On groups with uncountably many subgroups of finite index

  title={On groups with uncountably many subgroups of finite index},
  author={Daniel S. Silver and Susan G. Williams},
Let K be the kernel of an epimorphism χ : G → Z, for G a finitely presented group. If K has uncountably many normal subgroups of finite index r, then K has uncountably many subgroups (not necessarily normal) of any finite index greater than r. In particular, this is the case whenever G is subgroup separable and K is nonfinitely generated. Assume that G has an abelian HNN base contained in K. If K has infinitely many subgroups of a given finite index, then it has uncountably many. 

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