Let $F$ be the free group of rank $\aleph_0$, let $L \leq F$ be a finite index subgroup, and let $M$ be a proper characteristic subgroup of $L$. Is it possible that there exists a finite subset $S \subseteq F$ for which we have $\langle M,S \rangle = F$?
What is the corank of a proper char subgroup of a finite index subgroup of a free group?
Pablo
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