Abstract:
Let $G$ be a discrete group generated by reflections in hyperbolic or Euclidean space, and let $H\subset G$ be a finite index reflection subgroup. Suppose that the fundamental chamber of $G$ is a finite volume polytope with $k$ facets. We prove that the fundamental chamber of $H$ has at least $k$ facets.
Citation:
P. V. Tumarkin, A. A. Felikson, “Reflection Subgroups of Reflection Groups”, Funktsional. Anal. i Prilozhen., 38:4 (2004), 90–92; Funct. Anal. Appl., 38:4 (2004), 313–314