There may be answers in this freely downloadable publication of the AMS: <https://www.ams.org/publications/authors/mit-2.pdf>, "Mathematics Into Type" (updated edition). Dan Andy Latto wrote: ----- ... e doesn't have to show up unless you want to figure out what the constant you need to multiply by is. As an aside, how do you deal with sentences like the immediately proceeding one, which start with a mathematical expression, which means they can't start with a capital letter, which looks funny to my eye (even if typeset, so that the e is in math-italics). I usually rephrase; in this case, I might have said "The constant e" or "Euler's e" instead of just "e" to start the sentence. Is there some "style guide for mathematicians", and what does it say about starting a sentence with a formula? -----