math-modestacking-symbols
I was looking for a way to center text underneath math symbols such as constraints or regions. More specifically, from vector calculus, if we were to have a double integral bounded by some region D, then we would have the two integral signs with a D underneath. It looks like the D_1 and D_2 as seen below.
The flexisym package allows arbitrary commands to be inserted into math character definitions. It is usually used in conjunction with breqn for which it was written but here I use it directly. the definition of binary and relation functions are a bit opaque but just copied directly from the existing definition with the addition of \allowbreak as in your example, but applied to all characters of those classes (as long as they are defined via the standard LaTeX declarations).
\documentclass{article}
\usepackage{flexisym}
\begingroup
\catcode`\_11
\catcode`\:11
\gdef\math_bsym_Bin:Nn#1#2{%
\allowbreak\math_char:NNn 2#1{#2}\allowbreak}
\gdef\math_bsym_Rel:Nn#1#2{%
\allowbreak\math_char:NNn 3#1{#2}\allowbreak}
\endgroup
\begin{document}
text text text text text text text text text text text text \(x + x + x + x + x + x + x + x + x\)
i text text text text text text text text text text text text \(x + x + x + x + x + x + x + x + x\)
text text text text text text text text text text text text \(x + x + x + x + x + x + x + x + x\)
text text text text text text text text text text text text \(x = x = x = x = x = x = x = x = x\)
text text text text text text text text text text text text \(x = x = x = x = x = x = x = x = x\)
\end{document}
Use \limits:
$\bigcup\limits_{i=0}^{n}$
The above ^ and below _ parts may be empty, of course. This works also for \prod and \sum, for example (for all math operators to be exact).
But as mentioned already in the comments: It's not always good to force some output, because the line height could get to big. You have to try with your document because we don't know your settings (font, font size, line height, etc).
Best Answer
I cannot see this: