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What is the proper way of punctuating block formulae? There seem to be variance from “a mathematical text is, before everything else, a text” (qtd. via MathOverflow), to confusion whether parenthetically numbered equations follow the same rules, to publications like my current American physics textbook which uses none.

Personally, I used to punctuate my formulae, just like I separate statements in my algebra and my programming with semi-colons.

Another thing is that in practical application within LaTeX environment, some insist that punctuation, as part of the text, should be styled like the rest of the text, and be outside, whereas formulae have their own styling. But on Physics.SE MathJAX pushes outside punctuation into the next paragraph (it parses formula as a separate <div> breaking up the flow of text regardless of carriage returns?).

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The rule I was taught was that equations are just part of the text and that punctuation should never stand on its own.

So, if a block formatted equation comes immediately before punctuation you put the punctuation at the end of the formula. That means when I end a sentence with the quadratic equation I write $$ x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} .$$ That won't render on meta, but you can see how it looks in the Physics.SE FAQ. Notice that the full stop occurs inside equation markup delimiters ($$).

For a little OCD bonus you could use $$ x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \text{.}$$, but I don't generally bother

To avoid confusion this necessitates not using periods as mathematical operators and being careful about where how you use commas in formulae.

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  • $\begingroup$ In general I do the OCD \text{.} thing! But I refrain from doing it here, lest anyone editing my posts gets angry with me... Also, I wholeheartedly agree that equations are punctuated, but there are plenty of other formatting issues to be found on the internet that nag me more than this one. $\endgroup$ – user10851 Dec 12 '12 at 1:41
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    $\begingroup$ I usually put a \quad in front of punctuation (This results in the equation $$ a + b = c \quad .$$) to avoid confusion with operators. \mathrm{} is much more important when it comes to the d in d/dt. $\endgroup$ – Claudius Dec 12 '12 at 5:46
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There isn't a single proper way to do it - at least, for this site, there is no enforced or recommended convention. The only thing I would suggest is that you not put punctuation after a display-style MathJax formula, like $$x = y$$., because that spoils the flow of the text. But whether you include punctuation, $$x = y.$$, or don't, $$x = y$$, is up to you.

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