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127

Crossing things out Use \require{cancel} in the first formula in your post that requires cancelling; you need it only once per page. Then use: $$\require{cancel}\begin{array}{rl} \verb|y+\cancel{x}| & y+\cancel{x}\\ \verb|\cancel{y+x}| & \cancel{y+x}\\ \verb|y+\bcancel{x}| & y+\bcancel{x}\\ \verb|y+\xcancel{x}| & y+\xcancel{x}\\ \verb|y+\...


115

Provide Context Context matters. A question can sometimes be answered in one sentence when the discussion is between two experts familiar with each other's background, while the same question may take many paragraphs of detailed computation when being shown to an undergraduate student. By providing a context you help the potential responders to your ...


112

Colors Named colors are browser-dependent; if a browser doesn't know a particular color name, it may render the text as black. The following colors are standard in HTML4 and CSS2 and should be interpreted the same by most browsers: $$\begin{array}{|rc|} \hline \verb+\color{black}{text}+ & \color{black}{text} \\ \verb+\color{gray}{text}+ & \color{...


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Additional decorations $\def\demo#1#2{#1{#2}\ #1{#2#2}\ #1{#2#2#2}}$ \overline: $\demo\overline A$ \underline: $\demo\underline B$ \widetilde: $\demo\widetilde C$ \widehat: $\demo\widehat D$ \fbox: $\demo\fbox {$E$}$ \underleftarrow: $\demo\underleftarrow{F}\qquad$ variant: \xleftarrow{}: $\xleftarrow{abc}$ \underrightarrow: $\demo\underrightarrow{...


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Commutative diagrams AMScd diagrams must start with a "require": $\require{AMScd}$ \begin{CD} A @>a>> B\\ @V b V V= @VV c V\\ C @>>d> D \end{CD} to get this diagram: $\require{AMScd}$ \begin{CD} A @>a>> B\\ @V b V V= @VV c V\\ C @>>d> D \end{CD} @>>> is used for arrow right @<<< is used for arrow ...


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The reputation for an upvote on a question has been changed from +5 to +10, as announced in this blog post: We’re Rewarding the Question Askers. (So now reputation gained from an upvote on a question is the same as the reputation gained from an upvote on an answer.) You can check in your reputation tab or in the reputation breakdown that question upvotes ...


90

Tags & References For longer calculations (or referring to other post's results) it is convenient to use the tagging/labelling/referencing system. To tag an equation use \tag{yourtag}, and if you want to refer to that tag later on, add \label{somelabel} right after the \tag. It is not necessary that yourtag and somelabel are the same, but it usually is ...


90

Using \newcommand I would like to remark that it is possible to define LaTeX commands as you do in your TeX files. I felt so happy when I first discovered it! It's enough to insert something like $ \newcommand{\SES}[3]{ 0 \to #1 \to #2 \to #3 \to 0 } $ $ \newcommand{\SES}[3]{ 0 \to #1 \to #2 \to #3 \to 0 }$ at the top of your post (remember the dollars!). ...


86

I am the user who copied your answer Manx. My original post did not answer the OP’s question, so I edited it to yours. I apologise for copying your answer without mentioning you. I have edited my answer to give credit to you for the original post, and I have recently visited this page for advice on how, and when to edit an incorrect answer. In retrospect, ...


84

\implies ($\implies$) is a marginally preferable alternative to \Rightarrow ($\Rightarrow$) for implication. There's also \iff $\iff$ and \impliedby $\impliedby$. \to ($\to$) is preferable to \rightarrow or \longrightarrow for things like $f\colon A \to B$. The reverse is \gets ($\gets$).


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Big braces Use \left and \right to make braces - (round), [square] and {curly} - scale up to be the size of their arguments. Thus $$ f\left( \left[ \frac{ 1+\left\{x,y\right\} }{ \left( \frac{x}{y}+\frac{y}{x} \right) \left(u+1\right) }+a \right]^{3/2} \right) $$ renders as $$ f\left(\left[ \frac{...


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Limits To make a limit (like $\lim \limits_{x \to 1} \frac{x^2-1}{x-1}$), use this syntax: First, start off with $\lim. This renders as $\lim$. The backslash is there to prevent things like $lim$, where the letters are slanted. Second, add \limits_{x \to 1} inside. The code now looks like $\lim \limits_{x \to 1}$, and renders as $\lim \limits_{x \to 1}$. ...


63

A good title The title of a question is the first thing people see. Like headings in newspapers, book, song and album titles, their importance is not to be underestimated -- the presence of a good, descriptive title for your question often greatly improves the exposure (and hence the amount and quality of answers) it gets. To ensure maximal descriptiveness ...


62

Regardless of the practical consequences (or lack thereof) of your actions, there's one part of your complaint that I agree with: I find this a bit offensive, to be honest. It is! It's an unnecessarily sanctimonious phrasing in a message from a system that is known to have false positives on a semi-regular basis. The context in which this message is ...


61

Arbitrary operators If an operator is not available as a built-in command, use \operatorname{…}. So for things like $$\operatorname{arsinh}(x)$$ write \operatorname{arsinh}(x) since \arsinh(x) will give an error and arsinh(x) has wrong font and spacing: $arsinh(x)$. This was already mentioned in a comment by Charles Staats. You might consider this an ...


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Highlighting equation To highlight an equation, \bbox can be used. E.g, $$ \bbox[yellow] { e^x=\lim_{n\to\infty} \left( 1+\frac{x}{n} \right)^n \qquad (1) } $$ produces $$ \bbox[yellow] { e^x=\lim_{n\to\infty} \left( 1+\frac{x}{n} \right)^n \qquad (1) } $$ By default, the bounding box is "tight", so it doesn't extend beyond the characters used in the ...


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Avoid "no clue" questions Too many questions begin or end with "I don't even know how to begin with this problem". While this may be true (you may genuinely have no idea how to approach the problem), it is still not a valid reason to limit your post to the statement of the problem without any mention of your own thoughts. Such questions will most of the ...


51

Absolute values and norms The absolute value of some expression can be denoted as \lvert x\rvert or, more generally, as \left\lvert … \right\rvert. It renders as $\lvert x\rvert$. The norm of a vector (or similar) can be denoted as \lVert v\rVert or, more generally, as \left\lVert … \right\rVert. It renders as $\lVert v\rVert$. (You may also write \left\|…\...


50

Giving reasons on each line of a sequence of equations To produce this: \begin{align} v + w & = 0 &&\text{Given} \tag 1\\ -w & = -w + 0 && \text{additive identity} \tag 2\\ -w + 0 & = -w + (v + w) && \text{equations $(1)$ and $(2)$} \end{align} write this: \begin{align} v + w & = 0 &&\text{...


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Most cases of cereal downvoting are crunched by our automatic script that milks them away within about 24 hours. If someone is particularly persistent in breaking their voting fast in this manner, drop us a line via the "contact us" link at the bottom of any page and we can investigate further.


49

This was an automatic edit by the Community user as part of the recent network-wide change to https. In reality, an SE developer was responsible behind the scenes for this edit (and many many others in the same time period). The proper course of action is to simply ignore this edit, and all others like it: they were made for a reason.


47

Pack of cards If you are asking (or answering) a combinatorics question involving packs of cards you can make it look more elegant by using \spadesuit, \heartsuit, \diamondsuit, \clubsuit in math mode: $$\spadesuit\quad\heartsuit\quad\diamondsuit\quad\clubsuit$$ Or if you're really fussy: \color{red}{\heartsuit} and \color{red}{\diamondsuit} $$\color{red}{\...


46

While there was some doubt whether or not Revision 3 of this answer, directly copied gnometorule's answer, there were several other offending examples where the answer was copied verbatim, confirming amWhy's plagiarism. Notably Revision 1 of this answer directly copies Do complex numbers really exist? Revision 2 of this answer directly copies What is the ...


44

Mathematical typesetting using MathJax Mathematics.SE uses the emulation engine MathJax for providing $\TeX$-like mathematical typesetting. This means that you can use mathematical notation in your questions in a visually appealing way. How can I use MathJax? Basic information: This gives a quick start for people familiar with the $\TeX$-family of markup ...


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Tagging Tags are a way to help us organize posts on this website. People also use them to locate the questions they will find the most interesting. Thus, good tagging helps to attract the best potential answerers to your question. How do you select the best tags for your question Tags are about content. Tags (except "meta" tags, see below) are supposed to ...


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Why do we have a policy? First and foremost: we believe that the responsibility for the integrity of an exam, contest, competition, etc. ultimately falls on the shoulders of the organizers. That said, the Mathematics Stack Exchange community is not an island unto itself: rather we exist within a larger macrocosm of people who do or are interested by ...


41

This, I think, is the move to https links, where all the links which point to SE were silently edited in the background from http to https. For example, https://math.stackexchange.com/posts/29797/revisions, which points out to the date in question being April 13, 2017.


40

Left and Right Implication Arrows Another way to display the arrows for right and left implication instead of using $\Rightarrow$, $\Leftarrow$ and $\Leftrightarrow$ which produces $\Rightarrow$, $\Leftarrow$ and $\Leftrightarrow$ respectively, you can use $\implies$ for $\implies$, $\impliedby$ for $\impliedby$ and $\iff$ for $\iff$ The latter of which ...


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Formatting and writing This is not some random internet forum. We strive for well-composed questions and answers of lasting value -- so keep in mind that your question can be of interest to others as well. Posing a well-formatted question Use proper English to the best of your ability The use of proper spelling, grammar and punctuation makes your ...


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