Exercise 15, § 4

*a) Show that the nine Coxeter graphs below$^7$ are of compact hyperbolic type, and that they are, up to isomorphism, the only graphs of rank 4 with this property (use the classification of Chap. VI, § 4):

!Nine Coxeter graphs of rank 4

b) The same question for rank 5, the list consisting of the following five graphs:

!Five Coxeter graphs of rank 5

c) Show that there is no graph of compact hyperbolic type of rank $\geq 6$*