# Getting block matrices to be the same size

I want to produce equations with block matrices

\documentclass[a4paper,12pt,titlepage]{article}
\usepackage[fleqn]{amsmath}
\usepackage[pdftex]{graphicx}
\usepackage{blkarray}
\usepackage{mathtools}
\usepackage{multirow}
\newcommand{\rdots}{\hspace{.2ex}\raisebox{1ex}{\rotatebox{-12}{$\ddots$}}}

\begin{document}

\resizebox{0.9\textwidth}{!}{\begin{minipage}{\textwidth}
\begin{align}
\begin{blockarray}{(cccc|cccc)}
1 &  & 0 & \cdots & \BAmulticolumn{4}{c}{\multirow{4}{*}{\huge$0$}}\\
& \rdots &  &  & & & &\\
0 &  &  1 &  & & & &\\
\vdots &  &  & \rdots & & & &\\
\cline{1-8}
& & &  & 1 &  &  & \cdots \\
\BAmulticolumn{4}{c|}{\multirow{4}{*}{\huge$0$}} & & \rdots  &  \\
& & & & 0 &  & 1 &  \\
& & & & \vdots &  &  &\rdots \\
\end{blockarray}+
\begin{blockarray}{(cccc|cccc)}
\BAmulticolumn{4}{c|}{\multirow{4}{*}{\huge$0$}} &  \BAmulticolumn{4}{c}{\multirow{4}{*}{\huge$S-T$}}  \\
&     &   &   &    &   &   &   & \\
&     &   &   &    &   &   &   & \\
&     &   &   &    &   &   &   & \\
\cline{1-8}
\BAmulticolumn{4}{c|}{\multirow{4}{*}{\huge$T-S$}} & \BAmulticolumn{4}{c}{\multirow{4}{*}{\huge$0$}}   \\
&     &   &   &    &   &   &   & \\
&     &   &   &    &   &   &   & \\
&     &   &   &    &   &   &   & \\
\end{blockarray}
\end{align}
\end{minipage}}

\end{document}


My code produces the correct matrices, but they are not the same size:

the one on the lefthand size (with the S-T) is smaller! I don't get why, can anyone help?

• Why not simply using something like $\huge \left( \begin{array}{c|c} I & 0 \\ \hline 0 & I \end{array} \right) + \left( \begin{array}{c|c} 0 & S-T \\ \hline T-S & 0 \end{array} \right)$ (with a possible remark explaining that I denotes an identity matrix of the given size)? This is more concise and looks a lot better (in my opinion). – Gonzalo Medina Jan 16 '15 at 15:56

Here's a slightly simpler approach, omitting the unsightly internal rules in the matrices, and using a scalebox to shrink the identity matrix instead of trying to grow the other parts. The other trick used here is to insert an invisible vertical rule to control the spacing.
\documentclass{article}
\newcommand{\IM}{\scalebox{0.7}{$$\begin{matrix}1&0&\cdots\\0&1\\\vdots&&\ddots\end{matrix}$$}}
$\left( \begin{matrix} \IM & \BigO \\ \BigO & \IM \end{matrix} \right) + \left( \begin{matrix} \BigO & S-T \\ T-S & \BigO \end{matrix} \right)$