%% 
%%  A DANTE-Edition example
%% 
%% 
%%  Copyright (C) 2011 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[]{article}
\pagestyle{empty}
\setlength\textwidth{177.70511pt}
\setlength\parindent{0pt}
\AtBeginDocument{%
  \psset{mathLabel=false,labelFontSize=\small}}

\usepackage{pst-plot}

\begin{document}
\psset{unit=1.5}
\begin{pspicture}[showgrid](-1.2,-1.2)(1.2,1.2)
  \psset{plotpoints=200,linewidth=1.5pt}
  \def\PSfunc{t t mul 1 sub \param\space mul
      t t mul 1 add div t t mul 1 sub t mul
      \param\space mul t t mul 1 add div}
  \def\param{1}\parametricplot{-1.85}{1.85}{\PSfunc}
  \def\param{0.25}\parametricplot[linestyle=dashed]%
        {-4.5}{4.5}{\PSfunc}
  \def\param{0.75}\parametricplot[linestyle=dotted]%
        {-2.1}{2.1}{\PSfunc}
  \rput[l](-0.95,-0.4){$a=1;\ 0.75;\ 0.25$}
\end{pspicture}
\end{document}