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Results tagged with modular-group
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user 20787
6
votes
Are there noncongruence subgroups (of finite index) of the modular group generated only by 2...
I've compiled my serial comments on Misha's answer with this new answer, not containing any information unavailable in the other answers, but working out Wohlfart's example mentioned by Agol in more e …
- 15.4k
18
votes
Subgroups of $SL_2(\mathbb R)$ which contain $SL_2(\mathbb Z)$ as a finite index subgroup
$G = SL_2(\mathbb{Z})$ is indeed the only possibility. This follows from the structure of the quotient orbifold $\mathcal{O} = \mathbb{H}^2 / SL_2(\mathbb{Z})$, which is a once-punctured sphere with a …
- 15.4k