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The Manin-Drinfeld theorem asserts that for a modular curve $X_0(N)$ and Jacobian $J_0(N)$ with the former being embdedded in the latter under the map that takes $i\infty$ to $0$, the cusps are torsion.

The proof Manin-Drinfeld theorem seems to use Hecke operators and thus seems to be valid only for congruence subgroups of $PSL_2(\mathbb Z)$. Is there possibly a finite-index subgroup of $PSL_2(\mathbb Z)$ such that the statement of Manin-Drinfeld theorem is still true for the quotient?

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    You might have better luck with this question at mathoverflow.net2010-11-24

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