Braided Hopf algebra

Definition

Let H be a Hopf algebra over a field k, and assume that the antipode of H is bijective. A Yetter–Drinfeld module R over H is called a braided bialgebra in the Yetter–Drinfeld category H H Y D if

( R , ⋅ , η ) is a unital associative algebra, where the multiplication map ⋅ : R × R → R and the unit η : k → R are maps of Yetter–Drinfeld modules,

( R , Δ , ε ) is a coassociative coalgebra with counit ε , and both Δ and ε are maps of Yetter–Drinfeld modules,

the maps Δ : R → R ⊗ R and ε : R → k are algebra maps in the category H H Y D , where the algebra structure of R ⊗ R is determined by the unit η ⊗ η ( 1 ) : k → R ⊗ R and the multiplication map

Here c is the canonical braiding in the Yetter–Drinfeld category H H Y D .

A braided bialgebra in H H Y D is called a braided Hopf algebra, if there is a morphism S : R → R of Yetter–Drinfeld modules such that

where Δ R ( r ) = r ( 1 ) ⊗ r ( 2 ) in slightly modified Sweedler notation – a change of notation is performed in order to avoid confusion in Radford's biproduct below.

Examples

Any Hopf algebra is also a braided Hopf algebra over H = k

A super Hopf algebra is nothing but a braided Hopf algebra over the group algebra H = k [ Z / 2 Z ] .

The tensor algebra T V of a Yetter–Drinfeld module V ∈ H H Y D is always a braided Hopf algebra. The coproduct Δ of T V is defined in such a way that the elements of V are primitive, that is

The counit ε : T V → k then satisfies the equation ε ( v ) = 0 for all v ∈ V .

The universal quotient of T V , that is still a braided Hopf algebra containing V as primitive elements is called the Nichols algebra. They take the role of quantum Borel algebras in the classification of pointed Hopf algebras, analogously to the classical Lie algebra case.

Radford's biproduct

For any braided Hopf algebra R in H H Y D there exists a natural Hopf algebra R # H which contains R as a subalgebra and H as a Hopf subalgebra. It is called Radford's biproduct, named after its discoverer, the Hopf algebraist David Radford. It was rediscovered by Shahn Majid, who called it bosonization.

As a vector space, R # H is just R ⊗ H . The algebra structure of R # H is given by

where r , r ′ ∈ R , h , h ′ ∈ H , Δ ( h ) = h ( 1 ) ⊗ h ( 2 ) (Sweedler notation) is the coproduct of h ∈ H , and . : H ⊗ R → R is the left action of H on R. Further, the coproduct of R # H is determined by the formula

Here Δ R ( r ) = r ( 1 ) ⊗ r ( 2 ) denotes the coproduct of r in R, and δ ( r ( 2 ) ) = r ( 2 ) ( − 1 ) ⊗ r ( 2 ) ( 0 ) is the left coaction of H on r ( 2 ) ∈ R .