Underbrace Math Mode

Question

This is my code for creating the matrix in Latex

\begin{equation}
G = 
\begin{bmatrix}
g_0 \\ g_1 \\ . \\ . \\ . \\ g_k-1\\
\end{bmatrix}
=
\setcounter{MaxMatrixCols}{15}
\begin{bmatrix} 
p_{0,0}     & p_{1,0}     & . & . & . & p_{0,n-k-1}   & & & & 1 & 0 & . & . & . & 0 \\
p_{0,1}     & p_{1,1}     & . & . & . & p_{1,n-k-1}   & & & & 0 & 1 & . & . & . & 0 \\
.           & .           & . &   &   & .             & & & & . & . & . &   &   & . \\
.           & .           &   & . &   & .             & & & & . & . &   & . &   & . \\
.           & .           &   &   & . & .             & & & & . & . &   &   & . & . \\
p_{0,n-k-1} & p_{1,n-k-1} & . & . & . & p_{k-1,n-k-1} & & & & 0 & 0 & . & . & . & 1 \\
\end{bmatrix}
\end{equation}

How to add the underbrace for the highlighted part of the submatrix P ?

Thank you.

Answer 1

Here is one method using tikz overlays and markers

\documentclass[a4paper]{memoir}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{tikzmark}
\usetikzlibrary{decorations.pathreplacing}
\begin{document}
\begin{equation}
G = 
\begin{bmatrix}
g_0 \\ g_1 \\ . \\ . \\ . \\ g_k-1\\
\end{bmatrix}
=
\setcounter{MaxMatrixCols}{15}
\begin{bmatrix} 
p_{0,0}     & p_{1,0}     & . & . & . & p_{0,n-k-1}   & & & & 1 & 0 & . & . & . & 0 \\
p_{0,1}     & p_{1,1}     & . & . & . & p_{1,n-k-1}   & & & & 0 & 1 & . & . & . & 0 \\
.           & .           & . &   &   & .             & & & & . & . & . &   &   & . \\
.           & .           &   & . &   & .             & & & & . & . &   & . &   & . \\
.           & .           &   &   & . & .             & & & & . & . &   &   & . & . \\
\tikzmark{A} p_{0,n-k-1} & p_{1,n-k-1} & . & . & . & p_{k-1,n-k-1}\tikzmark{B} & & & & 0 & 0 & . & . & . & 1 \\
\end{bmatrix}
\end{equation}
\begin{tikzpicture}[remember picture,overlay,decoration={brace,amplitude=5pt,mirror,raise=5pt}]
  \draw[red,decorate] (pic cs:A) -- (pic cs:B)
  node [midway,below=9pt] {$p$};
\end{tikzpicture}

\end{document}