Bruno Poizat, Frank Olaf Wagner
If $G$ is an omega-stable group with a normal definable subgroup $H$, then the Sylow-$2$-subgroups of $G/H$ are the images of the Sylow-$2$-subgroups of $G$.
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We describe the nilpotent subgroups of the group Bir(P^2(C)) of birational transformations of the complex projective plane. Let N be a nilpotent subgroup of class k>1; then either each element of N has finite order, or N is virtually metabelian.
Let G be a branch group (as defined by Grigorchuk) acting on a tree T. A parabolic subgroup P is the stabiliser of an infinite geodesic ray in T. We denote by $ρ_{G/P}$ the associated quasi-regular representation. If G is discrete, these representations are irreducible, but if G is profinite, they split as a direct sum of finite-dimensionalrepresentations $ρ_{G/P_{n+1}}\ominusρ_{G/P_n}$, where P_n is the stabiliser of a level-n vertex in T. For a few concrete examples, we completely split $ρ_{G/P_n}$ in irreducible components. $(G,P_n)$ and $(G,P)$ are Gelfand pairs, whence new occurrences of …
Dans cet article, on montre que l'espace des groupes marqués est un sous-espace fermé d'un ensemble de Cantor dont la dimension de Hausdorff est infinie. On prouve que la dimension de Minkowski de cet espace est infinie en exhibant des sous-ensembles de groupes marqués à petite simplification dont les dimensions de Minkowski sont arbitrairement grandes. On donne une estimation des dimensions de Minkowski de sous-espaces de groupes à un relateur. On démontre enfin que les dimensions de Minkowski du sous-espace des groupes commutatifs marqués et d'un ensemble de Cantor défini par Grigorchuk sont…
The space of closed subgroups of a locally compact topological group is endowed with a natural topology, called the Chabauty topology. We completely describe the space of closed sugroups of the group RxZ, which is not trivial : for example, its fundamental group is uncountable.
In this paper, we present a simple proof of the fact that any compact subgroup of homeomorphisms of the 2-sphere is topologically conjugate to a closed subgroup of the orthogonal group O(3).
In this article we give a classification of the sub-groups in PSL(2,Z) and of the conjugacy classes of these sub-groups by the mean of an combinatorial invariant: some trivalent diagrams (dotted or not). We give explicit formulae enabling to count the number of isomorphism classes of these structures and of some of their variations, as function of the number of their arcs. Until now, the counting of non-dotted diagrams was an open problem, for it gives also the number of unrooted combinatorial maps, triangular or general respectively. The article ends with the description of a high performance…
Arithmétique des sous-groupes de Bruhat-Tits sur un anneau de valuation discrète hensélien
本文细化了Bruhat-Tits子群的上同调研究，补充1987年工作，并对半单单连通群得到Grothendieck-Serre型结果及拟分裂伴随群情形的障碍精确计算。
Nous explicitons les centralisateurs dans un sous-groupe discret cocompact d'isometrie du plan euclidien ----- We make explicit the centralizers of a discrete cocompact subgroup of isometries of the Euclidean plane.
Let G be the set of k-points of a connected semisimple algebraic group of k-rank one over a nonarchimedean local field k. We describe all finitely generated torsion-free discrete subgroups of G\times G acting properly discontinuously and cocompactly on G by left and right multiplication. We prove that after a small deformation in G\times G such a discrete subgroup keeps acting freely, properly discontinuously, and cocompactly on G. ----- Soit G l'ensemble des k-points d'un groupe algebrique semi-simple connexe de k-rang un sur un corps local ultrametrique k. Nous decrivons tous les sous-groupe…
Nous montrons la propriété du trou spectral pour la famille des graphes de Cayley obtenus par réduction modulo $q$ d'un sous-groupe de $\mathrm{SL}_d(\mathbb{Z})$ dont l'adhérence de Zariski est un $\mathbb{Q}$-groupe simple. -- We show a spectral gap property for the family of Cayley graphs obtained by reduction modulo $q$ of a subgroup of $\mathrm{SL}_d(\mathbb{Z})$ whose Zariski closure is a simple $\mathbb{Q}$-group.
By underlying the commutation relation in a right-angled Coxeter group W, we recover the fact that right-angled Coxeter groups are rigid and we describe the second subgroup of Aut(W) that appears in the decomposition of Aut(W) into a semi-direct product established by J. Tits in "Sur le groupe des automorphismes de certains groupes de Coxeter" (Journal of Algebra 113, (1988), 346-357). ----- En mettant l'accent sur la relation de commutation dans un groupe de Coxeter a angles droits W, on redemontre le fait que les groupes de Coxeter a angles droits sont rigides et on decrit le second sous-gro…
This is a thoroughly revised version of math.AG/0502516v1 (24 Feb. 2005). Let k be a field of characteristic zero. Let Y=G/H, where G is a connected linear algebraic group over k and H is a connected closed k-subgroup of G. Let X be a smooth compactification of Y over k. We prove the conjecture set forward in the previous version : the Galois-lattice given by the geometric Picard group of X is flasque. The previous version had only partial results in this direction. They were obtained at the expense of a long d'etour via local and global fields. We owe the drastic improvement to a suggestion b…
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