Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2025/iii/paper-202/4/c/solution

Let and be two solutions with the same initial value and Brownian motion, and put . Itô formula gives
Stop when either process or the stochastic integral becomes large. Taking expectations, using the assumed one-sided Lipschitz bound, and then removing the localization gives
The Gronwall inequality yields . Thus almost surely at every rational time, and path continuity makes the two processes indistinguishable. This proves pathwise uniqueness.

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