In the second case, Iq ◦ I p is of parabolic type, and the corresponding hyperbolic
isometry maps a horocycle with center at a to itself. The number 1
a−p − 1
a−q is proportional
to the translation length along the horocycle (the horocyclic coordinate can
be measured by the stereographic projection from the point a) so that (2) is again
equivalent to (5).