# Foster's theorem

Foster's theorem This article needs attention from an expert in Mathematics. The specific problem is: Needs a proof adding. WikiProject Mathematics may be able to help recruit an expert. (Février 2009) This article is about the theorem in Markov probability theory. For the theorem in electrical engineering, see Foster's reactance theorem.

En théorie des probabilités, Foster's theorem, named after Gordon Foster,[1] is used to draw conclusions about the positive recurrence of Markov chains with countable state spaces. It uses the fact that positive recurrent Markov chains exhibit a notion of "Lyapunov stability" in terms of returning to any state while starting from it within a finite time interval.

Theorem Consider an irreducible discrete-time Markov chain on a countable state space S having a transition probability matrix P with elements pij for pairs i, j in S. Foster's theorem states that the Markov chain is positive recurrent if and only if there exists a Lyapunov function {style d'affichage V:Sto mathbb {R} } , tel que {style d'affichage V(je)gq 0{texte{ }}forall {texte{ }}iin S} et {somme de style d'affichage _{jin S}p_{ij}V(j)<{infty }} for {displaystyle iin F} {displaystyle sum _{jin S}p_{ij}V(j)leq V(i)-varepsilon } for all {displaystyle inotin F} for some finite set F and strictly positive ε.[2] Related links Lyapunov optimization References ^ Foster, F. G. (1953). "On the Stochastic Matrices Associated with Certain Queuing Processes". The Annals of Mathematical Statistics. 24 (3): 355. doi:10.1214/aoms/1177728976. JSTOR 2236286. ^ Brémaud, P. (1999). "Lyapunov Functions and Martingales". Markov Chains. pp. 167. doi:10.1007/978-1-4757-3124-8_5. ISBN 978-1-4419-3131-3. This probability-related article is a stub. You can help Wikipedia by expanding it. Categories: Theorems regarding stochastic processesMarkov processesProbability stubs

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