Solution

ID: past-exam-of-the-algebra-analysis-and-differential-equations-2-course-of-the-university-of-oxford/2012/7/b/solution

The first component of the vector field vanishes only when or . The second vanishes only when or . Intersecting these alternatives gives exactly five equilibria. Their Jacobian matrix is
At ,
The real parts are negative and the imaginary parts are nonzero, so is a stable spiral.
At the Jacobian matrix is . Its real eigenvalues have opposite signs, so is a saddle equilibrium.
At ,
Both eigenvalues are positive, so is an unstable node.
At the Jacobian matrix is , giving a stable node.
At ,
so is a saddle equilibrium. All five equilibria are hyperbolic, so these linear classifications determine their nonlinear local stability as well.

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