Underbracket and overbracket intersecting each other

Question

I cannot seem to find a way to achieve the result I want which is best portrayed by this sketch diagram:

This is because if I add an overbracket it is closed by the underbracket } before it gets to the second group of underbracket.

$w = 
    \underbracket[0.75pt]{aaa...a}_{n}
    \underbracket[0.75pt]{aaa...a}_{m}
    \underbracket[0.75pt]{bbb...b}_{n}
    \underbracket[0.75pt]{aaa...a}_{m}
    \underbracket[0.75pt]{bbb...b}_{n}
$

Solution:

As suggested in this post: How to have overlapping under-braces and over-braces

$
w = 
    \rlap{\phantom{aaa}$\overbracket[0.75pt]{\phantom{aaaaa}}^x$}
    \underbracket[0.75pt]{aaa...a}_{n}
    \underbracket[0.75pt]{aaa...a}_{m}
    \underbracket[0.75pt]{bbb...b}_{n}
    \underbracket[0.75pt]{aaa...a}_{m}
    \underbracket[0.75pt]{bbb...b}_{n}
$\\

Note that the letters added in the phantom do not matter.

Answer 1

Probably the posts from Mico's link are the best way to go, but here is an alternative that allows you to avoid typing the text twice: tikzmark. I actually do not think that this method is superior in this very case because

1. you have to load another package,
2. you need to insert a \vphantom to get the vertical space right as an overlay tikzpicture does not insert space, and
3. you need to give each \tikzmarknode a unique label.

However, it is an alternative, and as soon as you want a more fancy bracket (bent, dashed, whatever) this method is likely to continue to give you what you want.

\documentclass{article}
\usepackage{mathtools}
\usepackage{tikz}
\usetikzlibrary{tikzmark}
\begin{document}
Some text to test the vertical space.
$
w = \vphantom{\overbracket[0.75pt]{a}^{x}}% to fix the vertical space
    \underbracket[0.75pt]{aa\tikzmarknode{a1}{a}\dots a}{n}
    \underbracket[0.75pt]{a\tikzmarknode{a2}{a}a\dots a}{m}
    \underbracket[0.75pt]{bbb\dots b}{n}
    \underbracket[0.75pt]{aaa\dots a}{m}
    \underbracket[0.75pt]{bbb\dots b}_{n}
    \begin{tikzpicture}[overlay,remember picture]
     \draw[line width=0.75pt] (a1.north east) -- ++ (0,0.8ex)
      -| node[pos=0.25,above]{$x$} (a2.north east);
    \end{tikzpicture}
$
Some text to test the vertical space.
[w = \vphantom{\overbracket[0.75pt]{a}^{x}}% to fix the vertical space
    \underbracket[0.75pt]{aa\tikzmarknode{a3}{a}\dots a}{n}
    \underbracket[0.75pt]{a\tikzmarknode{a4}{a}a\dots a}{m}
    \underbracket[0.75pt]{bbb\dots b}{n}
    \underbracket[0.75pt]{aaa\dots a}{m}
    \underbracket[0.75pt]{bbb\dots b}_{n}
    \begin{tikzpicture}[overlay,remember picture]
     \draw[line width=0.75pt] (a3.north east) -- ++ (0,0.8ex)
      -| node[pos=0.25,above]{$x$} (a4.north east);
    \end{tikzpicture}
]
Some text to test the vertical space.
\end{document}