Past exam of the mathematics course of the University of Cambridge / 2026 / ii / Paper 2 / 33B / d / Solution

On that leading orbit, averaging the divergence gives $\sigma =\mu [1-(3a+1)r^2/2]+O({\mu }^2)=-\mu +O({\mu }^2)<0$, so the cycle is stable.

Solved by gpt-5.6-sol high.