Geodesic
The normalizer of Γ0(N) in PSL(2, R)
Glasgow mathematical journal, Tome 32 (1990) no. 3, pp. 317-327

Voir la notice de l'article provenant de la source Cambridge University Press

Let Γ denote the modular group, consisting of the Möbius transformationsAs usual we denote the above transformation by the matrix remembering that V and – V represent the same transformation. If N is a positive integer we let Γ0(N) denote the transformations for which c ≡ 0 mod N. Then Γ0(N) is a subgroup of indexthe product being taken over all prime divisors of N. 
Akbas, M.; Singerman, D. The normalizer of Γ0(N) in PSL(2, R). Glasgow mathematical journal, Tome 32 (1990) no. 3, pp. 317-327. doi: 10.1017/S001708950000940X
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