# Birman–Wenzl algebra

In mathematics, the Birman–Murakami–Wenzl (BMW) algebra, introduced by Joan Birman and Hans Wenzl (1989) and Jun Murakami (1987), is a two-parameter family of algebras $\mathrm {C} _{n}(\ell ,m)$ of dimension $1\cdot 3\cdot 5\cdots (2n-1)$ having the Hecke algebra of the symmetric group as a quotient. It is related to the Kauffman polynomial of a link. It is a deformation of the Brauer algebra in much the same way that Hecke algebras are deformations of the group algebra of the symmetric group.

## Definition

For each natural number n, the BMW algebra $\mathrm {C} _{n}(\ell ,m)$ is generated by $G_{1}^{\pm 1},G_{2}^{\pm 1},\dots ,G_{n-1}^{\pm 1},E_{1},E_{2},\dots ,E_{n-1}$ and relations:
    $G_{i}G_{j}=G_{j}G_{i},\mathrm {if} \left\vert i-j\right\vert \geq 2,$
    $G_{i}G_{i+1}G_{i}=G_{i+1}G_{i}G_{i+1},$    $E_{i}E_{i\pm 1}E_{i}=E_{i},$
    $G_{i}+{G_{i}}^{-1}=m(1+E_{i}),$
    $G_{i\pm 1}G_{i}E_{i\pm 1}=E_{i}G_{i\pm 1}G_{i}=E_{i}E_{i\pm 1},$ $G_{i\pm 1}E_{i}G_{i\pm 1}={G_{i}}^{-1}E_{i\pm 1}{G_{i}}^{-1},$
    $G_{i\pm 1}E_{i}E_{i\pm 1}={G_{i}}^{-1}E_{i\pm 1},$ $E_{i\pm 1}E_{i}G_{i\pm 1}=E_{i\pm 1}{G_{i}}^{-1},$
    $G_{i}E_{i}=E_{i}G_{i}=l^{-1}E_{i},$ $E_{i}G_{i\pm 1}E_{i}=lE_{i}.$
These relations imply the further relations:
    $E_{i}E_{j}=E_{j}E_{i},\mathrm {if} \left\vert i-j\right\vert \geq 2,$
    $(E_{i})^{2}=(m^{-1}(l+l^{-1})-1)E_{i},$
    ${G_{i}}^{2}=m(G_{i}+l^{-1}E_{i})-1.$
This is the original definition given by Birman and Wenzl. However a slight change by the introduction of some minus signs is sometimes made, in accordance with Kauffman's 'Dubrovnik' version of his link invariant. In that way, the fourth relation in Birman & Wenzl's original version is changed to
* (Kauffman skein relation)
    $G_{i}-{G_{i}}^{-1}=m(1-E_{i}),$
Given invertibility of m, the rest of the relations in Birman & Wenzl's original version can be reduced to
* (Idempotent relation)
    $(E_{i})^{2}=(m^{-1}(l-l^{-1})+1)E_{i},$
* (Braid relations)
    $G_{i}G_{j}=G_{j}G_{i},{\text{if }}\left\vert i-j\right\vert \geqslant 2,{\text{ and }}G_{i}G_{i+1}G_{i}=G_{i+1}G_{i}G_{i+1},$
* (Tangle relations)
    $E_{i}E_{i\pm 1}E_{i}=E_{i}{\text{ and }}G_{i}G_{i\pm 1}E_{i}=E_{i\pm 1}E_{i},$
* (Delooping relations)
    $G_{i}E_{i}=E_{i}G_{i}=l^{-1}E_{i}{\text{ and }}E_{i}G_{i\pm 1}E_{i}=lE_{i}.$

## Properties

* The dimension of $\mathrm {C} _{n}(\ell ,m)$ is $(2n)!/(2^{n}n!)$.
* The Iwahori–Hecke algebra associated with the symmetric group $S_{n}$ is a quotient of the Birman–Murakami–Wenzl algebra $\mathrm {C} _{n}$.
* The Artin braid group embeds in the BMW algebra: $B_{n}\hookrightarrow \mathrm {C} _{n}$.

## Isomorphism between the BMW algebras and Kauffman's tangle algebras

It is proved by Morton & Wassermann (1989) that the BMW algebra $\mathrm {C} _{n}(\ell ,m)$ is isomorphic to the Kauffman's tangle algebra $\mathrm {KT} _{n}$. The isomorphism $\phi \colon \mathrm {C} _{n}\to \mathrm {KT} _{n}$ is defined by
and

## Baxterisation of Birman–Murakami–Wenzl algebra

Define the face operator as
    $U_{i}(u)=1-{\frac {i\sin u}{\sin \lambda \sin \mu }}(e^{i(u-\lambda )}G_{i}-e^{-i(u-\lambda )}{G_{i}}^{-1})$,
where $\lambda$ and $\mu$ are determined by
    $2\cos \lambda =1+(l-l^{-1})/m$
and
    $2\cos \lambda =1+(l-l^{-1})/(\lambda \sin \mu )$.
Then the face operator satisfies the Yang–Baxter equation.
    $U_{i+1}(v)U_{i}(u+v)U_{i+1}(u)=U_{i}(u)U_{i+1}(u+v)U_{i}(v)$
Now $E_{i}=U_{i}(\lambda )$ with
    $\rho (u)={\frac {\sin(\lambda -u)\sin(\mu +u)}{\sin \lambda \sin \mu }}$.
In the limits $u\to \pm i\infty$, the braids ${G_{j}}^{\pm }$ can be recovered up to a scale factor.

## History

In 1984, Vaughan Jones introduced a new polynomial invariant of link isotopy types which is called the Jones polynomial. The invariants are related to the traces of irreducible representations of Hecke algebras associated with the symmetric groups. Murakami (1987) showed that the Kauffman polynomial can also be interpreted as a function $F$ on a certain associative algebra. In 1989, Birman & Wenzl (1989) constructed a two-parameter family of algebras $\mathrm {C} _{n}(\ell ,m)$ with the Kauffman polynomial $K_{n}(\ell ,m)$ as trace after appropriate renormalization.