Labeling the Dimensions of a Matrix

Question

Included below is a picture of what I'm trying to set in Latex. In word, I have a matrix of size k x k, and I want to label the rows and columns as such. Creating the matrix is no issue, it's including the labels that's being a problem.

Answer 1

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\underbrace{
\begin{bmatrix}
1/2 & 1/2 & 0 & 0 & \cdots & 0\\
0 & 1/2 & 1/2 & 0 & \cdots & 0\\
0 & 0 & 1/2 & 1/2 & \cdots & 0\\
\vdots & \vdots & \vdots & \ddots & \vdots & \vdots\\
1/2 & 0 & 0 & 0 & \cdots & 1/2\\
\end{bmatrix}}_{\displaystyle k}
\left.\vphantom{\begin{bmatrix}
1/2 & 1/2 & 0 & 0 & \cdots & 0\\
0 & 1/2 & 1/2 & 0 & \cdots & 0\\
0 & 0 & 1/2 & 1/2 & \cdots & 0\\
\vdots & \vdots & \vdots & \ddots & \vdots & \vdots\\
1/2 & 0 & 0 & 0 & \cdots & 1/2\\
\end{bmatrix}}\right\}k\]
\end{document}

Answer 2

Not easy, but hey! It works!

\documentclass{article}
\usepackage{amsmath}

\usepackage{lipsum}

\begin{document}

\lipsum*[3]
\[
\left.
\begin{bmatrix}\,
\smash{
  \underbrace{
    \begin{matrix}
    1/2 & 1/2 &  0  &  0  & \dots &  0 \\
     0  & 1/2 & 1/2 &  0  & \dots &  0 \\
     0  &  0  & 1/2 & 1/2 & \dots &  0 \\
     \vdots & \vdots & \vdots & \vdots & \ddots & \vdots \\
    1/2 &  0  &  0  &  0  & \dots & 1/2
    \end{matrix}
  }_{k}
}
\vphantom{
  \begin{matrix}
  \smash[b]{\vphantom{\Big|}}
  0\\0\\0\\vdots\\0
  \smash[t]{\vphantom{\Big|}}
  \end{matrix}
}
\,\end{bmatrix}
\right\rbrace{\scriptstyle k}
\vphantom{\underbrace{\begin{matrix}0\\0\\0\\vdots\\0\end{matrix}}_{k}}
\]
\lipsum[4]

\end{document}

The inner part of the matrix is underbraced and smashed so the underbrace is not taken into account; for setting the vertical size I use a \vphantom containing a slightly oversized matrix. Next the right brace is easy to produce. Finally another phantom takes care of the vertical space taken by the underbrace.

Answer 3

It's simple with a TikZ matrix and brace decorations:

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{matrix}
\usetikzlibrary{decorations.pathreplacing}

\usepackage{mwe}% for testing purpose only

\begin{document}

\blindtext% for testing purpose only

Like this:
\[
\begin{tikzpicture}[decoration={brace,amplitude=6pt}]
    \matrix[matrix of math nodes,
    nodes={text width=2em, align=center},
    left delimiter={[},right delimiter={]},
    ] (m) {
    1/2 & 1/2 &  0  &  0  & \dots &  0 \\
     0  & 1/2 & 1/2 &  0  & \dots &  0 \\
     0  &  0  & 1/2 & 1/2 & \dots &  0 \\
     \vdots & \vdots & \vdots & \vdots & \ddots & \vdots \\
    1/2 &  0  &  0  &  0  & \dots & 1/2\\
    };
    \draw[decorate] ([xshift=16pt]m-1-6.north east) -- ([xshift=16pt]m-5-6.south east) node[midway, anchor=west, xshift=6pt]{$k$};
    \draw[decorate] ([yshift=-10pt]m-5-6.south east) -- ([yshift=-10pt]m-5-1.south west)  node[midway, anchor=north, yshift=-6pt]{$k$};
\end{tikzpicture}
\]
or, similarly, like this:
\[
\begin{tikzpicture}[decoration={brace,amplitude=6pt}]
    \matrix[matrix of math nodes,
    nodes={text width=2em, align=center},
    left delimiter={[},right delimiter={]},
    ] (m) {
    1/2 & 1/2 &  0  &  0  & \dots &  0 \\
     0  & 1/2 & 1/2 &  0  & \dots &  0 \\
     0  &  0  & 1/2 & 1/2 & \dots &  0 \\
     \vdots & \vdots & \vdots & \vdots & \ddots & \vdots \\
    1/2 &  0  &  0  &  0  & \dots & 1/2\\
    };
    \draw[decorate] ([xshift=12pt]m.north east) -- ([xshift=12pt]m.south east) node[midway, anchor=west, xshift=6pt]{$k$};
    \draw[decorate] ([yshift=-8pt]m.south east) -- ([yshift=-8pt]m.south west)  node[midway, anchor=north, yshift=-6pt]{$k$};
\end{tikzpicture}
\]
\blindtext% for testing purpose only

\end{document}