Combinatorial description of the principal congruence subgroup $\gamma$(2) in SL(2, z)

Flavien Mabilat

We characterize sequences of positive integers (c 1 , c 2 , ..., cn) for which the (2 x 2)-matrix c 1 --1 1 0 $\times$ $\times$ $\times$ cn --1 1 0 belongs to the principal congrue…