In Peskin on page 514, when deriving the Faddeev-Poppov ghosts, they arrive at the full Lagrangian for Yang-Mills: $$ \mathcal{L} = -\frac{1}{4}F^2 + \frac{1}{2\xi} (\partial \cdot A^a)^2+\textrm{Fermion Term + Ghosts Term} $$ I'm wondering, what's the correspondence (i.e., what's the way to derive) between the value of $\xi$ and the gauge condition on $A$? For example, Landau gauge is $\xi\to 0$. What kind of gauge-condition equation does that produce on $A$ and how can we derive it?  Compare with this page on Wikipedia:  which says that Landau gauge is when $A$ is only along the $y$-axis and has component there equal to $B\,x$ where $B$ is a constant and $x$ is the coordinate.
One of the most common gauges in QED computations are the $R_{\xi}$ gauges obtained by adding a term  \begin{equation} -\frac{(\partial_\mu A^{\mu})^2}{2\xi} \end{equation} to the Lagrangian. Different choices of $\xi$ correspond to different gauges ($\xi=0$ is Landau, $\xi=1$ is Feynman etc.) The propagator for the gauge field is different depending on the choice of gauge. The choice of Landau gauge forces $\partial_\mu A^\mu=0$, but I have never seen a similiar statement for the other gauges. I would like to know what constraint on the gauge field is produced by the other covariant gauges. For instance, what is the constraint on $A_\mu$ when $\xi=1,2,3,...$ etc. Is it still $\partial_\mu A^\mu=0$ or something different (it seems like it should be different)?
Tikzcd conflict with babel and cd I have this problem: when I try to compile a diagram I get the following message:     I think the culprit is a tikzcd arrow in ell 1-1   But I have not problem when, in the tikzlibrary, I just use cd, matrix and backgrounds. The problem is when using babel and cd at the same time.  \documentclass{article} \usepackage{amsfonts,amssymb,amsmath,tikz} \usepackage[spanish]{babel} \usepackage[latin1]{inputenc} \usepackage[mathcal]{euscript}  \usetikzlibrary{babel,cd,matrix,backgrounds}  \begin{document} \begin{center}   \begin{tikzcd}     X\rar[mapsto]{T}&amp;Y   \end{tikzcd} \end{center}  \end{document} Javascript multiple replace What's your preferred way of getting current system status (current CPU, RAM, free disk space, etc.) in Python? Bonus points for *nix and Windows platforms.  There seems to be a few possible ways of extracting that from my search:   Using a library such as  (that currently seems not actively developed and not supported on multiple platform) or something like  (again no activity since 2007 it seems and no support for Windows). Using platform specific code such as using a os.popen("ps") or similar for the *nix systems and MEMORYSTATUS in ctypes.windll.kernel32 (see ) for the Windows platform. One could put a Python class together with all those code snippets.   It's not that those methods are bad but is there already a well-supported, multi-platform way of doing the same thing? Rep line graph: Say "only 200+ accounts displayed" at all times As discovered/discussed , the cross-site reputation graph doesn't show accounts that have less than 200 reputation. The problem is that, unless you have zero accounts over that bar, it doesn't tell you that. I think the "Accounts must have a minimum of 200 reputation to appear in this graph" notice should appear either (A) at all times or (B) if you have any accounts below 200. What decides what to prepend the duplicate question list with on a duplicate question? On duplicate questions, I often see different headings for the duplicate question list. These include:     Possible Duplicate:    [duplicate question list]   and      This question already has an answer here:    [duplicate question list]   and more recently,     This question is an exact duplicate of:    [duplicate question list]   What decides which heading is placed before the duplicate question list? Can't login with Stack Exchange OpenID when HTTPS-Everywhere plugin is active - apparent protocol mismatch I can't seem to log in with my Stack Exchange OpenID. When I try, I get the error message:     Unable to log in with your OpenID provider:      The openid.return_to parameter (.) does not match the actual URL (.) the request was made with.   It works if I disable the HTTPS-Everywhere plugin in Firefox, but if I then re-enable the plugin and go to a site on which I am not logged in (in which case login is normally nearly seamless), it fails again with the same error message, or at least something very similar.  I'm not aware of having done anything that might have caused this, although I will admit I don't recall seeing https:// in the address bar on Stack Exchange before (I do now, and that's a good thing IMO!). I was using the site  and it worked fine then, but not now... A single row background colour I've tried latexdef which does not handle --- as the operand. And \texemdash is defined as a character. So that leaves me once again leaning on the collective wisdom of the group :) Note: the eventual outcome of this discussion involved dropping this particular off-topic reason, along with other changes. See:  &nbsp; &mdash;Shog     I was using the reason "demonstrate a minimal understanding" myself quite frequently, but the more I review, the more I have doubts about it and I think this reason can either be dropped entirely or at least rephrased, so I started to use "problems with code you've written must describe the specific problem" instead:     Questions asking for code must demonstrate a minimal understanding of the problem being solved.   So the poster has posted some description, maybe even a snippet of code and in his mind that already proves that he has that minimal understanding required. After all he described the problem and what he tried to solve. So telling him that he lacks that understanding is rather rude, and from the posters point of view doesn't even apply. What both version of the reason not emphasize is that a description of a problem is not various versions of     it doesn't work   but that we rather mean really specific details (see below). I can often see in the reopen review queue that the "improved" version just includes a more verbatim description, while what usually would be expected is some specific error message, a trace or a log, or whatever applies.  Then it continues with:     Include attempted solutions, why they didn't work, and the expected results. See also: Stack Overflow question checklist   comparing it to:     problems with code you've written must describe the specific problem — and include valid code to reproduce it — in the question itself. See SSCCE.org for guidance.   Which says essentially the same, only in more detail.   So the "minimum understanding" reason is IMO a more rude version of the reason telling the user that an SSCCE is missing.  The intent of giving this reason is to tell the user that he needs to give exact details to be able to help him, so IMO a checklist would be much better. Something like:     Your question was closed because it may not provide enough details. Things you can do to improve your question to get an answer include:         Format the code in a way that makes it readable and easily understandable.   Provide the smallest working sample of code demonstrating the problem (link to SSCCE)   Include your input and output.   If applicable provide logs (i.e. Android debugging log, stack trace, etc.).   If applicable provide the exact error messages (compiler, etc.).      And something like this should be best included already in the banner that appears when a question is closed instead of being hidden in some pages you often enough have to actively looking for.  It might be helpful to adjust this, based on the tags or close reason, as I realize that I'm more focused on the programming area, and the above list may not apply to other areas of SO.  Update  Another thing I forgot to mention (but wanted to) is this:     Questions asking for code   Is IMO essentially an invitation of the type of questions     I have a problem, can you provide the solution? (with an optional please)   while     Questions concerning problems with code you've written   puts at least some emphasis on the fact that the poster has to put some effort himself in trying to solve the problem by providing the code which he used to attempt the solution. Notation: Why write the differential first? From reading answers here, I've noticed that some people write integrals as $\int dx \; f(x)$, while other people write them as $\int f(x)\;dx$.  I realize that there is no mathematical difference between the two notation forms, but was wondering why some people choose the first method over the second.  Is there some place in higher maths that it becomes beneficial to write the differential first?  (I, personally, have always used the second method, just because I was taught that way...) Regression for an outcome (ratio or fraction) between 0 and 1 I am thinking of building a model predicting a ratio $a/b$, where $a \le b$ and $a &gt; 0$ and $b &gt; 0$. So, the ratio would be between $0$ and $1$.  I could use linear regression, although it doesn't naturally limit to 0..1.  I have no reason to believe the relationship is linear, but of course it is often used anyway, as a simple first model.  I could use a logistic regression, although it is normally used to predict the probability of a two-state outcome, not to predict a continuous value from the range 0..1.  Knowing nothing more, would you use linear regression, logistic regression, or hidden option c? Given $\frac{a}{c} + \frac{b}{d} = 1$ Prove that $$\lim_{(x,y)\rightarrow(0,0)} \frac{|x|^{a}|y|^{b}}{|x|^{c} + |y|^{d}}$$ does not exist.  So I have done the proof for strict inequalities. And the limit only exists when the fractions add up to an integer greater than $1$. I'm not sure how to approach this other than counter examples. C non-blocking keyboard input I'm trying to write a program in C (on Linux) that loops until the user presses a key, but shouldn't require a keypress to continue each loop.  Is there a simple way to do this? I figure I could possibly do it with select() but that seems like a lot of work.  Alternatively, is there a way to catch a ctrl-c keypress to do cleanup before the program closes instead of non-blocking io? Could anyone give me an explanation of the meaning of "get one's finger in the air" and the usage of it? I have Google Chrome Version 70.0.3538.77 (Official Build) (64-bit).  Today, after running sudo apt update and sudo apt full-upgrade, I saw:  E: Repository 'http://dl.google.com/linux/chrome/deb stable Release' changed its 'Origin' value from 'Google, Inc.' to 'Google LLC' N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.   I looked at the man page for apt-secure but it's pretty technical and difficult, at least for me, to follow.  So how do I fix this issue?    Edit: for now, I've removed the existing Google Chrome ppa from Software Sources and then ran sudo apt update and sudo apt full-upgrade successfully with the following being installed/upgraded successfully:  The following NEW packages will be installed:   python3-netifaces The following packages will be upgraded:   netplan.io nplan ppp 3 upgraded, 1 newly installed, 0 to remove and 0 not upgraded. Java String array: is there a size of method? I want to know the general formula for $\sum_{n=0}^{m}nr^n$ for some constant r and how it is derived.  For example, when r = 2, the formula is given by: $\sum_{n=0}^{m}n2^n = 2(m2^m - 2^m +1)$ according to   Thanks! Can sudo be reinstalled after being removed? Identifying a thin, black, worm-like creature with a hammer-shaped head I came across the creature shown in the photo below yesterday evening while walking the dogs. At first, I thought it was an earthworm, but a flashlight revealed it to be something I had never seen before.    It is approximately 12 inches (300 mm) in length and about 1/3 of an inch (10 mm) in diameter at its maximum. It is pitch black and very shiny—like piano black. It moves very slowly, using a snake-like motion. It had a very strange head, like a hammerhead shark.  I live in Durban, South Africa, if that helps to narrow down the possibilities.  Can anyone help in identifying this creature? Hard disk file system is shown as Raw file system; how can I access it?