Past exam of the mathematics course of the University of Cambridge 2023 iii Paper 209 1 iv Solution 2026-09-28
Let the square contain the Euclidean ball of radius , set , and define the logarithmic cutoffThen and on the boundary. On an edge at radius comparable to , the mean value theorem gives . There are edges in the annulus of radius , hence the discrete Dirichlet energy satisfiesThis logarithmic cutoff is the discrete manifestation of recurrence in two dimensions.