Consider a semigroup action on a set. We derive conditions, in terms of the induced action of the semigroup on {0,1}-valued probability charges, which ensure that all invariant probability charges are strongly continuous.

The martingale Hardy space $H_p\left({[0,1)}^2\right)$ and the classical Hardy space $H_p{(}^2)$ are introduced. We prove that certain means of the partial sums of the two-parameter Walsh-Fourier and trigonometric-Fourier series are uniformly bounded operators from $H_p$ to $L_p$ (0 < p ≤ 1). As a consequence we obtain strong convergence theorems for the partial sums. The classical Hardy-Littlewood inequality is extended to two-parameter Walsh-Fourier and trigonometric-Fourier coefficients. The dual inequalities are also verified and a...

This paper deals with feedback stabilization of second order equations of the form ytt + A0y + u (t) B0y (t) = 0, t ∈ [0, +∞[, where A0 is a densely defined positive selfadjoint linear operator on a real Hilbert space H, with compact inverse and B0 is a linear map in diagonal form. It is proved here that the classical sufficient ad-condition of Jurdjevic-Quinn and Ball-Slemrod with the feedback control u = ⟨yt, B0y⟩H implies the strong stabilization. This result is derived from a general compactness...

This paper deals with feedback stabilization of second order equations of the form ytt + A0y + u (t) B0y (t) = 0, t ∈ [0, +∞[, where A0 is a densely defined positive selfadjoint linear operator on a real Hilbert space H, with compact inverse and B0 is a linear map in diagonal form. It is proved here that the classical sufficient ad-condition of Jurdjevic-Quinn and Ball-Slemrod with the feedback control u = ⟨yt, B0y⟩H implies the strong stabilization. This result is derived from a general compactness theorem...

We introduce and study strongly invariant means m on commutative hypergroups, $m\left(T_x\phi ·\psi \right)=m\left(\phi ·T_{x̃}\psi \right)$, x ∈ K, $\phi ,\psi \in L^{\infty }\left(K\right)$. We show that the existence of such means is equivalent to a strong Reiter condition. For polynomial hypergroups we derive a growth condition for the Haar weights which is equivalent to the existence of strongly invariant means. We apply this characterization to show that there are commutative hypergroups which do not possess strongly invariant means.

En utilisant la structure infinitésimale des représentations unitaires irréductibles de ${\mathrm{PSL}}_2\left(ℝ\right)$, nous donnons une description complète de certaines $C^{*}$- algèbres associées aux réseaux de ${\mathrm{PSL}}_2\left(ℝ\right)$, répondant ainsi à certaines questions de Bekka–de La Harpe–Valette.

Polynomials on ${ℝ}^n$ with values in an irreducible ${Spin}_n$-module form a natural representation space for the group ${Spin}_n$. These representations are completely reducible. In the paper, we give a complete description of their decompositions into irreducible components for polynomials with values in a certain range of irreducible modules. The results are used to describe the structure of kernels of conformally invariant elliptic first order systems acting on maps on ${ℝ}^n$ with values in these modules.

Let G be a locally compact group, G* be the set of all extreme points of the set of normalized continuous positive definite functions of G, and a(G) be the closed subalgebra generated by G* in B(G). When G is abelian, G* is the set of Dirac measures of the dual group Ĝ, and a(G) can be identified as l¹(Ĝ). We study the properties of a(G), particularly its spectrum and its dual von Neumann algebra.

We consider the heat kernel ${\varphi }_t$ corresponding to the left invariant sub-Laplacian with drift term in the first commutator of the Lie algebra, on a nilpotent Lie group. We improve the results obtained by G. Alexopoulos in [1], [2] proving the “exact Gaussian factor” exp(-|g|²/4(1+ε)t) in the large time upper Gaussian estimate for ${\varphi }_t$. We also obtain a large time lower Gaussian estimate for ${\varphi }_t$.

In this survey article, I shall give an overview on some recent developments concerning the $L^p$-functional calculus for sub-Laplacians on exponential solvable Lie groups. In particular, I shall give an outline on some recent joint work with W. Hebisch and J. Ludwig on sub-Laplacians which are of holomorphic $L^p$-type, in the sense that every $L^p$-spectral multiplier for $p\ne 2$ will be holomorphic in some domain.

Un insieme $S$ in un gruppo $G$ si dice piccolo se esistono infiniti traslati di $S$ a due a due disgiunti. In questa nota dimostriamo in modo elementare che, sotto opportune ipotesi, $G$ non può essere l'unione di un numero finito di insiemi piccoli (e una generalizzazione di questo risultato).