Braided Hopf algebra

In mathematics, a braided Hopf algebra is a Hopf algebra in a braided monoidal category. The most common braided Hopf algebras are objects in a Yetter–Drinfeld category of a Hopf algebra H, particularly the Nichols algebra of a braided vector space in that category.

The notion should not be confused with quasitriangular 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 $$ {}^H_H\mathcal{YD}$$ if
 * $$ (R,\cdot ,\eta ) $$ is a unital associative algebra, where the multiplication map $$\cdot :R\times R\to R$$ and the unit $$ \eta :k\to R $$ are maps of Yetter–Drinfeld modules,
 * $$ (R,\Delta ,\varepsilon )$$ is a coassociative coalgebra with counit $$\varepsilon $$, and both $$ \Delta $$ and $$\varepsilon $$ are maps of Yetter–Drinfeld modules,
 * the maps $$\Delta :R\to R\otimes R $$ and $$ \varepsilon :R\to k $$ are algebra maps in the category $$ {}^H_H\mathcal{YD}$$, where the algebra structure of $$ R\otimes R $$ is determined by the unit $$ \eta \otimes \eta(1) : k\to R\otimes R$$ and the multiplication map
 * $$ (R\otimes R)\times (R\otimes R)\to R\otimes R,\quad (r\otimes s,t\otimes u) \mapsto \sum _i rt_i\otimes s_i u, \quad \text{and}\quad c(s\otimes t)=\sum _i t_i\otimes s_i. $$
 * Here c is the canonical braiding in the Yetter–Drinfeld category $$ {}^H_H\mathcal{YD}$$.

A braided bialgebra in $$ {}^H_H\mathcal{YD}$$ is called a braided Hopf algebra, if there is a morphism $$ S:R\to R $$ of Yetter–Drinfeld modules such that
 * $$ S(r^{(1)})r^{(2)}=r^{(1)}S(r^{(2)})=\eta(\varepsilon (r)) $$ for all $$ r\in R,$$

where $$\Delta _R(r)=r^{(1)}\otimes r^{(2)}$$ 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 $$ H=k[\mathbb{Z}/2\mathbb{Z}] $$.
 * The tensor algebra $$ TV $$ of a Yetter–Drinfeld module $$ V\in {}^H_H\mathcal{YD}$$ is always a braided Hopf algebra. The coproduct $$ \Delta $$ of $$ TV $$ is defined in such a way that the elements of V are primitive, that is
 * $$ \Delta (v)=1\otimes v+v\otimes 1 \quad \text{for all}\quad v\in V.$$
 * The counit $$\varepsilon :TV\to k$$ then satisfies the equation $$ \varepsilon (v)=0$$ for all $$ v\in V .$$


 * The universal quotient of $$ TV $$, 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 $$ {}^H_H\mathcal{YD}$$ 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 $$ R\otimes H $$. The algebra structure of $$ R\# H $$ is given by
 * $$ (r\# h)(r'\#h')=r(h_{(1)}\boldsymbol{.}r')\#h_{(2)}h', $$

where $$ r,r'\in R,\quad h,h'\in H$$, $$ \Delta (h)=h_{(1)}\otimes h_{(2)} $$ (Sweedler notation) is the coproduct of $$ h\in H $$, and $$ \boldsymbol{.}:H\otimes R\to R $$ is the left action of H on R. Further, the coproduct of $$ R\# H $$ is determined by the formula
 * $$ \Delta (r\#h)=(r^{(1)}\#r^{(2)}{}_{(-1)}h_{(1)})\otimes (r^{(2)}{}_{(0)}\#h_{(2)}), \quad r\in R,h\in H.$$

Here $$\Delta _R(r)=r^{(1)}\otimes r^{(2)}$$ denotes the coproduct of r in R, and $$ \delta (r^{(2)})=r^{(2)}{}_{(-1)}\otimes r^{(2)}{}_{(0)} $$ is the left coaction of H on $$ r^{(2)}\in R. $$