%% 
%%  Ein Beispiel der DANTE-Edition
%%  Mathematiksatz mit LaTeX
%%  3. Auflage
%%  Beispiel 09-11-1 auf Seite 182.
%%  Copyright (C) 2018 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[10pt]{screxa}
\pagestyle{empty}
\setlength\textwidth{170.40707pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\setcounter{equation}{0}
\renewcommand\theequation{9.\arabic{equation}}
%StartShownPreambleCommands
\usepackage{amsmath,cases}
\DeclareMathOperator{\sgn}{sgn}
%StopShownPreambleCommands
\begin{document}
\begin{numcases}{\sgn{x}=}
-1, & for $x < 0$\\ 0, & for $x = 0$\\ 1, & for $x > 0$
\end{numcases}
\end{document}