%% 
%%  Ein Beispiel der DANTE-Edition
%%  
%%  7. Auflage
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%%  Beispiel 14-03-30 auf Seite 230.
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%%  Copyright (C) 2016 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
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\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{121.16928pt}
\setlength\parindent{0pt}
%StartShownPreambleCommands
\usepackage{pst-plot}
%StopShownPreambleCommands
\begin{document}
\psset{unit=1.5}
\begin{pspicture}[showgrid](-1.2,-1.2)(1.2,1.2)
  \psset{plotpoints=200,linewidth=1.5pt,algebraic}
  \def\PSfunc{ (t^2-1)*param/(t^2+1) | (t^2-1)*t*param/(t^2+1) }
  \psparametricplot{-1.85}{1.85}[/param 1 def]{\PSfunc}
  \psparametricplot[linestyle=dashed]%
        {-4.5}{4.5}[/param 0.25 def]{\PSfunc}
  \psparametricplot[linestyle=dotted]%
        {-2.1}{2.1}[/param 0.75 def]{\PSfunc}
  \rput[l](-0.95,-0.4){$a=1;\ 0.75;\ 0.25$}
\end{pspicture}
\end{document}