# Combining rows and columns in array

I am trying to write some kind of blockmatrix

$A_n= \left( \begin{array}{cc|cc} && 0 & 0 \\ & A_{n-2}&n-2&0 \\ \hline\\ 0&-n+2&1&n-1 \\ 0&0&-n+1&1 \end{array} \right)$


How can I achieve that the entry in the upper left corner is written in center? Or rephrased: How is it possible two combine the columns (1,1)(1,2)(2,1) and (2,2)?

• Avoid using $$<math content>$$. Use instead $<math content>$ Jan 9, 2014 at 19:46

You're probably after the second of the following two instances (the first is yours):

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align*}
A_n &= \left(\begin{array}{cc|cc}
&         &     0 &      0 \\
& A_{n-2} & n - 2 &      0 \\
\hline \\
0 &  -n + 2 &      1 & n - 1 \\
0 &       0 & -n + 1 &     1
\end{array}\right) \\
A_n &= \left(\begin{array}{cc|cc}
&         &     0 &      0 \\
\multicolumn{2}{c|}{\smash{\raisebox{.5\normalbaselineskip}{$A_{n-2}$}}}
& n - 2 &      0 \\
\hline \\[-\normalbaselineskip]
0 &  -n + 2 &      1 & n - 1 \\
0 &       0 & -n + 1 &     1
\end{array}\right)
\end{align*}
\end{document}


Horizontal centering is achieved via \multicolumn{2}{c}, while vertical centering is obtained via \raisebox{.5\normalbaselineskip}. \smash removes any vertical distortion (from raising the boxed content), while the negative skip after \hrule vertically aligns the vertical rules (top and bottom).

With {pNiceArray} of nicematrix.

\documentclass{article}
\usepackage{nicematrix}

\begin{document}

$\begin{pNiceArray}{cc|cc}[margin=2pt] \Block{2-2}<\Large>{A_{n-2}} & & 0 & 0 \\ & & n - 2 & 0 \\ \hline 0 & -n + 2 & 1 & n - 1 \\ 0 & 0 & -n + 1 & 1 \end{pNiceArray}$

\end{document}