Paper 1, Section I, D

Dynamical Systems
Part II, 2010

Consider the 2-dimensional flow

x˙=μx+y,y˙=x21+x2νy\dot{x}=-\mu x+y, \quad \dot{y}=\frac{x^{2}}{1+x^{2}}-\nu y

where x(t)x(t) and y(t)y(t) are non-negative, the parameters μ\mu and ν\nu are strictly positive and μν\mu \neq \nu. Sketch the nullclines in the x,yx, y plane. Deduce that for μ<μc\mu<\mu_{c} (where μc\mu_{c} is to be determined) there are three fixed points. Find them and determine their type.

Sketch the phase portrait for μ<μc\mu<\mu_{c} and identify, qualitatively on your sketch, the stable and unstable manifolds of the saddle point. What is the final outcome of this system?