# How to set "bigger" elements with a box around in a matrix?

To be specific, I want to set the following matrix:

I already read Draw a box around matrix elements, but I dont know, how one makes a block "T" of this size.

How do I do this?

A normal pmatrix from amsmath can serve what you want. To construct that "T block", just use \boxed{~T~}. The tildes ~ are there to make the T wider, and you can also add the superscript 1 by doing \makebox[0pt]{$^1~~$} to cancel its width.

Finally, observe that \setlength\arraycolsep{0pt} for removing spacing between columns only for this matrix.

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

\newcommand{\tblk}{\makebox[0pt]{$^1~~$}\boxed{~T~}}

$\setlength\arraycolsep{0pt} A = \begin{pmatrix} \boxed{~T~} & & & \\ & \tblk & & \\ & & \ddots & \\ & & & \tblk \end{pmatrix}$

\end{document}


Since you tag this tikz-matrix, I use one of these. There it is pretty straightforward to put nodes with any desired property.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix,positioning}
\begin{document}
$A=\vcenter{\hbox{ \begin{tikzpicture}[T-block/.style={draw,minimum width=12mm,minimum height=5mm}] \matrix (m)[matrix of math nodes,nodes in empty cells, left delimiter=(,right delimiter=)] { |[T-block]| T & & & \\ & |[T-block]| T & & \\ & & |[T-block,draw=none]|& \\ & & & |[T-block]| T \\ } ; \draw[dotted,thick] (m-2-2) -- (m-4-4); \node[left=0pt of m-2-2.north west,anchor=north east,inner sep=1pt, font=\small] {1}; \node[left=0pt of m-4-4.north west,anchor=north east,inner sep=1pt, font=\small] {1}; \end{tikzpicture}}}$
\end{document}