In topology, braid theory is an abstract geometric theory studying the everyday braid concept, and some generalisations. The idea is that braids can be organised into groups, in which the group operation is 'do the first braid on a set of strings, and then follow it with a second on the twisted strings'. Such groups may be described by explicit presentations, as was shown by Emil Artin. For an elementary treatment along these lines, see the article on braid groups. Braid groups may also be given a deeper mathematical interpretation: as the fundamental group of certain configuration spaces.

To explain how to reduce a braid group in the sense of Artin to a fundamental group, we consider a connected manifold X of dimension at least two. The symmetric product of n copies of X means the quotient of the n-fold Cartesian product Xⁿ of X with itself, by the permutation action of the symmetric group on n letters operating on the indices of coordinates. That is, an ordered n-tuple is in the same orbit as any other that is a re-ordered version of it.

A path in the n-fold symmetric product is the abstract way of discussing n points of X, considered as an unordered n-tuple, independently tracing out n strings. Since we must require that the strings never pass through each other, it is necessary that we pass to the subspace Y of the symmetric product, of orbits of n-tuples of distinct points. That is, we remove all the subspaces of Xⁿ defined by conditions x_i = x_j. This is invariant under the symmetric group, and Y is the quotient by the symmetric group of the non-excluded n-tuples. Under the dimension condition Y will be connected.

With this definition, then, we can call the braid group of X with n strings the fundamental group of Y (for any choice of base point - this is well-defined up to isomorphism). The case of X the Euclidean plane is the original one of Artin. In some cases it can be shown that the higher homotopy groups of Y are trivial.

Closed braids

When X is the plane, the braid can be closed, i.e., corresponding ends can be connected in pairs, to form a link, i.e., a possibly intertwined union of possibly knotted loops in three dimensions. The number of components of the link can be anything from 1 to n, depending on the permutation of strands determined by the link. J. W. Alexander observed that every link can be obtained in this way from a braid. Different braids can give rise to the same link, just as different crossing diagrams can give rise to the same knot. The Jones polynomial is defined, a priori, as a braid invariant and then shown to depend only on the class of the closed braid.

See also

• knot theory
• braid group