote>
OK, let's do the details.
Start with any continuous function $f$ on $[-0.9,0.9]$ that is $0$ in some open neighborhood of the origin. Let $g$ be its even part. Then $f-g$ is odd and also vanishes near the origin. Thus... | Drawing some consequences from Franz Lemmermeyer's comment above, i.e. that the integer-valued polynomials $f_n:=\frac 1 2(x+x^n)$ converge to $\frac x 2$ uniformly on closed sub-intervals of $]-1,1[$. Then, in the same convergence, we also have that the $k$-fold iteration $f_n\circ\dots\circ f_n$ converges to $x/2^k$... | https://mathoverflow.net |
1,536,517 | [
"https://math.stackexchange.com/questions/1536517",
"https://math.stackexchange.com",
"https://math.stackexchange.com/users/203149/"
] | I was asked to show that if I have a generalized circle C, and two points not in C, then every circle going through these two points is orthogonal to C iff these two points are symmetric. Since I haven't come by anything yet but still have a hard time understanding the concept: is orthogonality between a line and a cir... | As A.S.'s comment indicates, both distributions relate to the same kind of process (a Poisson process), but they govern different aspects: The Poisson distribution governs how many events happen in a given period of time, and the exponential distribution governs how much time elapses between consecutive events.
By way... | Exponential is a continuous distribution while poisson is a discrete one.
| https://math.stackexchange.com |
532,689 | [
"https://physics.stackexchange.com/questions/532689",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/236986/"
] | I am a undergraduate physics student of open university I am in fear that I may forget derivation and even I already forget some of derivation of oscillation and waves but while I was learning them I understood each step and I can apply these formulas to solve problem.My question is it necessary to know and remember ea... | It is not always necessary. There are a few important derivations which you should have by heart (EOM of a harmonic oscillator in 1D for example) but that will happen automatically with time and practice. The most important thing to remember is the physics behind the system you are studying.
One of my teachers in schoo... | You should <em>understand</em> the derivations. Once you understand you can easily recreate them when you need them. Memorising without understanding is a useless strategy.
| https://physics.stackexchange.com |
230,171 | [
"https://stats.stackexchange.com/questions/230171",
"https://stats.stackexchange.com",
"https://stats.stackexchange.com/users/127910/"
] | A simple question. I know in theory, it is possible to calculate standard deviation for two numbers. I am wondering if it is plausible to do that. My objective is to compare two arbitrary time series data for the same phenomenon and plot mean and standard deviation as error bars for every time point. I know that you co... | Compilation and expansion of comments:
Let's presume your data is Normally distributed.
If you want to form two-sided error bars (or confidence intervals), say at the 95% level, you will need to base that on the Student t distribution with n-1 degrees of freedom, where n is the number of data points. You propose to ... | Setting aside your initial explanation of the time-series context, it might be useful to look at this as a simple case of observing two data points. For any two observed values $x_1 , x_2$ the sample standard deviation is $s = |x_2 - x_1| / \sqrt2$. This statistic is exactly as informative as giving the sample range... | https://stats.stackexchange.com |
45,808 | [
"https://security.stackexchange.com/questions/45808",
"https://security.stackexchange.com",
"https://security.stackexchange.com/users/32204/"
] | SAP Netweaver has its passwords hashed and also salted. But I read that a tool such as John the Ripper can bruteforce them.
So how does security varies when passwords are hashed, in particular in the sap system?
what if I have a password such is this... a12345 ?
Is it easy to bruteforce it, even though it is , lets ... | <blockquote>
Is there anything that a malicious user could still do even with these restrictions in place?
</blockquote>
It depends on your definition of "malicious". Running a tight loop or otherwise consuming huge amounts of resources is arguably malicious. Serving up a fake PayPal front is malicious, but certainl... | They seem to have taken fairly reasonable precautions. You can still reflect over the assemblies loaded in the AppDomain, so if you had time to poke through them, you might (or might not) find something in their custom assemblies that was interesting and exploitable, but it looks to me like the model is essentially wh... | https://security.stackexchange.com |
129,659 | [
"https://softwareengineering.stackexchange.com/questions/129659",
"https://softwareengineering.stackexchange.com",
"https://softwareengineering.stackexchange.com/users/44956/"
] | When designing a class should consistency in behaviour be favoured over common programming practice? To give a specific example:
A common convention is this: If a class owns an object (e.g. it created it) it is responsible of cleaning it up once it's done. A specific example would be in .NET that if your class owns an... | I use this technique too, I call it 'handoff', and I believe it deserves the status of a pattern. When an object A accepts a disposable object B as a construction time parameter, it also accepts a boolean called 'handoff', (which defaults to false,) and if it is true, then disposal of A cascades to disposal of B.
I a... | I think you're right, to be honest. I think that Microsoft messed up with the StreamWriter class, specifically, for the reasons you describe.
However, I've since seen a lot of code where people don't even try to dispose of their own Stream because the framework will do it for them, so fixing it now will do more harm t... | https://softwareengineering.stackexchange.com |
16,268 | [
"https://engineering.stackexchange.com/questions/16268",
"https://engineering.stackexchange.com",
"https://engineering.stackexchange.com/users/2947/"
] | Imagine some thing put in the presence of loud pure white noise, say at a level that might come from a teenager's HI-FI set to annoying. And the thing is like a champagne glass, sheet metal box or other thin walled vessel. I'm not considering anything like a cannon ball or anvil.
I can foresee two scenarios:-
<ol>
<... | You won't get much resonance because the phase the thing is being driven with keeps changing.
The resonant thing will act like a notch filter, so you are left with the frequencies near its resonance. However, for resonant energy to build up, the system has to get pumped for a while. While a short segment of the filt... | The thing will vibrate at all frequencies. It will vibrate more (be amplified) at frequencies that are close to a natural frequency. Effectively, the system will act as a band pass filter (NOT a band notch filter)
I did a very similar experiment in graduate school many times. It wasn't a teenagers hi fi set or a wi... | https://engineering.stackexchange.com |
178,172 | [
"https://electronics.stackexchange.com/questions/178172",
"https://electronics.stackexchange.com",
"https://electronics.stackexchange.com/users/80056/"
] | I need help identifying this 360 degree rotary encoder on my Roland SC-880 rack synthesizer. This one has become 'jumpy' and Roland claims the part is discontinued. So my only choice is to replace it. (or figure a way to open and clean it?)
This encoder has 20 detents and a push switch to select different modes. It's ... | I would probably use a small amount of tacky flux and pre-tin both the wire and the pins.
If you can secure the motor in a clamp with pins facing horizontally, that would be ideal for me (an average solderer).
I would feed the wire with my non dominant hand while resting my palm on the bench top and soldering with ... | The pins on each motor seem to be all in a single plane; perhaps you could make a custom circuit board that fits those pins and functions as a breakout board.
| https://electronics.stackexchange.com |
629,933 | [
"https://physics.stackexchange.com/questions/629933",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/296430/"
] | As electric field remains the zero inside the conductor so the potential at the surface should be the same as inside, but i came with a situation which is as follows: if a spherical conductor is placed inside (concentrically) a conducting shell which has greater dimensions than that of the first conductor and a some... | <blockquote>
I'm pretty sure the second method is the right one but I wonder why the first one is not valid.
</blockquote>
Both methods are right, but you are calculating different things with them. The first method you are ignoring the air so you are calculating the gauge pressure. If you include the air in the first ... | In the first case you just calculate the additional pressure of the water, wich is the only interesting thing, since th additional air pressure was on the floor even without water. Usually you calculate the pressure from the water only. When you weight yourself on a scales you also do not add the air pressure.
| https://physics.stackexchange.com |
36,426 | [
"https://scicomp.stackexchange.com/questions/36426",
"https://scicomp.stackexchange.com",
"https://scicomp.stackexchange.com/users/24246/"
] | Fluid simulations solving the hydrodynamic (HD) or the magneto-hydrodynamic (MHD) equations are very useful in physics, the latter being particularly useful for modeling plasmas.
Of course these equations are highly nonlinear and solving them accurately is difficult. My first question is, in the general case what some ... | At their core, simulation of realistic fluid behavior is difficult because of the <em>length scales</em> involved. If you have turbulent flow, as is common in hydrodynamic or magneto-hydrodynamic situations, then you have vortices at length scales that are often vastly smaller than the size of the domain you're interes... | Besides what Wolfgang mentions, all of your example problems have the potential for shocks which require us to capture waves that may not be present in other flow problems, and these waves can can span wide length, time, and amplitude scales which require high resolution and challenge our best numerical schemes to even... | https://scicomp.stackexchange.com |
718,094 | [
"https://physics.stackexchange.com/questions/718094",
"https://physics.stackexchange.com",
"https://physics.stackexchange.com/users/333740/"
] | I was recently reading about one capacitor being used to charge up another capacitor. In this example, a capacitor is charged by a battery, and then the battery is removed. The charged capacitor is then connected to another capacitor and charge is transferred between them. Since the wire is also made of conducting mate... | In special relativity (ie ignoring any curvature of spacetime), we each have a plane of simultaneity, which is a 3d 'slice' through spacetime corresponding to 'now' where we are. If you and I are moving relative to each other, our respective planes of simultaneity will be tilted relative to each other. Even for very sm... | The idea of "now" is dependent on relative velocity (this is the "relativity of simultaneity") and so it's entirely coordinate dependent, and thus not physically meaningful -- we have no way of knowing what's happening "now" at a distant point.
In relativity there is an absolute notion of ... | https://physics.stackexchange.com |
54,793 | [
"https://chemistry.stackexchange.com/questions/54793",
"https://chemistry.stackexchange.com",
"https://chemistry.stackexchange.com/users/31808/"
] | I thought of cleaning my headphones, which has an outer aluminium casing, with Dettol liquid which I believe has strong oxidising agents. This was a bad idea. It left a tarnish on the aluminium and I'd like to know if I can undo this.
| I would try any sort of weak acid that wouldn't cause damage to other parts of the headphones.
Citric Acid (found in lemon juice), Acetic Acid (found in vinegar), and Phosphoric Acid (found in cola) are probably all safe bets for sticking on a cloth and wiping off the casing. The primary difficulty you have here is th... | My friend once asked me to clean their silver chain. I used H2O2 as a school chemistry book suggested and it all turned black. They brought it to a jewelry store and the chain was cleaned ... with H2O2/NH3. A friend of mine brought a mixture of H2O2 and NH3 from lab to home to clean some silverware. Premixed, in one bo... | https://chemistry.stackexchange.com |
22,945 | [
"https://mechanics.stackexchange.com/questions/22945",
"https://mechanics.stackexchange.com",
"https://mechanics.stackexchange.com/users/13482/"
] | While driving, I faced some problem in my bike. when <code>I raise the accelerator suddenly my bike's speed goes down</code>. Anyone knows what is the problem?. I'm using <code>yamaha ss 125 cc</code> bike.
| If this is a carburated engine (not fuel injection) then it's likely that one or more of your carburettor jets is blocked or the diaphragm has a hole in it. Find a service manual for this motorcycle (there's probably a PDF of it somewhere on the internet) and follow the instructions for taking the top off the carb, you... | Twisting the throttle opens the butterfly valve in the carburetor which allows it to suck in more air. With the increased air flow, the carb <em>should</em> also suck in more fuel. So, if enough fuel isn't supplied the engine can't handle the extra air. I don't know enough about your bike to offer a solution, but th... | https://mechanics.stackexchange.com |
9,990 | [
"https://astronomy.stackexchange.com/questions/9990",
"https://astronomy.stackexchange.com",
"https://astronomy.stackexchange.com/users/6640/"
] | The Sun's rays hit our eyes around 8 minutes after they are emitted from the Sun.
Does this mean that the Sun that we see is always the Sun as it was some 8 minutes before? I strongly think this must be happening; is it really a fact? Do we always see the Sun's past?
| <strong>Yes</strong>, you are right. We don't only see the Sun 8 minutes in the past, we actually see the past of everything in space. We even see our closest companion, the Moon, 1 second in the past.
The further an object is from us the longer its light takes to reach us since the speed of light is finite and distan... | The speed of light in a vacuum is 299,792,458 meters per second, not infinite. Let's say, for example, particle of a beam of light, the photon, is emitted. It takes ~8 minutes to get to us; when it hits our eyes, we see it. This means that we see a photon that was emitted from the sun 8 minutes ago. We aren't, per se, ... | https://astronomy.stackexchange.com |
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