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]}
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Contents
Braids as fundamental groups
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 2. The symmetric product of n copies of X means the quotient of X^{n}, the nfold Cartesian product of X by the permutation action of the symmetric group on n strands operating on the indices of coordinates. That is, an ordered ntuple is in the same orbit as any other that is a reordered version of it.
A path in the nfold symmetric product is the abstract way of discussing n points of X, considered as an unordered ntuple, 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 ntuples of distinct points. That is, we remove all the subspaces of X^{n} 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 nonexcluded ntuples. 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 welldefined up to isomorphism). The case where X is 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. 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 singlemove 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.
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.^{[2]} Additionally, the "braid length" is the longest dimension of a braid.^{[3]}
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 spacetime trajectories formed by motion of physical rods, periodic orbits or "ghost rods", and almostinvariant sets has been used to estimate the topological entropy of several engineered and naturally occurring fluid systems, via the use of Nielsen–Thurston classification.^{[4]}
See also
 Braid group
 Braided monoidal category
 Change ringing software – how software uses braid theory to model bellringing patterns
 Knot theory
References
 ^ Artin, E. (1947). "Theory of Braids". Annals of Mathematics. 48 (1): 101–126. doi:10.2307/1969218.
 ^ Weisstein, Eric W. (August 2014). "Braid Index". MathWorld – A Wolfram Web Resource. Retrieved 20140806.
 ^ Weisstein, Eric W. (August 2014). "Length". MathWorld – A Wolfram Web Resource. Retrieved 20140806.
 ^ 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 9780691081496, 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.
 Fox, R.; Neuwirth, L. (1962), "The braid groups", Mathematica Scandinavica, 10: 119–126, MR 0150755.
 Gouillart, Emmanuelle; Thiffeault, JeanLuc; 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 3manifolds", Topology and its Applications, 78 (1–2): 95–122, doi:10.1016/S01668641(96)001514, 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 almostcyclic 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.