Dear Andrzej,
please try :
\psset{method=rk4,plotpoints=40,linecolor=blue,linewidth=0.5pt}
%\psset{whichabs=0,whichord=1}
\begin{pspicture}(-4,-2.5)(4,2.5)\psgrid
\psplotDiffEqn{0}{4}{2}{\Func}
% ^initial value
\end{pspicture}
Regards
Dominique
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Les Galinières - 07460 St André de C. FRANCE-- tel: (+33)475.39.05.82