The generalized triangle inequalities in symmetric spaces and buildings with applications to algebra

Abstract

In this paper we apply our results on the geometry of polygons in infinitesimal symmetric spaces, symmetric spaces and buildings, [KLM1, KLM2], to four problems in algebraic group theory. Two of these problems are generalizations of the problems of finding the constraints on the eigenvalues (resp. singular values) of a sum (resp. product) when the eigenvalues (singular values) of each summand (factor) are fixed. The other two problems are related to the nonvanishing of the structure constants of the (spherical) Hecke and representation rings associated with a split reductive algebraic group over Q and its complex Langlands’ dual. We give a new proof of the “Saturation Conjecture” for GL(l) as a consequence of our solution of the corresponding “saturation problem” for the Hecke structure constants for all split reductive algebraic groups over Q.