On Normal Subgroups of Coxeter Groups Generated by Standard Parabolic   Subgroups
  We discuss one construction of nonstandard subgroups in the category of Coxeter groups.   Two formulae for the growth series of such a subgroups are given.   As an application we construct a flag simple convex polytope, whose f-polynomial has non-real roots. 
Swiatoslaw R. Gal