Past exam of the mathematics course of the University of Cambridge / 2026 / ii / Paper 1 / 27L / c / Solution

If $S_n/n$ converged almost surely to a finite limit, thenBut $\mathbb{P}(X_n/n>1)=\mathbb{P}(X_1>n)\sim 1/(\pi n)$. These independent-event probabilities have divergent sum, so the second Borel-Cantelli lemmas says $X_n/n>1$ infinitely often almost surely, a contradiction.

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