Séminaire Géométrie et groupes discrets

The Topology of Compact Manifolds Arising from Anosov Representations

by Prof. Andrew Sanders (Universität Heidelberg)

Amphithéâtre Léon Motchane (IHES)

Amphithéâtre Léon Motchane



An Anosov representation of a word hyperbolic group Γ into a semisimple Lie group G is a dynamically defined strengthening of a quasi-isometric embedding of Γ into G, which serves as a flexible higher rank analogue of the notion of convex-cocompactness. In particular, Anosov representations yield interesting discrete subgroups of G. Guichard-Wienhard and Kapovich-Leeb-Porti constructed co-compact domains of proper discontinuity for these discrete subgroups lying in generalized flag manifolds G/P where P<G is a parabolic subgroup. Distinct domains of discontinuity are indexed by certain special subsets (ideals) in the Weyl group of G with respect to the Bruhat order. In this talk, we discuss the calculation of the homology groups of the quotient manifolds in the case when Γ is a closed surface group, and G is a complex simple Lie group. The formulas express the Betti numbers explicitly in terms of the combinatorial properties of the corresponding subset of the Weyl group of G. This yields a sufficient condition to distinguish the homotopy type of two quotient manifolds obtained from different ideals in the Weyl group. Time permitting, we will present some interesting special cases where the Poincaré polynomial can be expressed as a particularly simple rational function with the degree of the numerator and denominator depending on the genus of the surface.