Abstract
We determine the group structure of the normalizer of Γ0(N) in SL 2(ℝ) modulo Γ0(N). These results correct the Atkin–Lehner statement (Atkin and Lehner, Citation1970, Theorem 8).
ACKNOWLEDGMENTS
Work partially supported by MTM2006-11391.
After the article were accepted we learned that a similar result was obtained by M. Akbas and D. Singerman in The normalizer of Γ0(N) in PSL(2, R) with a different proof. See the postcript. We thank John McKay for drawing the references in the Postcript to our attention.
Notes
Communicated by J. L. Gomez Pardo.