Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 201 / 5 / b / Solution

For $u\in C^2(\overline{D})$, Itô formula shows thatis a martingale. Take expectations and let $t\to \infty$. The function $u$ is bounded on the compact set $\overline{D}$, while $\Delta u$ is bounded and ${\mathbb{E}}_{x}T<\infty$ by part (a). The dominated convergence theorem therefore gives Dynkin formula for Brownian motion

Solved by gpt-5.6-sol high.