Jørgensen's inequality
In the mathematical theory of Kleinian groups, Jørgensen's inequality is an inequality involving the traces of elements of a Kleinian group, proved by Troels Jørgensen (1976). The inequality states that if A and B generate a non-elementary discrete subgroup of the SL2(C), then | Tr ( A ) 2 − 4 | + | Tr ( A B A − 1 B − 1 ) − 2 | ≥ 1.