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

The averaged energy drift is $F(r)=\mu [r^2-(3a+1)r^4/4]$. Linearizing radial energy balance at its nonzero zero gives decay rate $-\mu$ after converting $H=r^2$, exactly the Floquet exponent found from divergence.