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A class of congruence subgroups of Hecke group $H( \lambda_{5})$
by Musa Demirci   İsmail Naci Cangül

Vol. 1 No. 4 (2006) P.549~P.556

ABSTRACT

In [2], it is shown that, for some Hecke groups, unlike the modular group, two de nitions of the principal congruence subgroups may not coincide and congruence and principal congruence subgroups of two important Hecke groups $H(\sqrt{m})$, for $m = 2$ or $3$, are classi ed and the quotients of $H(\sqrt{m})$ with these normal subgroups are given. Here we obtain a classi cation of the congruence subgroups obtained as the kernel of reduction homomorphism for another important Hecke group $H(\lambda_5)$ and also obtain the quotient groups. Finally the indices and abstract group structure of all these subgroups are determined.

KEYWORDS
Congruence subgroups, Hecke groups, level, principal congruence subgroups

MATHEMATICAL SUBJECT CLASSIFICATION 2010
Primary: 20H05, 11F06, 11F41, 11F46

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