N=Matrix(2,2,[[n,1],[1,0]])
where n integer >=1. Unlike modular group gamma the decomposition of
any element
Matrix(2,2,[[a,b],[c,d]])=N_1.N_2.N_3...N_k
is always unique. The other distinction is that the determinant is
always either 1 or -1. What is the name of this subgroup?