In 2005, the paper [KPT05] by Kechris, Pestov and Todorcevic provided a powerful tool to compute an invariant of topological groups known as the universal minimal flow. This immediately led to an explicit representation of this invariant in many concrete cases. However, in some particular situations, the framework of [KPT05] does not allow one to perform the computation directly, but only after a slight modification of the original argument. The purpose of the present paper is to supplement [KPT05]...

We prove several optimal Moser–Trudinger and logarithmic Hardy–Littlewood–Sobolev inequalities for systems in two dimensions. These include inequalities on the sphere $S^2$, on a bounded domain $\Omega \subset {ℝ}^2$ and on all of ${ℝ}^2$. In some cases we also address the question of existence of minimizers.

On présente des conditions suffisantes pour qu’une extension HNN soit intérieurement moyennable, respectivement CCI, qui donnent des critères nécessaires et suffisants parmi les groupes de Baumslag-Solitar. On en déduit qu’un tel groupe, vu comme groupe d’automorphismes de son arbre de Bass-Serre, possède des éléments non triviaux qui fixent des sous-arbres non bornés.

Il est bien connu qu’une fonction $f$ sur ${ℝ}^n$ est harmonique - Δf = 0 - si et seulement si sa moyenne sur toute sphère est égale à sa valeur au centre de cette sphère. De manière semblable, f vérifie l’équation de Helmholtz Δf + cf = 0 si et seulement si sa moyenne sur la sphère de centre x et de rayon r vaut $\Gamma (n/2){(r\surd c/2)}^{(2-n)/2}{J}_{(n-2)/2}\left(r\surd c\right)·f\left(x\right)$. Dans ce travail, nous généralisons ces résultats à l’opérateur ${(\Delta +c)}^k$ où k est un entier strictement positif et c une constante non nulle. Bien qu’une méthode pour y parvenir soit esquissée dans...

Let p,q be positive integers. The groups $U_p\left(ℂ\right)$ and $U_p\left(ℂ\right)\times U_q\left(ℂ\right)$ act on the Heisenberg group $H_{p,q}:=M_{p,q}\left(ℂ\right)\times ℝ$ canonically as groups of automorphisms, where $M_{p,q}\left(ℂ\right)$ is the vector space of all complex p × q matrices. The associated orbit spaces may be identified with ${\Pi }_q\times ℝ$ and ${\Xi }_q\times ℝ$ respectively, ${\Pi }_q$ being the cone of positive semidefinite matrices and ${\Xi }_q$ the Weyl chamber $x\in {ℝ}^q:x₁\ge \cdots \ge x_q\ge 0$. In this paper we compute the associated convolutions on ${\Pi }_q\times ℝ$ and ${\Xi }_q\times ℝ$ explicitly, depending on p. Moreover, we extend these convolutions by analytic continuation to series of convolution...

We examine multiple disjointness of minimal flows, that is, we find conditions under which the product of a collection of minimal flows is itself minimal. Our main theorem states that, for a collection ${X_i}_{i\in I}$ of minimal flows with a common phase group, assuming each flow satisfies certain structural and algebraic conditions, the product ${\prod }_{i\in I}{X}_i$ is minimal if and only if ${\prod }_{i\in I}{X}_i^{eq}$ is minimal, where $X_i^{eq}$ is the maximal equicontinuous factor of $X_i$. Most importantly, this result holds when each $X_i$ is distal. When the phase...

On montre, pour une classe particulière de groupes non-unimodulaires $G=N⋌ℝ$, où $N$ est un groupe de Lie stratifié et où l’action de $ℝ$ est définie par les dilatations naturelles de $N$, et pour les sous-laplaciens invariants à gauche correspondants $\Delta$, que toute fonction $m\in H^{2+ϵ}\left(ℝ\right)$ possédant un support compact dans ${ℝ}_{+}$ définit un opérateur $m\left(\Delta \right)$ borné sur les espaces de Lebesgue $L^p(G,d^{r}g)$ associés à la mesure de Haar invariante à droite sur $G$, $1\le p\le \infty$.

AMS Subj. Classification: MSC2010: 42C10, 43A50, 43A75We perform analysis of certain aspects of approximation in multiplicative systems that appear as duals of ultrametric structures, e.g. in cases of local fields, totally disconnected Abelian groups satisfying the second axiom of countability or more general ultrametric spaces that do not necessarily possess a group structure. Using the fact that the unit sphere of a local field is a Vilenkin group, we introduce a new concept of differentiation in...

We prove that on a product of generalized Heisenberg groups, a Hörmander type multiplier theorem for Rockland operators is true with the critical index n/2 + ϵ, ϵ>0, where n is the euclidean (topological) dimension of the group.