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We show that if two 3–manifolds with toroidal boundary are glued via a “sufficiently
complicated” map then every Heegaard splitting of the resulting 3–manifold
is weakly reducible. Additionally, suppose X ∪FY is a manifold obtained
by gluing X and Y , two connected small manifolds with incompressible
boundary, along a closed surface F. Then the following inequality on genera is
obtained:
Both results follow from a new technique to simplify the intersection between an
incompressible surface and a strongly irreducible Heegaard splitting.
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