A dynamical system satisfies the equations
x· = x - y - (x2 + y2)x,
y· = x + y - (x2 + y2)y.
Convert this system into polar form and find the polar equations of the phase paths that begin in the domain 0 < r < 1. Show that all these phase paths spiral anti-clockwise and tend to the limit cycle r = 1. Show also that the same is true for phase paths that begin in the domain r > 1. Sketch the phase diagram.