Contents

group theory

# Contents

## Idea

The braid group $Br(n)$ is the group whose elements are isotopy classes of $n$ 1-dimensional braids running vertically in 3-dimensional Cartesian space, the group operation being their concatenation.

Here a braid with $n$ strands is thought of as $n$ pieces of string joining $n$ points at the top of the diagram with $n$-points at the bottom.

(This is a picture of a 3-strand braid.)

We can transform / ‘isotope’ these braid diagrams just as we can transform knot diagrams, again using Reidemeister moves. The ‘isotopy’ classes of braid diagrams form a group in which the composition is obtained by putting one diagram above another.

The identity consists of $n$ vertical strings, so the inverse is obtained by turning a diagram upside down:

This is the inverse of the first 3-braid we saw.

There are useful group presentations of the braid groups. We will return later to the interpretation of the generators and relations in terms of diagrams.

## Definition

### Geometric definition

#### Ordinary braid group

Geometrically, one may understand the group of braids in $\mathbb{R}^3$ as the fundamental group of the configuration space of points in the plane $\mathbb{R}^2$ (traditionally regarded as the complex plane $\mathbb{C}$ in this context, though the complex structure plays no role in the definition of the braid group).

We say this in more detail:

Let $C_n \hookrightarrow \mathbb{C}^n$ denote the space of configurations of n ordered points in the complex plane, whose elements are those n-tuples $(z_1, \ldots, z_n)$ such that $z_i \neq z_j$ whenever $i \neq j$. In other words, $C_n$ is the complement of the fat diagonal:

$C_n \;\coloneqq\; \mathbb{C}^n \setminus \mathbf{\Delta}^n_{\mathbb{C}} \,.$

The symmetric group $S_n$ acts on $C_n$ by permuting coordinates. Let:

• $C_n/S_n$ denote the quotient by this group action, hence the orbit space (the space of $n$-element subsets of $\mathbb{C}$ if one likes),

• $[z_1, \ldots, z_n]$ denote the image of $(z_1, \ldots, z_n)$ under the quotient coprojection $\pi \colon C_n \to C_n/S_n$ (i.e. its the equivalence class).

We understand $p = (1, 2, \ldots, n)$ as the basepoint for $C_n$, and $[p] = [1, 2, \ldots n]$ as the basepoint for the configuration space of unordered points $C_n/S_n$, making it a pointed topological space.

###### Definition

The braid group is the fundamental group of the configuration space of n unordered points:

$Br(n) \;\coloneqq\; \pi_1 \big( C_n/S_n, [p] \big)$

The pure braid group is the fundamental group of the configuration space of n ordered points:

$PBr(n) \;\coloneqq\; \pi_1 \big( C_n, p \big) \,.$

Evidently a braid $\beta$ is represented by a path $\alpha: I \to C_n/S_n$ with $\alpha(0) = [p] = \alpha(1)$. Such a path may be uniquely lifted through the covering projection $\pi: C_n \to C_n/S_n$ to a path $\tilde{\alpha}$ such that $\tilde{\alpha}(0) = p$. The end of the path $\tilde{\alpha}(1)$ has the same underlying subset as $p$ but with coordinates permuted: $\tilde{\alpha}(1) = (\sigma(1), \sigma(2), \ldots, \sigma(n))$. Thus the braid $\beta$ is exhibited by $n$ non-intersecting strands, each one connecting an $i$ to $\sigma(i)$, and we have a map $\beta \mapsto \sigma$ appearing as the quotient map of an exact sequence

$1 \to PBr(n) \to Br(n) \to Sym(n) \to 1$

which is part of a long exact homotopy sequence corresponding to the fibration $\pi \colon C_n \to C_n/S_n$.

#### For general topological base spaces

Since the notion of a configuration space of points makes sense for points in any topological space, not necessarily the plane $\mathbb{R}^2$, the above geometric definition has an immediate generalization:

For $\Sigma$ any surface, the fundamental group of the (ordered) configuration space of points in $\Sigma$ may be regarded as generalized (pure) braid group. These surface braid groups are of interest in 3d topological field theory and in particular in topological quantum computation where it models non-abelian anyons.

Yet more generally, one may consider the fundamental group of the configuration space of points of any topological space $X$.

For example for $X$ a 1-dimensional CW-complex, hence an (undirected) graph, one speaks of graph braid groups (e.g. Farley & Sabalka 2009).

The following should maybe not be here in the Definition-section, but in some Properties- or Examples-section, or maybe in a dedicated entry on graph braid groups?:

It has been shown (An & Maciazek 2006, using discrete Morse theory and combinatorial analysis of small graphs) that graph braid groups are generated by particular particle moves with the following description:

1. Star-type generators: exchanges of particle pairs on vertices of the particular graph

2. loop type generators: circular moves of a single particle around a simple cycle of the graph

### Group-theoretic definition

#### Artin presentation

The Artin braid group, $Br({n+1})$, defined using $n+1$ strands is a group given by

• generators: $y_i$, $i = 1, \ldots, n$;

• relations:

• $r_{i,j} \equiv y_i y_j y_i^{-1} y_j^{-1}$ for $i+1 \lt j$

• $r_{i,i+1}\equiv y_i y_{i+1} y_i y_{i+1}^{-1} y_i^{-1} y_{i+1}^{-1}$ for $1 \leq i \lt n$.

#### In terms of automorphisms on free groups

The braid group $Br(n)$ may be alternatively described as the mapping class group of a 2-disk $D^2$ with $n$ punctures (call it $X_n$). Meanwhile, the fundamental group $\pi_1(X_n)$ (with basepoint on the boundary) is a free group $F_n$ on $n$ generators; the functoriality of $\pi_1$ implies we have an induced homomorphism

$Aut(X_n) \to Aut(\pi_1(X_n)) = Aut(F_n).$

If an automorphism $\phi: X_n \to X_n$ is isotopic to the identity, then of course $\pi_1(\phi)$ is trivial, and so the homomorphism factors through the quotient $MCG(X_n) = Aut(X_n)/Aut_0(X)$, so we get a homomorphism

$Br(n) = MCG(X_n) \to Aut(F_n)$

and this turns out to be an injection.

Explicitly, the generator $y_i$ used in the Artin presentation above is mapped to the automorphism $\sigma_i$ on the free group on $n$ generators $x_1, \ldots, x_n$ defined by

$\sigma_i(x_i) = x_{i+1}, \sigma_i(x_{i+1}) = x_{i+1}^{-1} x_i x_{i+1}, \; \else\; \sigma(x_j) = x_j.$

## Properties

### Relation to moduli space of monopoles

###### Proposition

(moduli space of monopoles is stably weak homotopy equivalent to classifying space of braid group)

For $k \in \mathbb{N}$ there is a stable weak homotopy equivalence between the moduli space of k monopoles (?) and the classifying space of the braid group $Br({2k})$ on $2 k$ strands:

$\Sigma^\infty \mathcal{M}_k \;\simeq\; \Sigma^\infty Br({2k})$

## Examples

The first few examples of the braid group $Br(n)$ for low values of $n$:

###### Example

The group $Br(1)$ has no generators and no relations, so is the trivial group:

$Br(1) \;\simeq\; 1 \,.$

###### Example

The group $Br(2)$ has one generator and no relations, so is the infinite cyclic group of integers:

$Br(2) \;\simeq\; \mathbb{Z} \,.$

###### Example

The group $Br(3)$ (we will simplify notation writing $u = y_1$, $v = y_2$) has presentation

$Br(3) \;\simeq\; \mathcal{P} \coloneqq \big( u,v : r \equiv u v u v^{-1} u^{-1} v^{-1} \big).$

This is also known as the “trefoil knot group”, i.e., the fundamental group of the complement of a trefoil knot.

###### Example

The group $Br(4)$ (simplifying notation as before) has generators $u,v,w$ and relations:

• $r_u \equiv v w v w^{-1} v^{-1} w^{-1}$,
• $r_v \equiv u w u^{-1} w^{-1}$,
• $r_w \equiv u v u v^{-1} u^{-1} v^{-1}$.

###### Example

The Hurwitz braid group (or sphere braid group) is the surface braid group for $\Sigma$ the 2-sphere $S^2$. Algebraically, the Hurwitz braid group $H_{n+1}$ has all of the generators and relations of the Artin braid group $Br({n+1})$, plus one additional relation:

$y_1 y_2 \dots y_{n-1} y_n^2 y_{n-1}\dots y_2 y_1$

## References

### General

Classical references:

• Joan S. Birman, Braids, links, and mapping class groups, Princeton Univ Press, 1974.

• R. H. Fox, L. Neuwirth, The braid groups, Math. Scand. 10 (1962) pp.119-126. pdf, MR150755

Textbook accounts:

• Wikipedia: Braid group

• Joshua Lieber, Introduction to Braid Groups, 2011 (pdf)

• Juan González-Meneses, Basic results on braid groups, Annales Mathématiques Blaise Pascal, Tome 18 (2011) no. 1, pp. 15-59 (ambp:AMBP_2011__18_1_15_0)

• Alexander I. Suciu, He Wang, The pure braid groups and their relatives, Perspectives in Lie theory, 403-426, Springer INdAM series, vol. 19, Springer, Cham, 2017 (arXiv:1602.05291)

On the group homology and group cohomology of braid groups:

For orderings of the braid group see

• Patrick Dehornoy, Braid groups and left distributive operations , Transactions AMS 345 no.1 (1994) pp.115–150.

• H. Langmaack, Verbandstheoretische Einbettung von Klassen unwesentlich verschiedener Ableitungen in die Zopfgruppe , Computing 7 no.3-4 (1971) pp.293-310.

On geometric presentations of braid groups:

• An, Byung Hee and Maciazek, Tomasz, Geometric Presentations of Braid Groups for Particles on a Graph, Communications in Mathematical Physics (2021) volume 384, pp. 1109-1140,(http://dx.doi.org/10.1007/s00220-021-04095-x)

### Braid group representations (as topological quantum gates)

On linear representations of braid groups (see also at braid group statistics and interpretation as quantum gates in topological quantum computation):

Review:

in relation to modular tensor categories:

• Colleen Delaney, Lecture notes on modular tensor categories and braid group representations, 2019 (pdf, pdf)

• R. B. Zhang, Braid group representations arising from quantum supergroups with arbitrary $q$ and link polynomials, Journal of Mathematical Physics 33, 3918 (1992) (doi:10.1063/1.529840)

### Graph braid groups

• Daniel Farley, Lucas Sabalka, Presentations of Graph Braid Groups (arXiv:0907.2730)

• Ki Hyoung Ko, Hyo Won Park, Characteristics of graph braid groups (arXiv:1101.2648)

• Byung Hee An, Tomasz Maciazek, Geometric presentations of braid groups for particles on a graph (arXiv:2006.15256)

### Relation to moduli space of monopoles

On moduli spaces of monopoles related to braid groups:

### Braid group cryptography

Partly motivated by the possibility of quantum computation eventually breaking the security of cryptography based on abelian groups, such as elliptic curves, there are proposals to use non-abelian braid groups for purposes of cryptography (“post-quantum cryptography”).

An early proposal was to use the Conjugacy Search Problem in braid groups as a computationally hard problem for cryptography. This approach, though, was eventually found not to be viable.

Original articles:

• Iris Anshel, M. Anshel and D. Goldfeld, An algebraic method for public-keycryptography, Math. Research Letters 6 (1999), 287–291 (pdf)

• K.H. Ko, S.J. Lee, J.H. Cheon , J.W. Han, J. Kang, C. Park , New Public-Key Cryptosystem Using Braid Groups, In: M. Bellare (ed.) Advances in Cryptology — CRYPTO 2000 Lecture Notes in Computer Science, vol 1880. Springer 2000 (doi:10.1007/3-540-44598-6_10)

Review:

#### Via E-multiplication

A followup proposal was to use the problem of reversing E-multiplication in braid groups, thought to remedy the previous problems.

Original article:

• Iris Anshel, Derek Atkins, Dorian Goldfeld and Paul E Gunnells, WalnutDSA(TM): A Quantum-Resistant Digital Signature Algorithm (eprint:2017/058)

Review:

• Magnus Ringerud, WalnutDSA: Another attempt at braidgroup cryptography, 2019 (pdf)

But other problems were found with this approach, rendering it non-viable.

Original article:

• Matvei Kotov, Anton Menshov, Alexander Ushakov, An attack on the Walnut digital signature algorithm, Designs, Codes and Cryptography volume 87, pages 2231–2250 (2019) (doi:10.1007/s10623-019-00615-y)

Review:

• José Ignacio Escribano Pablos, María Isabel González Vasco, Misael Enrique Marriaga and Ángel Luis Pérez del Pozo, The Cracking of WalnutDSA: A Survey, in: Interactions between Group Theory, Symmetry and Cryptology, Symmetry 2019, 11(9), 1072 (doi:10.3390/sym11091072)

#### Further developments

The basic idea is still felt to be promising:

• Xiaoming Chen, Weiqing You, Meng Jiao, Kejun Zhang, Shuang Qing, Zhiqiang Wang, A New Cryptosystem Based on Positive Braids (arXiv:1910.04346)

• Garry P. Dacillo, Ronnel R. Atole, Braided Ribbon Group $C_n$-based Asymmetric Cryptography, Solid State Technology Vol. 63 No. 2s (2020) (JSST:5573)

But further attacks are being discussed:

• James Hughes, Allen Tannenbaum, Length-Based Attacks for Certain Group Based Encryption Rewriting Systems (arXiv:cs/0306032)

As are further ways around these:

• Xiaoming Chen, Weiqing You, Meng Jiao, Kejun Zhang, Shuang Qing, Zhiqiang Wang, A New Cryptosystem Based on Positive Braids (arXiv:1910.04346)
category: knot theory

Last revised on June 24, 2021 at 09:12:52. See the history of this page for a list of all contributions to it.