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We find a geometric invariant of isotopy classes of strongly irreducible Heegaard
splittings of toroidal 3–manifolds. Combining this invariant with a theorem of R
Weidmann, proved here in the appendix, we show that a closed, totally orientable
Seifert fibered space M has infinitely many isotopy classes of Heegaard splittings of
the same genus if and only if M has an irreducible, horizontal Heegaard splitting, has
a base orbifold of positive genus, and is not a circle bundle. This characterizes
precisely which Seifert fibered spaces satisfy the converse of Waldhausen’s
conjecture.
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