Prime knot

In knot theory, a prime knot or prime link is a knot that is, in a certain sense, undecomposable. Specifically, it is a non-trivial knot which cannot be written as the knot sum of two non-trivial knots. Knots that are not prime are said to be composite knots or composite links. It can be a nontrivial problem to determine whether a given knot is prime or not.

Simplest prime link

A family of examples of prime knots are the torus knots. These are formed by wrapping a circle around a torus p times in one direction and q times in the other, where p and q are coprime integers.

The simplest prime knot is the trefoil with three crossings. The trefoil is actually a (2, 3)-torus knot. The figure-eight knot, with four crossings, is the simplest non-torus knot. For any positive integer n, there are a finite number of prime knots with n crossings. The first few values (sequence A002863 in the OEIS) are given in the following table.

n 12345678910111213141516
Number of prime knots
with n crossings		 0011237214916555221769988469722532931388705
Composite knots 00000214............
Total 001125825............

Enantiomorphs are counted only once in this table and the following chart (i.e. a knot and its mirror image are considered equivalent).

A chart of all prime knots with seven or fewer crossings, not including mirror-images, plus the unknot (which is not considered prime).

Schubert's theorem

A theorem due to Horst Schubert states that every knot can be uniquely expressed as a connected sum of prime knots.[1]

See also

References
  1. Schubert, H. "Die eindeutige Zerlegbarkeit eines Knotens in Primknoten". S.-B Heidelberger Akad. Wiss. Math.-Nat. Kl. 1949 (1949), 57–104.
  • Weisstein, Eric W. "Prime Knot". MathWorld.
  • "Prime Links with a Non-Prime Component", The Knot Atlas.
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