# Yetter–Drinfeld category

In mathematics a Yetter–Drinfeld category is a special type of braided monoidal category. It consists of modules over a Hopf algebra which satisfy some additional axioms.

## Definition

Let H be a Hopf algebra over a field k. Let $\Delta$ denote the coproduct and S the antipode of H. Let V be a vector space over k. Then V is called a (left left) Yetter–Drinfeld module over H if
* $(V,{\boldsymbol {.}})$ is a left H-module, where ${\boldsymbol {.}}:H\otimes V\to V$ denotes the left action of H on V,
* $(V,\delta \;)$ is a left H-comodule, where $\delta :V\to H\otimes V$ denotes the left coaction of H on V,
* the maps ${\boldsymbol {.}}$ and $\delta$ satisfy the compatibility condition
       $\delta (h{\boldsymbol {.}}v)=h_{(1)}v_{(-1)}S(h_{(3)})\otimes h_{(2)}{\boldsymbol {.}}v_{(0)}$ for all $h\in H,v\in V$,
    where, using Sweedler notation, $(\Delta \otimes \mathrm {id} )\Delta (h)=h_{(1)}\otimes h_{(2)}\otimes h_{(3)}\in H\otimes H\otimes H$ denotes the twofold coproduct of $h\in H$, and $\delta (v)=v_{(-1)}\otimes v_{(0)}$.

## Examples

* Any left H-module over a cocommutative Hopf algebra H is a Yetter–Drinfeld module with the trivial left coaction $\delta (v)=1\otimes v$.
* The trivial module $V=k\{v\}$ with $h{\boldsymbol {.}}v=\epsilon (h)v$, $\delta (v)=1\otimes v$, is a Yetter–Drinfeld module for all Hopf algebras H.
* If H is the group algebra kG of an abelian group G, then Yetter–Drinfeld modules over H are precisely the G-graded G-modules. This means that
       $V=\bigoplus _{g\in G}V_{g}$,
    where each $V_{g}$ is a G-submodule of V.
* More generally, if the group G is not abelian, then Yetter–Drinfeld modules over H=kG are G-modules with a G-gradation
       $V=\bigoplus _{g\in G}V_{g}$, such that $g.V_{h}\subset V_{ghg^{-1}}$.
* Over the base field $k=\mathbb {C} \;$ all finite-dimensional, irreducible/simple Yetter–Drinfeld modules over a (nonabelian) group H=kG are uniquely given through a conjugacy class $[g]\subset G\;$ together with $\chi ,X\;$ (character of) an irreducible group representation of the centralizer $Cent(g)\;$ of some representing $g\in [g]$:
    $V={\mathcal {O}}_{[g]}^{\chi }={\mathcal {O}}_{[g]}^{X}\qquad V=\bigoplus _{h\in [g]}V_{h}=\bigoplus _{h\in [g]}X$
** As G-module take ${\mathcal {O}}_{[g]}^{\chi }$ to be the induced module of $\chi ,X\;$:
       $Ind_{Cent(g)}^{G}(\chi )=kG\otimes _{kCent(g)}X$
    (this can be proven easily not to depend on the choice of g)
** To define the G-graduation (comodule) assign any element $t\otimes v\in kG\otimes _{kCent(g)}X=V$ to the graduation layer:
       $t\otimes v\in V_{tgt^{-1}}$
** It is very custom to directly construct $V\;$ as direct sum of X´s and write down the G-action by choice of a specific set of representatives $t_{i}\;$ for the $Cent(g)\;$-cosets. From this approach, one often writes
       $h\otimes v\subset [g]\times X\;\;\leftrightarrow \;\;t_{i}\otimes v\in kG\otimes _{kCent(g)}X\qquad {\text{with uniquely}}\;\;h=t_{i}gt_{i}^{-1}$
    (this notation emphasizes the graduation $h\otimes v\in V_{h}$, rather than the module structure)

## Braiding

Let H be a Hopf algebra with invertible antipode S, and let V, W be Yetter–Drinfeld modules over H. Then the map $c_{V,W}:V\otimes W\to W\otimes V$,
       $c(v\otimes w):=v_{(-1)}{\boldsymbol {.}}w\otimes v_{(0)},$
    is invertible with inverse
       $c_{V,W}^{-1}(w\otimes v):=v_{(0)}\otimes S^{-1}(v_{(-1)}){\boldsymbol {.}}w.$
    Further, for any three Yetter–Drinfeld modules U, V, W the map c satisfies the braid relation
       $(c_{V,W}\otimes \mathrm {id} _{U})(\mathrm {id} _{V}\otimes c_{U,W})(c_{U,V}\otimes \mathrm {id} _{W})=(\mathrm {id} _{W}\otimes c_{U,V})(c_{U,W}\otimes \mathrm {id} _{V})(\mathrm {id} _{U}\otimes c_{V,W}):U\otimes V\otimes W\to W\otimes V\otimes U.$
A monoidal category ${\mathcal {C}}$ consisting of Yetter–Drinfeld modules over a Hopf algebra H with bijective antipode is called a Yetter–Drinfeld category. It is a braided monoidal category with the braiding c above. The category of Yetter–Drinfeld modules over a Hopf algebra H with bijective antipode is denoted by $_{H}^{H}{\mathcal {YD}}$.