can someone please tell me? I couldn't find a reference in the paper I was reading.
1 Answer
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Subgroups of finite index in free groups. Canadian J.Math., 1:187–190, 1949.
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1$\begingroup$ This is not a result of Hall; Hall counted precisely the number of such subgroups. Here are links to the paper: cms.math.ca/cjm/v1/p187 ams.org/mathscinet-getitem?mr=28836 $\endgroup$ Feb 16, 2010 at 5:00