\mid within \left and \right delimiters is expected to be automatically scaled, but today I noticed such behavior:
L = \sup \left\{ \sum_{x \in F} a(x) \mid F \subset X,\, |F| < \infty \right\},
I suppose the optimal middle bar should be somewhat taller. (Or is this just an illusion?) Any ideas? Thanks.
BTW, this behavior was triggered in amsart. I didn't test it in other document classes (nevertheless, I believe that the behavior should be the same as long as \mid is defined).
Here are some possible choices
\documentclass{article}
\usepackage{mleftright}
\begin{document}
\[
L = \sup \mleft\{\, \sum_{x \in F} a(x) \;\middle|\; F \subset X,\, |F| < \infty \,\mright\},
\]
\[
L = \sup \biggl\{\, \sum_{x \in F} a(x) \mathrel{\bigg|} F \subset X,\, |F| < \infty \,\biggr\},
\]
\[
L = \sup \biggl\{\, \sum_{x \in F} a(x) \mathrel{\Big|} F \subset X,\, |F| < \infty \,\biggr\},
\]
\end{document}
I'd choose the third one, probably, ruling out the first which has too high braces.
\middleas the delimiter-sizing macro. More specifically,\middle|.\middle|yields terrible result in this case, I would well go with\mid...\,\middle|\,. It looks terrible, but it is correct in terms of the\leftand\rightbraces. Correct in the sense that it matches the height of the outer elements. Inside, of course, there's nothing as big close to it, so it looks excessively ugly. Suggestions would be to not use\leftand\right, but rather\bigland\bigrsay, and perhaps stick to\mid.\bigr: tex.stackexchange.com/questions/15894/…