George M. Bergman,
Supports of derivations, free factorizations,
and ranks of fixed subgroup in free groups,
Trans. Amer. Math. Soc., 351 (1999) 1531-1550.
Warren Dicks has found that Example 16, which he
and Stallings showed me, had already appeared in the literature:
R. G. Burns, T. C. Chau and D. Solitar,
On the intersection of free factors of a free group,
Proc. Amer. Math. Soc. 64 (1977) 43--44.
MR 56#5717 .
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