On the Index of Congruence Subgroups of Aut(F_n)

Details On the Index of Congruence Subgroups of Aut(F_n) On the Index of Congruence Subgroups of Aut(F_n)

2008-04-16

arxiv.org/abs/0804.2578v1

J. Algebra 321 (2009), 2875-2889.

Mathematics

Group Theory

Scientific paper

For an epimorphism pi of the free group F_n onto a finite group G write Gamma(G,pi) for the group of all automorphisms f of F_n for which pi*f = pi. This is called the standard congruence subgroup of Aut(F_n) associated to G and pi. In the case n = 2 we present formulas for the index of Gamma(G,pi) where G is abelian or dihedral. Moreover, we show that congruence subgroups associated to dihedral groups provide a family of subgroups of arbitrary large index in Aut(F_2) generated by a fixed number of elements. This implies that finite index subgroups of Aut(F_2) cannot be written as free products.

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