%% 
%%  An UIT Edition example
%% 
%%  Example 09-17-7 on page 189.
%% 
%%  Copyright (C) 2010 Herbert Voss
%% 
%%  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.
%% 
%% 
%% ==== 
% Show page(s) 1
%% 
\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{201.70511pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands
\usepackage{mathtools}
\StopShownPreambleCommands
\begin{document}
\[ \mathop{\mathrm{sgn}}x =
   \begin{dcases*}
                -1  & $x$ negative \\
     \phantom{+} 0  & $x=0$ \\
                +1  & $x$ positive
 \end{dcases*} \]
\end{document}