Exercise 16, § 4

*Show that any Coxeter graph of hyperbolic type of rank $\geq 4$ that has an edge with coefficient 6 is isomorphic to one of the following eleven graphs (which are non-compact and of rank 4):

!Eleven Coxeter graphs of rank 4

(7) In these graphs, any edge that has no label next to it is taken to have coefficient 3 (cf. Chap. IV, § 1, no. 9).