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How do you use an underbrace with a right brace?

This is what I've cobbled together but the right brace shouldn't contain the underbrace. They should look like siblings, so to speak. Is there a way of doing that?

enter image description here

\(

    \left.
    \underbrace{
    \left[

            \begin{matrix}
                a_{11}  & a_{12}  & a_{13} & \cdots & a_{1n} \\
                a_{21}  & a_{22}  & a_{23} & \cdots & a_{2n} \\
                a_{31}  & a_{32}  & a_{33} & \cdots & a_{3n} \\
                \vdots  & \vdots  & \vdots &        & \vdots \\
                a_{m1}  & a_{m2}  & a_{m3} & \cdots & a_{mn}
            \end{matrix}

    \right]    
    }_{\text{ n columns}}
    \right\} \text{m rows}

\)
Solution

  • Here is a complete hack to get this for you.

    <script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.3/MathJax.js?config=TeX-AMS_HTML"></script>

\(
    \kern .4em
    \underbrace{
      \kern-.4em
      \left.
      \left[
        \begin{matrix}
            a_{11}  & a_{12}  & a_{13} & \cdots & a_{1n} \\
            a_{21}  & a_{22}  & a_{23} & \cdots & a_{2n} \\
            a_{31}  & a_{32}  & a_{33} & \cdots & a_{3n} \\
            \vdots  & \vdots  & \vdots &        & \vdots \\
            a_{m1}  & a_{m2}  & a_{m3} & \cdots & a_{mn}
        \end{matrix}
      \right]    
      \right\} m\text{ rows}
      \Rule{0em}{0em}{3.6em}
      \kern -4.5em
    }_{\textstyle n \text{ columns}}
    \kern 4.5em
\)

    The kerning has to be worked out by hand. If you are planning to use this in actual LaTeX, the \Rule is non-standard, and would have to be replaced by a corresponding \rule. Or it could be left out entirely (as it is only for a little space between the matrix and the underbrace).