Past exam of the mathematics course of the University of Cambridge 2022 iii Paper 203 4 b Solution 2026-09-28
For , the centered Loewner imageis the Boundary-point Bessel flow for SLE, a Bessel process of dimensionWhen , one has , and the Hitting-zero classification for a Bessel process says that hits zero almost surely. Thus every fixed nonzero boundary point is swallowed in finite time.
A hull generated by a continuous trace cannot swallow a real point while the trace remains strictly inside : before any contact with the real line, the trace is a crosscut-free interior curve and no boundary interval is disconnected from infinity. Consequently finite swallowing forces the trace to meet away from its initial point. SLE therefore almost surely intersects the boundary for every .
Past exam of the mathematics course of the University of Cambridge 2023 iii Paper 203 2 b Solution 2026-09-28
For the Bessel processthe scale function of a one-dimensional diffusion is when . If , the boundary hitting probability from a diffusion scale function gives
If , then and . Letting and then shows that the process cannot escape to infinity before reaching zero. The exit time from each bounded interval is finite almost surely, so almost surely.
If , then in absolute value as . Equivalently,Thus the process does not hit zero. The borderline case has scale function and also does not hit zero. This is the Hitting-zero classification for a Bessel process.