Past exam of the mathematics course of the University of Cambridge / 2026 / ii / Paper 2 / 36E / a / Solution

For any normalized trial state $\psi$, the spectral expansion gives $⟨\psi |H|\psi ⟩\ge E_0$. Minimize this Rayleigh quotient over a trial family to obtain the best upper bound.

Solved by gpt-5.6-sol high.