Satellite knot

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In the mathematical theory of knots, a satellite knot is a knot which contains an incompressible, non-boundary parallel torus in its complement ^[1]. The class of satellite knots include composite knots, cable knots and Whitehead doubles.

A satellite knot K can be picturesquely described as follows: start by taking a nontrivial knot K' lying inside an unknotted solid torus V. Here "nontrivial" means that the knot K' is not allowed to sit inside of a 3-ball in V, also V is not allowed to be a disc bundle over K'. Then tie up the solid torus into a nontrivial knot.

This means there is a non-trivial embedding and K = f(K'). The construction ensures that is a non-boundary parallel incompressible torus in the complement of K. In this case, K' is called the companion knot to the satellite knot K. Composite knots contain a certain kind of incompressible torus called a swallow-follow torus, which can be visualized as swallowing one summand and following another summand.

Since V is an unknotted solid torus, is a tubular neighbourhood of an unknot J. The 2-component link together with the embedding f is called the pattern associated to the Satellite operation.

A convention: people usually demand that the embedding is untwisted in the sense that f must send the standard longitude of V to the standard longitude of f(V). Said another way, given two disjoint curves , f must preserve their linking numbers ie: lk(f(c₁),f(c₂)) = lk(c₁,c₂).

Contents 1 Basic families 2 Examples 3 Origins 4 Follow-up work 5 Uniqueness of satellite decomposition 6 References

[edit] Basic families

When is a torus knot, then K is called a cable knot. Examples 3 and 4 are cable knots.

If K' is a non-trivial knot in S³ and if a compressing disc for V intersects K' in precisely one point, then K is called a connect-sum. Another way to say this is that the pattern is the connect-sum of a non-trivial knot K' with a Hopf link.

If the link is the Whitehead link, K is called a Whitehead double. If f is untwisted, K is called an untwisted Whitehead double.

[edit] Examples

Example 1: The connect-sum of a figure-8 knot and trefoil.

Example 2: Untwisted Whitehead double of a figure-8.

Example 3: Cable of a connect-sum.

Example 4: Cable of trefoil.

Examples 5 and 6 are variants on the same construction. They both have two non-parallel, non-boundary-parallel incompressible tori in their complements, splitting the complement into the union of three manifolds. In Example 5 those manifolds are: the Borromean rings complement, trefoil complement and figure-8 complement. In Example 6 the figure-8 complement is replaced by another trefoil complement.

[edit] Origins

In 1949 ^[2] Horst Schubert proved that every oriented knot in S³ decomposes as a connect-sum of prime knots in a unique way, up to reordering, making the monoid of oriented isotopy-classes of knots in S³ a free commutative monoid on countably-infinite many generators. Shortly after, he realized he could give a new proof of his theorem by a close analysis of the incompressible tori present in the complement of a connect-sum. This led him to study general incompressible tori in knot complements in his epic work Knoten und Vollringe ^[3], where he defined satellite and companion knots.

[edit] Follow-up work

Schubert's demonstration that incompressible tori play a major role in knot theory was one several early insights leading to the unification of 3-manifold theory and knot theory. It attracted Waldhausen's attention, who later used incompressible surfaces to show that a large class of 3-manifolds are homeomorphic if and only if their fundamental groups are isomorphic ^[4]. Waldhausen conjectured what is now the Jaco-Shalen-Johannson-decomposition of 3-manifolds, which is a decomposition of 3-manifolds along spheres and incompressible tori. This later became a major ingredient in the development of geometrization, which can be seen as a partial-classification of 3-dimensional manifolds.

[edit] Uniqueness of satellite decomposition

In Knoten und Vollringe, Schubert proved that in some cases, there is essentially a unique way to express a knot as a satellite. But there are also many known examples where the decomposition is not unique ^[5]. With a suitably-enhanced notion of satellite operation called splicing, the JSJ-decomposition gives a proper uniqueness theorem for satellite knots ^{[6] [7]}.

[edit] References

1. ^ Colin Adams, The Knot Book: An Elementary Introduction to the Mathematical Theory of Knots, (2001), ISBN 0-7167-4219-5
2. ^ Schubert, H. Die eindeutige Zerlegbarkeit eines Knotens in Primknoten. S.-B Heidelberger Akad. Wiss. Math.-Nat. Kl. 1949 (1949), 57--104.
3. ^ Schubert, H. Knoten und Vollringe. Acta Math. 90 (1953), 131--286.
4. ^ Waldhausen, F. On irreducible 3-manifolds which are sufficiently large.Ann. of Math. (2) 87 (1968), 56--88.
5. ^ Motegi, K. Knot Types of Satellite Knots and Twisted Knots. Lectures at Knots '96. World Scientific.
6. ^ Eisenbud, D. Neumann, W. Three-dimensional link theory and invariants of plane curve singularities. Ann. of Math. Stud. 110
7. ^ Budney, R. JSJ-decompositions of knot and link complements in S^3. L'enseignement Mathematique 2e Serie Tome 52 Fasc. 3-4 (2006). arXiv:math.GT/0506523