In the second case, Iq ◦ Ip 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).
Finally, in the third case the map Iq ◦ Ip is of elliptic type and corresponds to a
rotation with center
◦. The angle paq is half the rotation angle (to see this, use the
Poincar´e model); hence ∠paq = ∠sar is equivalent to (5).