# Yang-Baxter operators

In their most familiar form, Yang–Baxter operators are certain invertible linear endomorphisms which have applications to physics and topology. In physics these operators often provide solutions to the quantum Yang–Baxter equation, an equation which has its roots in statistical mechanics [a30], [a7], [a28], [a31], [a35] (cf. also Statistical mechanics, mathematical problems in). In topology quite often they can be used to construct invariants of knots, links or three-dimensional manifolds (cf. also Knot theory; Link; Three-dimensional manifold); cf. [a30], [a17], [a18], [a26], [a27], [a32], [a33].

Closely related to the quantum Yang–Baxter equation is the braid equation. There are natural categorical structures associated with the braid and quantum Yang–Baxter equations which play an important role in quantum groups and their applications [a21], [a23], [a5], [a34].

Yang–Baxter operators in the category $\square _ { k }\operatorname {Mod}$ of left modules over a commutative ring $k$ are certain $k$-linear mappings $R : V \otimes _ { k } V \rightarrow V \otimes _ { k } V$. Let $R _ { 12 } = R \otimes _ { k } 1$, $R _ { 23 } = 1 \otimes _ { k } R$ and $R _ { 13 } = ( 1 \otimes _ { k } \tau _ { V , V } ) ( R \otimes _ { k } 1 ) ( 1 \otimes _ { k } \tau _ { V , V } )$, where $\tau _ { U , V } : U \otimes _ { k } V \rightarrow V \otimes _ { k } U$ is the "twist" mapping defined for $k$-modules $U$ and $V$ by $\tau_{ U , V } ( u \otimes v ) = v \otimes u$ for all $u \in U$ and $v \in V$. Then $R$ satisfies the quantum Yang–Baxter equation in $\square _ { k }\operatorname {Mod}$ if

\begin{equation} \tag{a1} R _ { 12 } R _ { 13 } R _ { 23 } = R _ { 23 } R _ { 13 } R _ { 12 }. \end{equation}

Note that $R$ satisfies (a1) if and only if $B = \tau _ { V , V } R$ satisfies the braid equation in $\square _ { k }\operatorname {Mod}$, which is

\begin{equation} \tag{a2} B _ { 12 } B _ { 23 } B _ { 12 } = B _ { 23 } B _ { 12 } B _ { 23 }. \end{equation}

If $R$ is invertible and satisfies (a2), that is, $R _ { 12 } R _ { 23 } R _ { 12 } = R _ { 23 } R _ { 12 } R _ { 23 }$, then $R$ is a Yang–Baxter operator in $\square _ { k }\operatorname {Mod}$ (see [a5]). There are other formulations of the notion of Yang–Baxter operator in the context of modules; see, e.g., [a8] and [a9].

Observe that the quantum Yang–Baxter and braid equations have natural formulations in any category $\mathcal{C}$ with a suitable notion of tensor product and in which the tensor product of morphisms is defined [a16], [a20], [a34]. The notion of quantum Yang–Baxter operator thus has a natural generalization to categories $\mathcal{C}$ with such additional structure; see, e.g., [a16], [a5].

A good source of solutions to (a1) in $\square _ { k }\operatorname {Mod}$ are certain elements $R = \sum _ { s = 1 } ^ { n } a _ { s } \otimes b _ { s } \in A \otimes _ { k } A$, where $A$ is an algebra over $k$. For $R \in A \otimes _ { k } A$ and a $k$-module $V$, let $R _ { V } : V \otimes _ { k } V \rightarrow V \otimes _ { k } V$ be defined by $R _ { V } ( u \otimes v ) = R \cdot ( u \otimes v )$ for all $u , v \in V$, where $V \otimes _ { k } V$ is regarded as a left $A \otimes _ { k } A$-module under component multiplication. Then $R _ { V }$ is a solution to (a1) for all left $k$-modules $V$ if and only if

(a3) |

When $A = M _ { n } ( k )$ is the algebra of $( n \times n )$-matrices over $k$, then an $R \in A \otimes _ { k } A$ which satisfies (a1), or equivalently (a3), is called an $R$-matrix. Suppose that $A = M _ { n } ( k )$ and , where $r _ { t \text{l}} ^ { s k } \in k $ and $\{ E _ { t } ^ { s } \} _ { 1 \leq s , t \leq n}$ is the standard basis for $A$. Then (a3) is equivalent to

(a4) |

for all $1 \leq p , q , r , a , b , c \leq n$, which is probably the most familiar form of the quantum Yang–Baxter equation. Ordinarily, (a4) is written using the Einstein summation convention, that is, summation signs are omitted with the understanding that indices that appear as upper and lower indices are summed over their full range of values.

Certain $k$-algebras $A$ with an $R \in A \otimes _ { k } A$ which satisfies (a1) can be used to construct invariants. Quasi-triangular Hopf algebras, in particular quantum algebras (cf. also Quasi-triangular Hopf algebra), give rise to regular isotopy invariants of $1$-$1$ tangles. Ribbon Hopf algebras give rise to regular isotopy invariants of knots and links, and under mild restrictions they give rise to invariants of three-dimensional manifolds. Let $k$ be a field. In this case a finite-dimensional Hopf algebra $A$ over $k$ is closely linked to these structures. The Hopf algebra $A$ is a subHopf algebra of the quantum double $D ( A )$ of $A$, which is a quasi-triangular Hopf algebra [a1]. Every finite-dimensional quasi-triangular Hopf algebra over $k$ is a subHopf algebra of a ribbon Hopf algebra; in particular, $A$ is a subHopf algebra of a ribbon Hopf algebra [a6].

The classification of $R$-matrices seems to be a very daunting task, and most work to date (1998) has involved symbolic computation. Suppose that $k$ is the field of complex numbers. Then the $R$-matrices are completely classified in the $n = 2$ case [a13] and the classification of one basic family is known in the $n = 3$ case [a14].

Some of the more important examples of $R$-matrices, those related to the quantized enveloping algebras, are formal infinite sums or belong to a completed tensor product. See [a4], [a1] for discussion of this important part of the theory.

There is a category $\square _ { A } ^ { A } \mathcal{C}$ with a pre-braiding structure, defined and studied in [a34], associated to a bi-algebra $A$ over $k$ which gives rise to Yang–Baxter operators. Here, the formal variant $\square _ { A } \mathcal{C} ^ { A }$ is considered, whose objects are left $A$-modules and right $A$-comodules $V$ which satisfy the condition

for all $a \in A$ and $v \in V$, where $\rho ( v ) = v ^ { ( 1 ) } \otimes v ^ { ( 2 ) } \in V \otimes _ { k } A$ denotes the coproduct $\rho$ applied to $v \in V$. For an object $V$ of $\square _ { A } \mathcal{C} ^ { A }$, define $\mathcal{R} _ { V } : V \otimes _ { k } V \rightarrow V \otimes _ { k } V$ by $\mathcal{R} _ { V } ( u \otimes v ) = u ^ { \{ 1 \} } \otimes u ^ { ( 2 ) } . v$ for all $u , v \in V$. Then $\mathcal{R} _ { V }$ satisfies (a1). The pre-braiding structure on $\square _ { A } \mathcal{C} ^ { A }$ is the collection of morphisms of the form $\sigma _ { U , V } : U \otimes _ { k } V \rightarrow V \otimes _ { k } U$ which are defined for all pairs of objects $U$, $V$ by $\sigma_{ U , V} ( u \otimes v ) = u ^ { ( 2 ) } . v \otimes u ^ { ( 1 ) }$ for all $u \in U$ and $v \in V$. Observe that $\sigma_{ V , V } = \tau_{ V , V} R _ { V }$ is a solution to the braid equation (a2). When $A$ is a Hopf algebra, the morphism $\sigma_{ U, V}$ is invertible, and the collection of all $\sigma_{ U, V}$ is referred to as a braiding structure. When $k$ is a field and $A$ is a finite-dimensional Hopf algebra over $k$, the category $\square _ { D(A) } \mathcal{C}^{D ( A )}$ can be identified with $\square _{D(A)} \operatorname{Mod}$, the category of left modules of the quantum double [a23].

The FRT construction of [a11], [a10] has an interesting interpretation in light of the preceding paragraph. Suppose that $k$ is a field and $R : V \otimes _ { k } V \rightarrow V \otimes _ { k } V$ is a solution to (a1), where $V$ is a finite-dimensional vector space over $k$. The FRT construction $A ( R )$ is a certain bi-algebra over $k$ associated with $R$. There is a natural way of turning $V$ into an object of $\square _ { A ( R ) } {\cal C} ^ { A ( R) }$ such that $R = \mathcal{R} _ { \mathcal{V} }$, described in [a24]. For a universal description of the FRT construction associated with certain Yang–Baxter operators, see [a5], [a21]. See also [a8] for a discussion of algebras associated with Yang–Baxter operators.

There is a certain quotient $\widetilde { A ( R )}$ of $A ( R )$ which is more closely tied to $R$ from a computational point of view. If $V \neq ( 0 )$, then it is never the case that $A ( R )$ is a Hopf algebra, whereas $\widetilde { A ( R )}$ may very well be a Hopf algebra [a25]. Determining new families of solutions to (a1) of the type described in the preceding paragraph may very well involve using a combination of bi-algebra techniques involving $\widetilde { A ( R )}$ and computer methods [a3], [a25].

There are parametrized versions of (a1), and hence parametrized versions of Yang–Baxter operators. Let $X$ be a set, $R : X \rightarrow \operatorname { End } _ { k } ( V \otimes _ { k } V )$ be a function and suppose that $Z \subseteq X \times X$ is a non-empty subset with a (multiplication) mapping $Z \rightarrow X$. Then $R$ satisfies the $1$-parameter quantum Yang–Baxter equation if

\begin{equation*} R ( x ) _ { 12 } R ( x y ) _ { 13 } R ( y ) _ { 23 } = R ( y ) _ { 23 } R ( x y ) _ { 13 } R ( x ) _ { 12 } \end{equation*}

holds for all $( x , y ) \in Z$. There is an FRT construction for $1$-parameter families [a3]. A $2$-parameter family of solutions to the quantum Yang–Baxter equation is a function $R : X \times X \rightarrow \operatorname { End } _ { k } ( V \otimes _ { k } V )$ which satisfies

\begin{equation*} R ( x , y ) _ { 12 } R ( x , z ) _ { 13 } R ( y , z ) _ { 23 } = \end{equation*}

\begin{equation*} = R ( y , z ) _ { 23 } R ( x , z ) _ { 13 } R ( x , y ) _ { 12 } \end{equation*}

for all $x , y , z \in X$. For examples and discussion, see [a1], [a12], [a11], [a15], [a19].

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Yang-Baxter operators.

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