38

If I write

If $x \rightarrow y$, then $y \rightarrow z$.

it looks fine. But if the sentence is more involved and formulas longer, I tend to change $...$ to \[...\]. In many cases I think the result is ok, especially if there's only one equation in the end of the sentence.

However, in the above example it would be like:

If

     x→y

, then

     y→z

.

The comma and period doesn't look very nice.

Sure, I could break it up and say If formula (1) holds, then formula (2) holds, but this is usually an overkill in my opinion.

TL;DR: What do you do with periods after sentences ending with a \[...\]-equation?

lockstep
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aioobe
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    The question as to whether you should do this or not is ... contentious. Take a look at http://tex.stackexchange.com/questions/11821/punctuation-in-equations and follow the links. – Andrew Stacey Oct 27 '11 at 08:54

2 Answers2

45

You insert the period as part of the equation and therefore also in math mode, as opposed to putting it outside the equation.

enter image description here

\documentclass{article}
\begin{document}
If \[x\rightarrow y,\] then \[y\rightarrow z.\]
\end{document}

Naturally, this may lead to a slightly off-center spacing due to the punctuation (albeit marginally visible). However, to correct for this, you can use \phantom{<punct>} on the opposite side:

\documentclass{article}
\begin{document}
If \[\phantom{,}x\rightarrow y,\] then \[\phantom{.}y\rightarrow z.\]
\end{document}
Werner
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    Ok, yea, that's obviously an option. You know for sure that this common practice? – aioobe Oct 27 '11 at 08:00
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    Since equations (or mathematics in general, whether in-line \textstyle or in \displaystyle) form part of the textual flow, it is my opinion that they can include punctuation. If you want to correct for the possible centered misalignment, see my updated answer. – Werner Oct 27 '11 at 08:09
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    Nice update :-) – aioobe Oct 27 '11 at 08:14
  • Would it not be easier to make use of mathtools's \mathrlap command to make the punctuations left-aligned with no width, keeping the centering? – sodd Aug 11 '15 at 22:59
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    @nordev: Sure, or a regular \rlap as there is no difference in your use of punctuation in the equation compared to a regular text-based period .. Hopefully your equation is not just oversized so the \rlap stretches into the margin. – Werner Aug 11 '15 at 23:24
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    Why not use \text{.} (or maybe \text. if you want to keep it short)? Depending on your setup (an extreme case being using OpenDyslexic as the main text font, for example), this might lead to much more consistent punctuation between normal sentences and “sentences that end with math stuff”. – Alice M. Oct 10 '17 at 16:44
  • TeX is such a hack... – André Caldas Dec 25 '20 at 14:58
21

Punctuation in displayed formulas should stay within the formula:

If
\[
x\to y,
\]
then
\[
y\to z.
\]

Some people leave a space before the punctuation (I don't); others completely suppress punctuation in displayed formulas, on the basis that the display is doing the work and the reader can infer commas or periods from the context.

Which style to use is a question of personal taste; the most important thing is to be coherent along the document.

Mew
  • 843
egreg
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  • +1, nice answer. I like the "reader can infer commas" part :-) (seems slightly nastier to let the reader infer periods though) – aioobe Oct 27 '11 at 08:06
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    @aioobe That's why I always use punctuation also in displayed formulas; in rare cases this needs rephrasing in order to avoid ambiguities in formulas, but I prefer to be consistent. In the "no punctuation" style, the presence of a period can be inferred by a following capital letter. – egreg Oct 27 '11 at 08:09