%%
%% The LaTeX Companion, 2ed (second printing August 2004)
%%
%% Example 8-3-1 on page 486.
%%
%% Copyright (C) 2004 Frank Mittelbach, Michel Goossens,
%%    Johannes Braams, David Carlisle, and Chris Rowley
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3
%% of this license or (at your option) any later version.
%%
%% See http://www.latex-project.org/lppl.txt for details.
%%

\documentclass{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{207.0pt}

\StartShownPreambleCommands
\usepackage{amsmath}
\StopShownPreambleCommands
\begin{document}
\begin{equation}    P_{r - j} =
 \begin{cases}
    0   &  \text{if $r - j$ is odd,} \\
    r! \, (-1)^{(r - j)/2}
        &  \text{if $r - j$ is even.}
 \end{cases}                \end{equation}
\end{document}
