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Modular arithmetic of free subgroups

  • Thomas W. Müller and Jan-Christoph Schlage-Puchta
Published/Copyright: July 27, 2005
Forum Mathematicum
From the journal Volume 17 Issue 3

Abstract

Denote by ƒλ(G ) the number of free subgroups of index λmG , where mG is the least common multiple of the orders of the finite subgroups in G. The present paper develops a general theory for the p-divisibility of ƒλ(G ), where p is a prime dividing mG. Among other things, we obtain an explicit combinatorial description of ƒλ(G ) modulo p, leading to an optimal generalisation of Stothers’ explicit formula for the parity of ƒλ(PSL2 (ℤ)).

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Published Online: 2005-07-27
Published in Print: 2005-05-25

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