%%
%% An UIT Edition example
%%
%% Example 10-10-5 on page 224.
%%
%% Copyright (C) 2010 Herbert Voss
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3
%% of this license or (at your option) any later version.
%%
%% See
http://www.latex-project.org/lppl.txt for details.
%%
%%
%% ====
% Show page(s) 1
%%
\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{375.57637pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands
\usepackage{amsmath,amssymb}
\makeatletter
\def\underbraceMatrix{\@ifnextchar[{\underbraceM@i}{\underbraceM@i[4pt]}}
\def\underbraceM@i[#1]#2{\underbraceM@ii{#2}{#1}}%
\def\underbraceM@ii#1#2{\mathop{\vtop{\m@th\ialign{##\crcr
$\hfil\displaystyle{#1}\hfil$\crcr\noalign{\kern3\p@\nointerlineskip}%
\kern#2\upbracefill\kern#2\crcr\noalign{\kern3\p@}}}}\limits}
\makeatother
\StopShownPreambleCommands
\begin{document}
\[ \underbrace{%
\left(\begin{array}{cc}
A_{\cal E E} & A_{\cal E I}\\ A_{\cal I E} & A_{\cal I I}
\end{array}\right)}_{\textstyle=A}
\underbraceMatrix{%
\left(\begin{array}{cc}
A_{\cal E E} & A_{\cal E I}\\ A_{\cal I E} & A_{\cal I I}
\end{array}\right)}_{\textstyle=A}
\underbraceMatrix[1em]{%
\left(\begin{array}{cc}
A_{\cal E E} & A_{\cal E I}\\ A_{\cal I E} & A_{\cal I I}
\end{array}\right)}_{\textstyle=A} \]
\end{document}
