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On an Optional Semimartingale Decomposition and the Existence of a Deflator in an Enlarged Filtration

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Book cover In Memoriam Marc Yor - Séminaire de Probabilités XLVII

Part of the book series: Lecture Notes in Mathematics ((SEMPROBAB,volume 2137))

Abstract

Given a reference filtration \(\mathbb{F}\), we consider the cases where an enlarged filtration \(\mathbb{G}\) is constructed from \(\mathbb{F}\) in two different ways: progressively with a random time or initially with a random variable. In both situations, under suitable conditions, we present a \(\mathbb{G}\)-optional semimartingale decomposition for \(\mathbb{F}\)-local martingales. Our study is then applied to the question of how an arbitrage-free semimartingale model is affected when stopped at the random time in the case of progressive enlargement or when the random variable used for initial enlargement satisfies Jacod’s hypothesis. More precisely, we focus on the No-Unbounded-Profit-with-Bounded-Risk (NUPBR) condition, also called non arbitrages of the first kind in the literature. We provide alternative proofs of some results from Aksamit et al. (Non-arbitrage up to random horizon for semimartingale models, short version, preprint, 2014 [arXiv:1310.1142]), incorporating a different methodology based on our optional semimartingale decomposition.

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Notes

  1. 1.

    The random variable \(\xi\) can take values in a more general space without any difficulty.

  2. 2.

    The upper script “pr” stands for progressive.

  3. 3.

    A set \(A \subset \varOmega \times [0,\infty [\) is thin if, for all ω ∈ Ω the set A(ω) is countable.

  4. 4.

    The upper script “i stands for initial.

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Acknowledgements

The authors are thankful to the Chaire Marchés en Mutation (Fédération Bancaire Française) for financial support and to Marek Rutkowski for valuable comments that helped to improve this paper.

We thank also the anonymous referee for his(her) helpful comments.

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Correspondence to Monique Jeanblanc .

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Aksamit, A., Choulli, T., Jeanblanc, M. (2015). On an Optional Semimartingale Decomposition and the Existence of a Deflator in an Enlarged Filtration. In: Donati-Martin, C., Lejay, A., Rouault, A. (eds) In Memoriam Marc Yor - Séminaire de Probabilités XLVII. Lecture Notes in Mathematics(), vol 2137. Springer, Cham. https://doi.org/10.1007/978-3-319-18585-9_9

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