Pencils of geodesics in symmetric spaces, Karpelevich boundary, and associahedron-like polyhedra

Abstract

There are several ways of a construction of a boundary of a symmetric space using pencils of geodesics: the Karpelevich boundary, the visibility boundary, the associahedral boundary, and the sea urchin. We give explicit descriptions of these boundaries. We obtain some moduli space like polyhedra as sections of these compactifications by Cartan subspaces. For simplicity, we consider only the space $GL_n/O_n$ of ellipsoids in $R^n$.