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- W1744745307 abstract "A Markov process is defined by its transition matrix. A skip-free Markov process is a stochastic system defined by a level that can only change by one unit either upwards or downwards. A regular perturbation is defined as a modification of one or more parameters that is small enough not to change qualitatively the model.This thesis focuses on a category of methods, called matrix analytic methods, that has gained much interest because of good computational properties for the analysis of a large family of stochastic processes. Those methods are used in this work in order i) to analyze the effect of regular perturbations of the transition matrix on the stationary distribution of skip-free Markov processes; ii) to determine transient distributions of skip-free Markov processes by performing regular perturbations.In the class of skip-free Markov processes, we focus in particular on quasi-birth-and-death (QBD) processes and Markov modulated fluid models.We first determine the first order derivative of the stationary distribution - a key vector in Markov models - of a QBD for which we slightly perturb the transition matrix. This leads us to the study of Poisson equations that we analyze for finite and infinite QBDs. The infinite case has to be treated with more caution therefore, we first analyze it using probabilistic arguments based on a decomposition through first passage times to lower levels. Then, we use general algebraic arguments and use the repetitive block structure of the transition matrix to obtain all the solutions of the equation. The solutions of the Poisson equation need a generalized inverse called the deviation matrix. We develop a recursive formula for the computation of this matrix for the finite case and we derive an explicit expression for the elements of this matrix for the infinite case.Then, we analyze the first order derivative of the stationary distribution of a Markov modulated fluid model. This leads to the analysis of the matrix of first return times to the initial level, a charactersitic matrix of Markov modulated fluid models.Finally, we study the cumulative distribution function of the level in finite time and joint distribution functions (such as the level at a given finite time and the maximum level reached over a finite time interval). We show that our technique gives good approximations and allow to compute efficiently those distribution functions.----------Un processus markovien est defini par sa matrice de transition. Un processus markovien sans sauts est un processus stochastique de Markov defini par un niveau qui ne peut changer que d'une unite a la fois, soit vers le haut, soit vers le bas. Une perturbation reguliere est une modification suffisamment petite d'un ou plusieurs parametres qui ne modifie pas qualitativement le modele.Dans ce travail, nous utilisons des methodes matricielles pour i) analyser l'effet de perturbations regulieres de la matrice de transition sur le processus markoviens sans sauts; ii) determiner des lois de probabilites en temps fini de processus markoviens sans sauts en realisant des perturbations regulieres. Dans la famille des processus markoviens sans sauts, nous nous concentrons en particulier sur les processus quasi-birth-and-death (QBD) et sur les files fluides markoviennes. Nous nous interessons d'abord a la derivee de premier ordre de la distribution stationnaire – vecteur cle des modeles markoviens – d'un QBD dont on modifie legerement la matrice de transition. Celle-ci nous amene a devoir resoudre les equations de Poisson, que nous etudions pour les processus QBD finis et infinis. Le cas infini etant plus delicat, nous l'analysons en premier lieu par des arguments probabilistes en nous basant sur une decomposition par des temps de premier passage. En second lieu, nous faisons appel a un theoreme general d'algebre lineaire et utilisons la structure repetitive de la matrice de transition pour obtenir toutes les solutions a l’equation. Les solutions de l'equation de Poisson font appel a un inverse generalise, appele la matrice de deviation. Nous developpons ensuite une formule recursive pour le calcul de cette matrice dans le cas fini et nous derivons une expression explicite des elements de cette derniere dans le cas infini.Ensuite, nous analysons la derivee de premier ordre de la distribution stationnaire d'une file fluide markovienne perturbee. Celle-ci nous amene a developper l'analyse de la matrice des temps de premier retour au niveau initial – matrice caracteristique des files fluides markoviennes. Enfin, dans les files fluides markoviennes, nous etudions la fonction de repartition en temps fini du niveau et des fonctions de repartitions jointes (telles que le niveau a un instant donne et le niveau maximum atteint pendant un intervalle de temps donne). Nous montrerons que cette technique permet de trouver des bonnes approximations et de calculer efficacement ces fonctions de repartitions." @default.
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- W1744745307 date "2015-06-19" @default.
- W1744745307 modified "2023-09-27" @default.
- W1744745307 title "Skip-free markov processes: analysis of regular perturbations" @default.
- W1744745307 hasPublicationYear "2015" @default.
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