Control the width of an underbrace

Question

I would like to shorten this underbrace so that it terminates directly underneath the vertical bars without extending past them.

Minimal Working Example:

\documentclass{report}
\newcommand{\Underbrace}[2]{{\underbrace{#2}_{#1}}}
\begin{document}
    [\Underbrace{a}{|x+y|}]
\end{document}

The underbrace covers the entire width of the object, which includes the space around the vertical bars. You can remove the outer spaces using \Underbrace{\!|x+y|\!}. But the use case would change dependent on the input. — Werner, Feb 22 '21 at 18:43
@Werner That's perfect, thanks! What do you mean about the use case? — simple jack, Feb 22 '21 at 19:42
@simplejack: The spacing correction will depend on what you supply. Take this as an example. Even with \! the \underbrace still stretches too wide because of the additional space around the extensible delimeters. — Werner, Feb 22 '21 at 19:53
@Werner I see. Also, it gets messed up when trying to write a product such as $|x||y|$ with an underbrace under each factor. — simple jack, Feb 22 '21 at 19:54
@simplejack: Riiight. For that you'll need $\,\Underbrace{..}{\!|x|\!}\,\,\Underbrace{..}{\!|y|\!}\,$ . Here \! is the negative of \,. See What commands are there for horizontal spacing? — Werner, Feb 22 '21 at 20:00
The answers in these questions address this problem: How to prevent an underbrace, with a subscript, from affecting the sizes of delimiters and \underbrace disturbs balanced vertical spacing — barbara beeton, Feb 22 '21 at 21:22

score 1 · Accepted Answer · answered Feb 22 '21 at 20:01

You can try one of these solutions – the second is similar to @Werner's suggestion, with the negative thin spaces compensated with thin spaces befor and after the underbrace:

\documentclass{report}
\newcommand{\Underbrace}[2]{{\underbrace{#2}_{#1}}}
\begin{document}
\[\Underbrace{a}{|x+y|} \neq |\Underbrace{a}{x+y}|\neq\,\Underbrace{a}{\!|x+y|\!}\,\]


\end{document}

Control the width of an underbrace

1 Answers1