In topology, a branch of mathematics, **braid theory** is an abstract geometric theory studying the everyday braid concept, and some generalizations. The idea is that braids can be organized 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'.^{[citation needed]} Such groups may be described by explicit presentations, as was shown by Emil Artin (1947). For an elementary treatment along these lines, see the article on braid groups. Braid groups are also understood by a deeper mathematical interpretation: as the fundamental group of certain configuration spaces.^{[1]}

## Contents

## Braids and configuration spaces

To explain how to reduce a braid group in the sense of Artin to a fundamental group, we consider a connected manifold of dimension at least 2. The *symmetric product* of copies of means the quotient of , the -fold Cartesian product of by the permutation action of the symmetric group on strands operating on the indices of coordinates. That is, an ordered -tuple is in the same orbit as any other that is a re-ordered version of it.

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

With this definition, then, we can call **the braid group of with strings** the fundamental group of (for any choice of base point – this is well-defined up to isomorphism). The case where is the Euclidean plane is the original one of Artin. In some cases it can be shown that the higher homotopy groups of 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. A theorem of J. W. Alexander demonstrates that every link can be obtained in this way as the "closure" of a braid. Compare with string links.

Different braids can give rise to the same link, just as different crossing diagrams can give rise to the same knot. Markov (1935) describes two moves on braid diagrams that yield equivalence in the corresponding closed braids. A single-move version of Markov's theorem, was published by Lambropoulou & Rourke (1997).

Vaughan Jones originally defined his polynomial as a braid invariant and then showed that it depended only on the class of the closed braid.

The Markov theorem gives necessary and sufficient conditions under which the closures of two braids are equivalent links.^{[2]}

### Braid index

The "braid index" is the least number of strings needed to make a closed braid representation of a link. It is equal to the least number of Seifert circles in any projection of a knot.^{[3]} Additionally, the "braid length" is the longest dimension of a braid.^{[4]}

## Applications

Braid theory has recently been applied to fluid mechanics, specifically to the field of chaotic mixing in fluid flows. The braiding of (2 + 1)-dimensional space-time trajectories formed by motion of physical rods, periodic orbits or "ghost rods", and almost-invariant sets has been used to estimate the topological entropy of several engineered and naturally occurring fluid systems, via the use of Nielsen–Thurston classification.^{[5]}

## See also

- Braid group
- Braided monoidal category
- Change ringing software – how software uses braid theory to model bell-ringing patterns
- Knot theory

## References

**^**Artin, E. (1947). "Theory of Braids".*Annals of Mathematics*.**48**(1): 101–126. doi:10.2307/1969218. JSTOR 1969218.**^**J. S. Birman,*Knots, links, and mapping class groups*, Annals of Math Study, no. 82, Princeton University Press (1974).**^**Weisstein, Eric W. (August 2014). "Braid Index".*MathWorld*– A Wolfram Web Resource. Retrieved 2014-08-06.**^**Weisstein, Eric W. (August 2014). "Length".*MathWorld*– A Wolfram Web Resource. Retrieved 2014-08-06.**^**Boyland, Aref & Stremler (2000); Gouillart, Thiffeault & Finn (2006); Stremler et al. (2011).

## Notes

- Birman, Joan S. (1974),
*Braids, links, and mapping class groups*, Annals of Mathematics Studies,**82**, Princeton, N.J.: Princeton University Press, ISBN 978-0-691-08149-6, MR 0375281. - Boyland, Philip L.; Aref, Hassan; Stremler, Mark A. (2000), "Topological fluid mechanics of stirring" (PDF),
*Journal of Fluid Mechanics*,**403**: 277–304, Bibcode:2000JFM...403..277B, doi:10.1017/S0022112099007107, MR 1742169, archived from the original (PDF) on 2011-07-26. - Fox, Ralph; Neuwirth, Lee (1962), "The braid groups",
*Mathematica Scandinavica*,**10**: 119–126, doi:10.7146/math.scand.a-10518, MR 0150755. - Gouillart, Emmanuelle; Thiffeault, Jean-Luc; Finn, Matthew D. (2006), "Topological mixing with ghost rods",
*Physical Review E*,**73**(3): 036311, arXiv:nlin/0510075 , Bibcode:2006PhRvE..73c6311G, doi:10.1103/PhysRevE.73.036311, MR 2231368. - Lambropoulou, Sofia; Rourke, Colin P. (1997), "Markov's theorem in 3-manifolds",
*Topology and its Applications*,**78**(1–2): 95–122, doi:10.1016/S0166-8641(96)00151-4, MR 1465027. - Markov, Andrey (1935), "Über die freie Äquivalenz der geschlossenen Zöpfe",
*Recueil Mathématique de la Société Mathématique de Moscou*(in German and Russian),**1**: 73–78. - Stremler, Mark A.; Ross, Shane D.; Grover, Piyush; Kumar, Pankaj (2011), "Topological chaos and periodic braiding of almost-cyclic sets",
*Physical Review Letters*,**106**(11): 114101, Bibcode:2011PhRvL.106k4101S, doi:10.1103/PhysRevLett.106.114101.

## External links

- "Braids - the movie" A movie in computer graphics to explain some of braid theory (group presentation, word problem, closed braids and links, braids as motions of points in the plane).
- WINNER of Science magazine 2017 Dance Your PhD: Representations of the Braid Groups. Nancy Scherich.
- Behind the Math of Dance Your PhD, Part 1: The Braid Groups. Nancy Scherich. Explanation of braid groups as used in the movie.