[This is a completely revised version of an earlier preprint which was circulated under the title Reflection triangles and parallel walls in Coxeter complexes, available here: pdf ] |
[Based on two series of lectures, the first one given in the Advanced Class in Algebra at the University of Oxford, Hilary term 2007 and the second given jointly with Bertrand Rémy at the conference Buildings and Groups in Ghent, May 2007. ] |