Centering in matrices

Question

I am building a big matrix within the which I have block matrices, which I want to indicate by boxes. My current state looks like this:

 $$\begin{matrix} \framebox[3.\width]{J}& \begin{matrix} 0&0&\\0&0&\end{matrix}&\\
    \begin{matrix} 0&0&\\0&0&\end{matrix}& \framebox[3.\width]{J}&\end{matrix}$$

but this makes the frameboxes aligned to the right. I want them centered in their matrix-cell.

How can I solve this?

Answer 1

The columns are centred, but each of your 'inner' matrices has an empty column, generated by a spurious & at the end of the row. Delete these, and the problem goes away.

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\[
\begin{matrix}
\framebox[3.\width]{J} &
\begin{matrix}
0 & 0  \\
0 & 0
\end{matrix}
& \\
\begin{matrix}
0 & 0 \\
0 & 0
\end{matrix}
& \framebox[3.\width]{J} &
\end{matrix}
\]
\end{document}