Normalizers of congruence groups in SL<sub>2</sub>(โ) and automorphisms of lattices
Normalizers of congruence groups in SL<sub>2</sub>(โ) and automorphisms of lattices
We determine the normalizer in SL 2 (โ) of several families of congruence subgroups of SL 2 (โค). In addition, we show how these tools can be used to evaluate the groups of automorphisms and the discriminant kernels of a large family of isotropic lattices of signature (2,1).