Coxeter writes the rank 2 complex group, p [ q ] r represents Coxeter diagram . The p and r should only be suppressed if both are 2, which is the real case [ q ]. The order of a rank 2 group p [ q ] r is g = 8 / q ( 1 / p + 2 / q + 1 / r − − 1 ) − − 2 .