Évènements

Après avoir introduit les U-statistiques à  deux échantillons,
nous présenterons
une version empirique de ces-dernières. Ceci permet de détecter un
potentiel changement de loi
dans un échantillon. Nous allons donner des conditions suffisantes pour
la convergence
des U-statistiques à  deux échantillons dans un espace fonctionnel
approprié ainsi qu’une description du processus limite.
Il s’agit d’un travail réalisé en collaboration avec Herold Dehling
(Ruhr-Universität Bochum) et Olimjon Sharipov (National University of
Uzbekistan)

We consider Lie groups $G$ endowed with a pair of anticommuting left-invariant abelian complex structures $(J_1,J_2)$ and a left-invariant, possibly indefinite, metric $g$ such that $(G,J_1,J_2,g)$ results to be a hyperkähler manifold. We study the algebraic structure and geometric properties of such Lie groups with an abelian hyperkähler structure. It results that such groups are always 3-step nilpotent and there is a correspondence between the associated hyperkähler Lie algebras and certain triples $(V,Omega, J_s)$ defined for a complex (associative) commutative algebra $V$ such that $V^3={0}$. This correspondence allows us to compute the Riemannian curvature of the pseudo-metric, describe the holonomy algebra and show that hyperkähler Lie groups with abelian complex structures are complete and locally symmetric. This clearly implies that every simply-connected Lie group endowed with an abelian hyperkähler structure is actually a symmetric space. In constrast to the definite case, there exist non-flat examples of abelian hyperkähler Lie groups; they cannot be 2-step nilpotent and their dimension is always equal to or greater than 16. Moreover, using the triple description, we classify up to Lie algebra isomorphism all Lie algebras $g$ admitting an abelian hyperkähler structure for $dimgle 12$. Some remarks on their classification up to triholomorphic symplectomorphism will also be mentioned. [BS_HK] I. Bajo, E. Sanmart'{i}n, « Indefinite hyperkähler metrics on Lie groups with abelian complex structures », 2019, to appear in Transformation Groups.

Let (X=G/K) be a noncompact complex Grassmann manifold of rank (r). Let (tau_l) be a character of (K), and (Ktimes_M{C}) the homogeneous line bundle associated with (tau_{l_{mid M}}). We give an image characterization for the Poisson transform (P_{lambda,l}) of (L^2)-sections of (Ktimes_M{C}). More precisely, for real and regular spectral parameter (lambda) in (mathfrak{a}^ast) we prove that (P_{lambda,l}) is an isomorphism from (L^2(Ktimes_M{C})) onto a space of joint eigensections (F) of the algebra of (G)-invariant differential operators on (Gtimes_K{C}) that satisfy (displaystylesup_{R>1}frac{1}{R^r}int_{B(R)}mid F(g)mid^2, {rm d}g<infty.) This generalizes a conjecture by Strichartz which corresponds to (tau_l) trivial.\