function tracevdp(a, t0, T, Nptmil, Node)
// Approximation VDP
u = pointmilieu(a, t0, T, Nptmil, vdp);
// Approximation ODE
t = linspace(t0, T, Node+1);
x = ode(a, t0, t, vdp)
plot(u(1,:), u(2,:));
plot(x(1,:), x(2,:), 'r');
title('VDP');
endfunction