On the intersection of subgroups of a free group
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- Tardos, G. Invent Math (1992) 108: 29. doi:10.1007/BF02100597
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The Hanna Neumann Conjecture says that the intersection of subgroups of rankn+1 andm+1 of a free group has rank at mostnm+1. This paper proves the conjecture for the casem=1. (See Theorem 1.) Our methods imply that the strengthened Hanna Neumann Conjecture is also true in this case (Theorem 2′).