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Doob's upcrossing inequality for Countable and NNReal - #361

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Doob's upcrossing inequality for Countable and NNReal#361
WojciechCzernous wants to merge 10 commits into
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@WojciechCzernous WojciechCzernous commented Jan 20, 2026

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First on Countable index sets - via finite ones and thanks to the existing code for Nat - and then in continuous case, the Doob's upcrossing inequality has been formalized and proved, short of measurability in continuous case, which requires the debut theorem. With this latter restriction, the number of upcrossings is a.s. finite, which is important for Martingale Convergence Theorem.

As the recursive-hitting-time definition of upcrossing number is unsuitable for NNRat, we use an equivalent one for the sake of proving the Countable case: the one found in the Kallenberg's "Foundations of Modern Probability" (2021), Theorem 9.18.

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Comment thread BrownianMotion/StochasticIntegral/Upcrossing.lean Outdated
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WojciechCzernous marked this pull request as draft April 24, 2026 11:46
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WojciechCzernous marked this pull request as ready for review April 27, 2026 11:34
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