No CrossRef data available.
Published online by Cambridge University Press: 06 December 2006
Let $\Delta$ denote the (2,3,7)-group. We establish an upper bound for the number of congruence subgroups of index $n$ and a lower bound for the total number of subgroups of index $n$. Since the latter grows more quickly, there exist non-congruence subgroups of index $n$ for all $n$ greater than some $n_0$.