Séminaire de Mathématique

# Geometric construction of buildings for hyperbolic Kac-Moody groups

## by Prof. Alex FEINGOLD (Binghamton University, SUNY & IHÉS)

Europe/Paris
Amphithéâtre Léon Motchane (IHES)

### Amphithéâtre Léon Motchane

#### IHES

Le Bois Marie 35, route de Chartres 91440 Bures-sur-Yvette
Description
A twin building is a simplicial complex associated to a group G with a twin BN-pair. A complex hyperbolic Kac-Moody (KM) group G is associated with a hyperbolic KM Lie algebra g = g(A), where A is a hyperbolic type Cartan matrix. The invariant symmetric bilinear form (. , .) on the standard Cartan subalgebra h in g has signature (n-1,1) on the split real form of h, providing a Lorentzian geometry. The Cartan-Chevalley involution on g gives a compact" real form k of g, a real Lie algebra whose complexification is g, whose Cartan subalgebra t also has Lorentzian form (. , .), and there is also a corresponding compact real group K. We are able to embed the twin building for G inside the union of all lightcones" {x in k | (x,x) <= 0} which is in the union of all K conjugates of t.  This provides a geometrical realization of the twin building of G closely related to the structure of all Cartan subalgebras in k, and sheds light on the geometry of the infinite dimensional groups G and K. This is especially interesting in the case of rank 3 hyperbolic algebras whose Weyl groups are hyperbolic triangle groups, so that the building is a union of copies of the tesselated Poincaré disk with certain boundary lines identified. This is joint work with Lisa Carbone (Rutgers University) and Walter Freyn (TU-Darmstadt).
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