In this paper we consider Jørgensen's inequalities for classical Schottky groups of the real type. The infimum of Jørgensen's numbers for groups of types II, V and VII are 16, 4 and 4 , respectively, each of which is the best possible for Jørgensen's inequality.
Hiroki SATO. "Jørgensen's inequality for classical Schottky groups of real type." J. Math. Soc. Japan 50 (4) 945 - 968, October, 1998. https://doi.org/10.2969/jmsj/05040945