On maximal nonparabolic normal subgroups of the modular group

Mason, A. W.; Dickson, N. K.

doi:10.1090/S0002-9939-1979-0542079-1

On maximal nonparabolic normal subgroups of the modular group
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by A. W. Mason and N. K. Dickson PDF

Proc. Amer. Math. Soc. 77 (1979), 167-173 Request permission

Abstract:

Let F be a free normal subgroup of finite index in the modular group $\Gamma$ with $F \ne \Gamma ’.F’$ is known to be a nonparabolic normal subgroup of $\Gamma$. We show that, with two exceptions, there exists another nonparabolic normal subgroup of $\Gamma$ properly containing $F’$.

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