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For example, if I want to write a closure of a set, which is denoted A_1, should I use $\overline{A_1}$ or $\overline A_1$ (or something completely different)?

For me, the more logical choice seems to be $\overline{A_1}$ (since I read \overline command as "closure of"). But maybe from typographical point of view this can be considered differently.

Here is a MathB.in link for comparison. (I am aware that rendering in MathJax and LaTeX is not necessarily the same, but I do not know what is standard way to make images with symbols for TeX.SE posts.)

Here is also a picture which I tried to create from pdf output using the method described in this answer on meta.

$\overline{A_1}$ vs. $\overline A_1$

I have seen that there exists a question \overline and \widehat with subscript. But that question asks about subscript added to the bar symbol, not subscript added to the symbol(s) which are under the bar.

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Certainly something like $\overline{A_1}$, because it is closure of $A_1$. But I understand, that the rule may look a bit too long for you. Indeed, there is no italic correction for math font. If you prefer a bit shorter version, see, e.g. answers to Appearance issues with \bar{} and \underline{}.

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You could think of ${\overline A}_1$ as being defined as $\overline{A_1}$ in which case you have the shorter line and can just write \overline A_1.

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You can set up things so that you can decide at the last moment what style you prefer.

\documentclass{article} \usepackage{amsmath,amssymb} \DeclareFontFamily{U}{mathx}{} \DeclareFontShape{U}{mathx}{m}{n}{ <-> mathx10 }{} \DeclareFontSubstitution{U}{mathx}{m}{n} \DeclareSymbolFont{mathx}{U}{mathx}{m}{n} \DeclareMathAccent{\widebar}{\mathalpha}{mathx}{"73} % first version (subscript under bar) \NewDocumentCommand{\closureA}{me{_}}{% \widebar{#1\IfValueT{#2}{_{#2}}}% } % second version (subscript outside bar) \NewDocumentCommand{\closureB}{m}{% \widebar{#1}% } \begin{document} First version: $\closureA{A}\ne\closureA{A}_{1}$ \medskip Second version: $\closureB{A}\ne\closureB{A}_{1}$ \end{document}

output

Of course in your document you'll use \closure deciding upon one style, but keeping at hand the alternative definition, so you can change it at will even at the very last moment, because the callouts are the same.

\documentclass{article} \usepackage{amsmath,amssymb} % see https://tex.stackexchange.com/a/391193/4427 \DeclareFontFamily{U}{mathx}{} \DeclareFontShape{U}{mathx}{m}{n}{ <-> mathx10 }{} \DeclareFontSubstitution{U}{mathx}{m}{n} \DeclareSymbolFont{mathx}{U}{mathx}{m}{n} \DeclareMathAccent{\widebar}{\mathalpha}{mathx}{"73} % first version (subscript under bar) \NewDocumentCommand{\closure}{me{_}}{% \widebar{#1\IfValueT{#2}{_{#2}}}% } % second version (subscript outside bar) %\NewDocumentCommand{\closure}{m}{% % \widebar{#1}% %} \begin{document} $\closure{A}\ne\closure{A}_{1}$ \end{document}
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