Hello, I am trying to trace a phaseportrait of this system: sys:=D(x)(t)=-a*x(t)+b*y(t), D(y)(t)=-c*(p1*x(t)+q1*(x(t))^3)+V0*d*cos(2*Pi*10^(-4)*f*t); where a:=0.1785714286, b:=1, c:=20000, d:=10, V0 varies and f:=1273. For instance, lets take V0=4.08. I did the following procedure: >with(DEtools): with(plots): >p1:=0.18*10^(-5): q1:=0.12*10^(-5): >sys:=D(x)(t)=-a*x(t)+b*y(t), D(y)(t)=-c*(p1*x(t)+q1*(x(t))^3)+V0*d*cos(2*Pi*10^(-4)*f*t); >ic:=[[x(0)=-0.09,y(0)=-0.09],[x(0)=-0.09,y(0)=0.09],[x(0)=0.09,y(0)=-0.09],[x(0)=0.09,y(0)=0.09]];