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I have noticed from different published articles that mathematical equations may or may not be ended by a comma or a period (depending on their position in the text). Which is the best practice?

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    Looks like a web search could have solved this. – Federico Poloni Sep 16 '17 at 12:59
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    Very, very relevant – Wojowu Sep 16 '17 at 15:11
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    @FedericoPoloni, and as far as I understand one of the goals of SE is that the primary result of a web search will lead to a SE Q&A that addresses the query in question. – Lamar Latrell Sep 16 '17 at 17:44
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    @LamarLatrell That was not my understanding; I thought that if a web search leads to, say, a Wikipedia page or some trustworthy reference that directly answers the question, there is no need to duplicate that information with an SE question. – David Z Sep 16 '17 at 18:33
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    @DavidZ, It's possible I have incorrect information on the matter, but my first thoughts were what if someone edits that Wiki or the webpage dies? (the same reasons why link only answers are frowned upon). Also, it may not affect you but my recent experience of living and working in a country with internet censorship has highlighted how helpful it has been for information to have levels of redundancy. – Lamar Latrell Sep 16 '17 at 19:27
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In-line formulas are punctuated as required by the grammar. For displayed formulas there are two conventions: punctuate as required by the grammar; or no punctuation. See if the journal specifies this in their style (most don't). Example

jahnke_and_emde_-_dover_publications

[Jahnke & Emde, Dover Publications]

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    I wouldn't take a German text as an example to back up your claim: different languages may have different conventions here, and German, in particular, notoriously has different rules than English for comma placement. – Federico Poloni Sep 17 '17 at 7:24
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    @FedericoPoloni You didn't notice that the text was bilingual? – Mr Lister Sep 17 '17 at 8:02
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    @MrLister No, I didn't, but that makes it even weirder as an example. Mathematical conventions do not translate well among languages (case in point: that decimal comma). It's not like there is a shortage of mathematical texts in English that OP could have used instead... – Federico Poloni Sep 17 '17 at 8:03
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    @FedericoPoloni No, it doesn't make it weirder; on the contrary, it shows that formulas are simply part of the sentence structure, no matter the language. – Mr Lister Sep 17 '17 at 12:24
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    @MrLister No, it shows that they were treated that way in this example of a bilingual text. There's no reason to assume the convention followed in English is the same as in German, and if they're not then the fact that this uses "," as a decimal separator suggests German conventions are being preferred over English. This is how I've always treated them, but nevertheless I agree with Federico that this is a very poor choice of example to illustrate that. – Chris H Sep 18 '17 at 7:01
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The practice I'm familiar with is that equations are part of the text, and end with a period if they are the end of the sentence and with a comma or other punctuation mark if the sentence continues and calls for it.

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  • Yes, that's a common approach. In other words, think of offset formulas as if they were part of the regular sentence structure. You will then know how to terminate them: with a period, if they end a sentence; and a comma, a semicolon, or nothing if the sentence continues in some way. – Wolfgang Bangerth Sep 16 '17 at 19:02
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    There shouldn't be a comma just because the sentence continues; there should only be a comma if one is required in that position in the sentence. – David Richerby Sep 16 '17 at 19:58
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    And, in some cases, there could be other punctuation ... colon, question mark, etc. Whatever is required by the grammar. – GEdgar Sep 17 '17 at 0:23
  • @GEdgar : I agree, if $3 = 3$! – Eric Towers Sep 17 '17 at 2:18
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The best practice is to follow the style guidelines of the publication you are writing for. An editor will make sure that your manuscript fits within the style guidelines of the publisher. If there are no specific guidelines for your case or you aren't writing for a specific audience, do whatever seems right to you. Just make sure that your adopted style is clear and consistent, and be ready to defend it if necessary.

Personally, I think that punctuation can be unsightly and confusing in displayed math, but is usually helpful for clarity of inline math. This is one of my few points of disagreement with Chicago, whose chapter 12 advocates punctuating all displayed and inline math as integral to its containing sentence.

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  • Note conventions differ. In pure math, punctuation is standard, whereas in (at least in some parts of) engineering, no punctuation is standard. So for a pure mathematician, it looks weird to see it without when the equation ends the sentence. – Kimball Sep 17 '17 at 20:58
  • @Kimball I don't think that the standard in engineering is no punctuation. I just think engineers is not as familiar with math standards and it gets overlooked by many. – Ian Sep 18 '17 at 6:26
  • @Ian Maybe I misunderstood and it's just there's no standard. I was on an engineering student's PhD committee (in radar) and I pointed out that there was no punctuation after equations in the thesis, but the engineering profs on the committee said that's normal for them. – Kimball Sep 18 '17 at 7:21
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Yes it is good practice. You should read the papers of people are well known to be excellent expositors like Serre and his students Atiyah and Grothendieck (who also had quite a few students). They all endorse commas and periods. Perhaps the best guide to mathematical writing is https://www.youtube.com/watch?v=ECQyFzzBHlo (Serre). Let me emphasize how authoritative Serre's opinion is: he is easily the most influential mathematician of the postwar period.

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