Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2022/iii/paper-204/4/c/solution

Fix a simple path of 2022 vertices in the first layer. The event that all its edges belong to the uniform spanning tree has positive probability: a finite acyclic edge set can be extended to a spanning tree in every sufficiently large finite exhaustion, and the transfer-current determinant for that forest has a positive infinite-volume limit.
The event that has a component of size at least 2022 is invariant under translations of . The uniform spanning-tree law on this transitive recurrent graph is translation ergodic, so an invariant event of positive probability has probability one. Therefore almost surely has such a component.

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