We determine the normalizer in SL2(R) of several families of congruence subgroups of SL2(Z). 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).
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© 2017 World Scientific Publishing Company.
- Congruence subgroups
- automorphisms of lattices
- generalized Atkin-Lehner involutions