Generalized cusps in real projective manifolds: classification

Abstract

We study a generalized cusp C that is diffeomorphic to [ 0 , ∞ ) times a closed Euclidean manifold. Geometrically, C is the quotient of a properly convex domain in R P n by a lattice, Γ, in one of a family of affine Lie groups G ( ψ ) , parameterized by a point ψ in the (dual closed) Weyl chamber for SL ( n + 1 , R ) , and Γ determines the cusp up to equivalence. These affine groups correspond to certain fibered geometries, each of which is a bundle over an open simplex with fiber a horoball in hyperbolic space, and the lattices are classified by certain Bieberbach groups plus some auxiliary data. The cusp has finite Busemann measure if and only if G ( ψ ) contains unipotent elements. There is a natural underlying Euclidean structure on C unrelated to the Hilbert metric.