Past exam of the mathematics course of the University of Cambridge / 2026 / ii / Paper 3 / 35E / b / Solution

Stirling and replacing the magnetization sum by an integral gives the stated form with $P(m)=-\beta (Bm+3J{m}^2+2K{m}^3)$, up to subexponential constants.

Solved by gpt-5.6-sol high.