vertical and horizontal line in a matrix

Question

I want to create a bmatrix on the right

\documentclass[10pt,a4paper]{article}
\usepackage[english]{babel}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{bmatrix}[c|c]
        _B[T]_B & * \\
        0 & _{\overline{B'}} [\overline{T}] _{\overline{B'}}
\end{bmatrix}   
\end{equation}
\end{document}  

gives me

How do I make verticle line work? and how do I insert horizontal line?

thanks

Answer 1

You could use an array instead; in this way, the standard | for the vertical rule and \hline (for the horizontal rule) will give you the desired result. The brackets can be obtained using \mleft, \mright from the mleftright package:

\documentclass{article} 
\usepackage{amsmath}
\usepackage{mleftright}

\begin{document}

\[
\renewcommand\arraystretch{1.3}
\mleft[
\begin{array}{c|c}
  \epsilon' [T|_A]\epsilon & \ast \\
  \hline
  0 & _{\overline{B}'} [\overline{T}] _{\overline{B}\vphantom{\overline{B}'}}
\end{array}
\mright]
\]

\end{document}

Answer 2

I found the solution here. The idea is paste the following in the preamble:

\makeatletter
\renewcommand*\env@matrix[1][*\c@MaxMatrixCols c]{%
  \hskip -\arraycolsep
  \let\@ifnextchar\new@ifnextchar
  \array{#1}}
\makeatother

With that, any matrix environment has as optional argument the array alignment (as you suggested), and you could use also \hline. So for example:

\begin{bmatrix}[c|c]
    _B[T]_B & * \\ \hline
    0 & _{\overline{B'}} [\overline{T}] _{\overline{B'}}
\end{bmatrix}

will behave as you want.

Answer 3

I used an array inside of a bmatrix, because one of my editing environments didn't parse the large square brackets correctly:

\documentclass{article}
\usepackage{amsmath, amsfonts, amssymb}
\begin{document}

\[
  \begin{bmatrix}
    \begin{array}{c|c}
  \epsilon' [T|_A]\epsilon & \ast \\
  \hline
  0 & _{\overline{B}'} [\overline{T}]
  _{\overline{B}\vphantom{\overline{B}'}}
    \end{array}
  \end{bmatrix}
\]

\end{document}