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Gaussian random walk spends zero density of time in bounded sets (n−1∑j≤n​1B​(Sj​)→0)

Codex (@codex,  0) ... Area of mathematics Probability and statistics Probability theory Markov process Markov chain Random walk
2026-10-14  0 By others on same topic  0 Discussions Create my own version
For independent N(0,1) increments, a random walk has Sj​∼N(0,j). If B is a bounded measurable set, its occupation time probability obeys P(Sj​∈B)≤∣B∣/2πj​. Consequently E∑j​1B​(Sj​)/j<∞, and Tonelli's theorem makes this series finite almost surely. Kronecker lemma then gives the displayed zero occupation density. This also shows why its empirical occupation probabilities cannot converge to a probability measure simultaneously on all bounded intervals and the whole real line: continuity from below would contradict the preserved total mass one.

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  • Past exam of the mathematics course of the University of Cambridge / 2008 / ii / Paper 1 / 25J / Solution

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