Question: DEplot: Plot not appearing when doing a phase plot


I am trying to do a phase plot of an autonomous system using DEplot command. However, no phase plot appears. Could you tell me what am I doing wrong? The same code worked for my professor in class, but it's not working for me. I am using Maple 16. The code is posted below.


> with DEtools

> sys := diff(x(t), t) = y(t), diff(y(t), t) = x(t)*(1-x(t)*x(t))+y(t);

> DEplot([sys], [x(t), y(t)], t = 0 .. 0.1e-8, x = -3 .. 3, y = -3 .. 3, color = black)

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