Reflection subgroups of Coxeter systems (Q923730)

Let \((W,R)\) be a Coxeter system, \(T\) the set of reflections, \(W'=\langle W'\cap T\rangle\) a reflection subgroup. The author shows that a certain subset of \(T\) is a set of Coxeter generators of \(W'\). He also gives a geometric criterion for a set of reflections to be the set of canonical generators of a reflection subgroup and classifies the isomorphism types of reflection subgroups of affine Weyl groups.