The 4-dimensional double rotations, [2 p,2,2 q ] (with gcd (p, q)=1), which include a central group, and are expressed by Conway as ±[C p ×C q ], order 2 pq. From Coxeter diagram , generators {0,1,2,3}, requires two generator for [2 p,2,2 q ], as {0123,0132}. Half groups, [2 p,2,2 q ], or cyclic graph, [(2 p,2,2 q,2)], expressed by Conway is [C p ×C q ], order pq, with one generator, like {0123}.