In the mathematical theory of knots, a **satellite knot** is a knot that contains an incompressible, non boundary-parallel torus in its complement.^{[1]} Every knot is either hyperbolic, a torus, or a satellite knot. The class of satellite knots include composite knots, **cable knots** and **Whitehead doubles.** (*See* Basic families, below for definitions of the last two classes.) A satellite *link* is one that orbits a companion knot *K* in the sense that it lies inside a regular neighborhood of the companion.^{[2]}^{:217}

A satellite knot can be picturesquely described as follows: start by taking a nontrivial knot lying inside an unknotted solid torus . Here "nontrivial" means that the knot is not allowed to sit inside of a 3-ball in and is not allowed to be isotopic to the central core curve of the solid torus. Then tie up the solid torus into a nontrivial knot.

This means there is a non-trivial embedding and . The central core curve of the solid torus is sent to a knot , which is called the "companion knot" and is thought of as the planet around which the "satellite knot" orbits.The construction ensures that is a non-boundary parallel incompressible torus in the complement of . 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 is an unknotted solid torus, is a tubular neighbourhood of an unknot . The 2-component link together with the embedding is called the *pattern* associated to the satellite operation.

A convention: people usually demand that the embedding is *untwisted* in the sense that must send the standard longitude of to the standard longitude of . Said another way, given two disjoint curves , must preserve their linking numbers i.e.: .

## Contents

## Basic families

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

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

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

## 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.

## Origins

In 1949 ^{[3]} Horst Schubert proved that every oriented knot in 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 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*,^{[4]} where he defined satellite and companion knots.

## 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.^{[5]} 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. The ramifications for knot theory were first described in the long-unpublished manuscript of Bonahon and Siebenmann.^{[6]}

## 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.^{[7]} With a suitably enhanced notion of satellite operation called splicing, the JSJ decomposition gives a proper uniqueness theorem for satellite knots.^{[8]}^{[9]}

## See also

## References

**^**Colin Adams,*The Knot Book: An Elementary Introduction to the Mathematical Theory of Knots*, (2001), ISBN 0-7167-4219-5**^**Menasco, William; Thistlethwaite, Morwen, eds. (2005).*Handbook of Knot Theory*. Elsevier. ISBN 0080459544. Retrieved 2014-08-18.**^**Schubert, H. Die eindeutige Zerlegbarkeit eines Knotens in Primknoten. S.-B Heidelberger Akad. Wiss. Math.-Nat. Kl. 1949 (1949), 57–104.**^**Schubert, H. Knoten und Vollringe. Acta Math. 90 (1953), 131–286.**^**Waldhausen, F. On irreducible 3-manifolds which are sufficiently large.Ann. of Math. (2) 87 (1968), 56–88.**^**F.Bonahon, L.Siebenmann, New Geometric Splittings of Classical Knots, and the Classification and Symmetries of Arborescent Knots, [1]**^**Motegi, K. Knot Types of Satellite Knots and Twisted Knots. Lectures at Knots '96. World Scientific.**^**Eisenbud, D. Neumann, W. Three-dimensional link theory and invariants of plane curve singularities. Ann. of Math. Stud. 110**^**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