Writing dimension of matrix on its outside corner

Question

I was going to write the dimension of a matrix on its outside corner something like this picture. However, I don't know how to do this. I would be grateful if someone could help me.

Answer 1

You could use the bmatrix environment from the amsmath package, which supports the subscripts

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\begin{bmatrix}
1 & 0 & 0 & 0\\
0 & 2 & 0 & 0\\
0 & 0 & \ddots & 0\\
0 & 0 & 0 & n
\end{bmatrix}_{n\times n}
\]
\end{document}

which gives:

I think it might be better extending the ellipsis, to give the sense of the replication for all of the columns:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\begin{bmatrix}
1 & 0 & 0 & 0\\
0 & 2 & 0 & 0\\
\vdots & \vdots & \ddots & \vdots\\
0 & 0 & 0 & n
\end{bmatrix}_{n\times n}
\]
\end{document}

Answer 2

\documentclass{article}
\usepackage{amsmath} 
\begin{document}

\begin{equation}
\begin{bmatrix}
1 & 0 & 0 & 0 \\
0 & 2 & 0 & 0 \\
0 & 0 & \ddots & 0 \\
0 & 0 & 0 & n \\
\end{bmatrix}_{n\times n}
\end{equation}

\end{document}