Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2024/iii/paper-201/3/a/solution

Let . The function is a harmonic function on the annulus , so Itô formula shows that
is a bounded martingale. By the optional sampling theorem for a supermartingale applied to this martingale,
where . Solving gives the planar Brownian annulus hitting probability

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