Solution

ID: past-exam-of-the-differential-equations-2-course-of-the-university-of-oxford/2014/1/a/solution

At an equilibrium point, and . The first equation gives or . Substituting in the second gives, respectively,
These are all the equilibrium points. The Jacobian matrix of the vector field is
At ,
The eigenvalue has algebraic multiplicity two and eigenspace . For the linearized coordinates , ,
Both components tend to zero. The repeated eigenvalue with one independent eigenvector makes this a stable improper degenerate node, a sink equilibrium. The term is the signature of the nontrivial Jordan block. Since it is a hyperbolic equilibrium point, the linearization stability theorem gives local asymptotic stability of the nonlinear system. Approaching trajectories are tangent to the vertical eigendirection.
At ,
One eigenvalue is positive and one negative, so this point is a saddle equilibrium. Its stable manifold is tangent to , and its unstable manifold is tangent to .
For the phase portrait, the nullclines are
Above the second line the vertical flow is downward, and below it upward. For , horizontal flow is to the right below and to the left above it; for the directions reverse. The line is invariant, with , so it flows toward . Together with the saddle's two invariant directions, these signs determine the sketch.
Figure 1.
Phase portrait with the stable improper degenerate node at , the saddle equilibrium at , and their nullclines
. Details.

New to topics? Read the docs here!