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Published online by Cambridge University Press: 09 April 2009
We shall be concerned with problem 7 of Hanna Neumann [7], which asks: Prove or disprove that if U and B are varieties, and neither of U and B is contained in the other, then U ∪ B and [U, B] are decomposable if and only if U and B have a common non-trivial right hand factor.