can someone please tell me? I couldn't find a reference in the paper I was reading.

Subgroups of finite index in free groups. Canadian J.Math., 1:187–190, 1949. 


can someone please tell me? I couldn't find a reference in the paper I was reading. 


Subgroups of finite index in free groups. Canadian J.Math., 1:187–190, 1949. 

