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

Question

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?

Answer 1

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}

Answer 2

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}