## Linear representations of braid groups and classical Yang-Baxter equations.(English)Zbl 0661.20026

Braids, AMS-IMS-SIAM Jt. Summer Res. Conf., Santa Cruz/Calif. 1986, Contemp. Math. 78, 339-363 (1988).
[For the entire collection see Zbl 0651.00010.]
The author studies linear representations of the pure braid groups and the braid groups. For this purpose he considers the following notations: $$B_ n:$$ braid group on n strings with generators $$\sigma_ i$$, $$1\leq i\leq n-1$$, represented by a braid interchanging strings i and $$i+1$$. $$P_ n:$$ pure braid group on n strings with generators $\gamma_{ij}=\sigma_ i\sigma_{i+1}...\sigma_{j-1}\sigma^ 2_ j\sigma^{-1}_{j-1}...\sigma_ i^{-1},\quad 1\leq i<j\leq n.$ $$X_ n=\{(z_ 1,...,z_ n)\in {\mathbb{C}}^ n$$; $$z_ i\neq z_ j$$ if $$i\neq j\}$$. $$\Omega =\sum_{1\leq i<j\leq n}\Omega_{ij}d\log (z_ i-z_ j)$$ defined over $$X_ n$$, where $$\Omega_{ij}$$ are $$m\times m$$ matrices with $${\mathbb{C}}$$ coefficients.
After giving some characterizations of the braid groups in details, he proves the following theorems: Theorem 1: Let $$\theta$$ : $$P_ n\to GL(m,{\mathbb{C}})$$ be a linear representation such that each $$\| \theta (\gamma_{ij})-1\|$$ is sufficiently small for $$1\leq i<j\leq n$$. Then there exist constant matrices $$\Omega_{ij}$$, $$1\leq i<j\leq n$$, close to 0, satisfying the infinitesimal pure braid relations, such that the monodromy of the connection $$\Omega =\sum_{1\leq i<j\leq n}\Omega_{ij}d\log (z_ i-z_ j)$$ is the given $$\theta$$. Theorem 2: Let $$\theta_{\lambda}: B_ n\to End(V^{\otimes n})$$ be the one parameter family of the monodromy representation of the connection $$\sum_{1\leq i<j\leq n}\lambda \tau_{ij}d\log (z_ 1-z_ j)$$ defined above by means of $${\mathfrak g}=sl(2,{\mathbb{C}})$$ and its two dimensional irreducible representation. Then $$\theta_{\lambda}$$ is equivalent to the linear representation of $$B_ n$$ defined by $$\theta_{\lambda}(\sigma_ i)=-q^{1/4}\{qe_ i-(1-e_ i)\}$$, $$1\leq i\leq n-1$$, where $$q=e^{-2\pi i\lambda}.$$
At the end of this paper the author gives some open questions concerning the subject studied. We think that this work makes some important contributions to this field. Still one can hope for some generalizations.
Reviewer: T.Başkan

### MSC:

 20F36 Braid groups; Artin groups 35F05 Linear first-order PDEs 55P62 Rational homotopy theory 20F38 Other groups related to topology or analysis 57M25 Knots and links in the $$3$$-sphere (MSC2010)

Zbl 0651.00010