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Suppose F is a compact orientable surface, K is a knot in F × I, and (F × I)surg is
the 3–manifold obtained by some nontrivial surgery on K. If F ×{0} compresses in
(F ×I)surg, then there is an annulus in F ×I with one end K and the other end an
essential simple closed curve in F ×{0}. Moreover, the end of the annulus at K
determines the surgery slope.
An application: Suppose M is a compact orientable 3–manifold that
fibers over the circle. If surgery on K ⊂ M yields a reducible manifold, then
either
* the projection K ⊂ M → S1 has nontrivial winding number,
* K lies in a ball,
* K lies in a fiber, or
* K is cabled.
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