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A congruence test for subgroups of $SL_2(Z)$

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Speaker: 
A. Posingies
Affiliation: 
MPI
Date: 
Wed, 2010-11-24 14:15 - 15:15
Location: 
MPIM Lecture Hall
Parent event: 
Number theory lunch seminar

The subgroups of $SL_2(Z)$ can be divided in two types. There are the congruence subgroup, i.e. subgroups that contain a principal congruence subgroup $\Gamma(N)$, and the non-congruence subgroups. Often, the form a subgroup is given in does not allow an easy and direct determination of the type. In this talk I will present a method to check for a subgroup given by permutations if it is a congruence subgroup or not.

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