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Theses

Théorèmes ergodiques, actions de groupes et représentations unitaires

Abstract : In this thesis, we first study the notion of discrepance, which measures the rate of convergence of ergodic means. We prove estimations for the discrepancy of actions on the sphere, the torus and the Bernoulli shift, as well as for actions of locally compact groups. Moreover, we prove an inequality that allows us to locate these discrepancies in the larger framework of the Monte-Carlo method. We consider the action of the free group on the boundary of its Cayley tree. We prove a convergence theorem of some means associated with this action, that only preserves the class of the natural measures on this boundary. We recover the previously known result that the unitary representation associated to it is irreducible. We then investigate the Howe-Moore property. Groups that satisfy it have the property that whenever they act ergodically on some probability space, then the action is mixing ; unfortunately, this property is not stable by direct products. We formulate a generalization of the Howe-Moore property, relying on an axiomatization of the Mautner phenomenon, that allows us to treat the case of products. Finally, we prove that every lattice inherits the radial rapid decay property, and give an explicit example of a discrete group, endowed with a natural length function which is quasi-isometric to a word-length, that has RRD but doesn't have RD.
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Submitted on : Thursday, April 2, 2020 - 8:37:00 PM
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Antoine Pinochet Lobos. Théorèmes ergodiques, actions de groupes et représentations unitaires. Mathématiques [math]. Aix-Marseille Université (AMU), 2019. Français. ⟨tel-02530349⟩

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