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150 W. MENASCO AND X. ZHANG
on a non-trivial knot in S 3 produces a reducible manifold, then the surgery
slope is not 0 [Ga], in fact it must be an integer [GL2] with absolute value
larger than one [GL3], and the reducible manifold contains a lens space
summand [GL3]. In Section 4, we look at the conjecture from the point of
view of tangle sums. Namely, if some surgery on a knot in S 3 produces a
reducible manifold, then a reducing 2-sphere will decompose the resulting
manifold into a union of two tangles whose strands come from the core of
the filling solid torus. We give some partial results along this line. Notably,
we show that such tangle decomposition cannot give summands which are
2-strand tangles.
Our investigation of the cabling conjecture is extended to Section 5 but
restricted to the class of knots in S 3 whose exteriors do not contain meridionally-incompressible
closed essential surfaces. Note that this class of knots
include all alternating knots [M], almost alternating knots, toroidally alternating
knots and Montesinos knots [A]. We show that if such knot exterior
admits a filling yielding a reducible manifold, then the reducible manifold
must be a connected sum of two lens spaces. We also show that such knot
exterior does not admit any Dehn filling producing a large Seifert fibered
space. It is a conjecture that every knot in S 3 does not admit a surgery yielding
a large Seifert fibered 3-manifold. This conjecture can be considered as
an extension of the cabling conjecture, in some sense.
Studying exceptional Dehn surgery (filling) on hyperbolic knot (exterior),
i.e., surgery (filling) which produces non-hyperbolic manifolds, is a basic
subject in 3-manifold topology. Most of the problems and results described
above belong to this subject. Given a knot K in a compact orientable 3manifolds
W with non-empty boundary such that the exterior of K in W
is a simple manifold, an exceptional surgery on W along K will produce a
manifold which contains either a reducing 2-sphere, or essential annulus, or
∂-reducing disk or essential torus. Sharp upper bounds on the distances (i.e.,
the geometric intersection numbers) between all these types of exceptional
surgery slopes have been found. (See [GW, Introduction] for a table summary
of these bounds.) But if one singles out Seifert Dehn surgery–surgery
which yields a Seifert fibered space– as a special type of exceptional surgery,
then it is still unknown what are the optimal upper bounds on the distances
between a Seifert Dehn surgery slope and other types of exceptional surgery
slopes. There are 5 bounds to be determined. In Section 6, the last section
of the paper, we resolve this issue in four of the total five cases. We show
that if one surgery on W along K produces a Seifert surgery and another
surgery on W along K produces a reducible or ∂-reducible manifold, then
the distance between the two surgery slopes is at most one. Previous known
bounds in both cases were 2, obtained in [W2] and [GW] respectively. The
new bound one is optimal as it can be realized by infinitely many examples
found in [EW, Section 2]. We also show that if one surgery on W along