, a link
is a collection of knots
which do not intersect, but which may be linked (or knotted) together. A knot can be described as a link with one component. Links and knots are studied in a branch of mathematics called knot theory
. Implicit in this definition is that there is a trivial
reference link, usually called the unlink
, but the word is also sometimes used in context where there is no notion of a trivial link.
For example, a co-dimension two link in 3-dimensional space is a subspace of 3-dimensional Euclidean space (or often the 3-sphere) whose connected components are homeomorphic to circles.
The simplest nontrivial example of a link with more than one component is called the Hopf link, which consists of two circles (or unknots) linked together once. Borromean rings form a link with three components each equivalent to the unknot. The three loops are collectively linked despite the fact that no two of them are directly linked.
Frequently the word link is used to describe any submanifold of the sphere diffeomorphic to a disjoint union of a finite number of spheres, .
In full generality, the word link is essentially the same as the word knot -- the context is that one has a submanifold M of a manifold N (considered to be trivially embedded) and a non-trivial embedding of M in N, non-trivial in the sense that the 2nd embedding is not isotopic to the 1st. If M is disconnected, the embedding is called a link (or said to be linked). If M is connected, it is called a knot.