2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/77091The main result of this paper describes the normalizer of a finite parabolic subgroup of a (possibly infinite) Coxeter group. We use this to compute the automorphism groups of some Lorentzian lattices and K3 surfaces.16 pages, plain tex. References added, minor errors correctedGroup TheoryAlgebraic Geometrytext