How to write the given block matrix
With use of the nicematrix
package:
\documentclass[12pt,oneside]{article}
\usepackage{nicematrix}
\begin{document}
\[
P = \begin{bNiceArray}{C CCCC}
a & 0 & 0 & \dots & 0 \\
0 & \Block{4-4}<\huge>{Q} \\
0 & & & & \\
\vdots & & & & \\
0 & & & &
\end{bNiceArray}
\]
\end{document}
For showed result you need to compile above MWE at least twice.
Nest matrices:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
P=\begin{bmatrix}
a & \begin{matrix} 0 & 0 & \dots & 0 \end{matrix} \\
\begin{matrix} 0 \\ 0 \\ \vdots \\[1ex] 0 \end{matrix} & \text{\Huge$Q$}
\end{bmatrix}
\]
\end{document}
Don't use center
around \[...\]
.
One more easy method is using \multicolumn
and \multirow
and the MWE
is:
\documentclass{article}
\usepackage{amsmath,multirow}
\begin{document}
\[
P=\begin{bmatrix}
a & 0 & 0 & \dots & 0 \\
0 \\
0 &\multicolumn{4}{c}{\multirow{2}{*}{\Huge Q}}\\
\vdots \\
0 &
\end{bmatrix}
\]
\end{document}