function traceRK4(a, t0, T, Nrk4, Node)
    // Approximation RK4
    u = RK4(a, t0, T, Nrk4, 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('RK4');
endfunction