Let's talk about the category of modules {_H\mathcal{M}} for a braided Hopf algebra {H} for a minute. If {V} and {W} are {H}-modules, then we have an easy action of {H} on {V \otimes W} as well, by:
\displaystyle h \triangleright(v\otimes w) = \sum h_1\triangleright v \otimes h_2\triangleright w
Hence we can think of {\otimes} as a functor {_H\mathcal{M} \times _H\mathcal{M} \rightarrow _H\mathcal{M}}. This sort of functor is what turns {_H\mathcal{M}} into a monoidal category. We also have the opposite function {\otimes^\text{op}}, which does what one would expect: {V \otimes^\text{op} W = W \otimes V}.
We have another operation on this category, the flip map {\tau}, which carries the action of one functor {\otimes} to another {\otimes^\text{op}}. We'd like it to be a natural transformation, actually a natural isomorphism (one can see already that it's invertible) from {\otimes \rightarrow \otimes^\text{op}}. For this to work we must have, for each {f: V \rightarrow V'} and {g: W \rightarrow W'} in the category, a morphism {\tau_{V \otimes W}: V \otimes W \rightarrow W \otimes V} such that the square
commutes. But before we check that, let's first verify that {\tau_{V \otimes W}} is even a morphism in the category (to take a step back from category-theoretic language, this means that we need to check that {\tau_{V \otimes W}(v\otimes w) = w \otimes v} is an {H}-linear map). But it's not necessarily true that {h \triangleright \tau_{V \otimes W}(v\otimes w) = \tau_{V \otimes W}(\sum h_{(1)}\triangleright v \otimes h_{(2)}\triangleright w)}. In particular, this is only true when {H} is co-commutative.
But {H} is a braided Hopf algebra, meaning that its failure to be co-commutative is controlled by an invertible element {\mathcal{R} \in H \otimes H}. Can we use this to construct an natural transformation {\otimes \rightarrow \otimes^{op}}? Yes, we can, and this is exactly the structure that gives us a braided category.
To be brief, we can define a braided category to be a monoidal category together with a natural isomorphism {\Psi : \otimes \rightarrow \otimes^\text{op}} that obeys the following conditions: { \Psi_{V \otimes W, Z} = \Psi_{V,Z} \circ \Psi_{W,Z}} and {\Psi_{V,W\otimes Z} = \Psi_{V,Z} \circ \Psi_{V,W}}. (These are the ``hexagon conditions''. I'm not writing the structure maps of the monodial category, but when you include them, you express these as two hexagonal commutative diagrams.)
In the case of a braided, or quasitriangular Hopf algebra, our {\Psi_{V,W} = \tau(\mathcal{R} \triangleright v\otimes w) = \sum R_2 \triangleright w \otimes R_1 \triangleright v}. The reader can verify that this meets the axioms of a braided category.
There's more we can say about the braiding {\Psi} and it's connection to braid groups, et cetera, but I don't want to go down that route this evening. Rather, I want to discuss the dual version of this theory. First, I need find the dual notion of a braided Hopf algebra. We'll call this a co-braided Hopf algebra. By this I mean that we want to control how far {H} is from being commutative, instead of co-commutative.
A co-braided Hopf algebra is a Hopf algebra {H} together with a linear functional {R: H\otimes H \rightarrow k}. We require {R} to be convolution invertible, id est, that there exists another linear functional {R^{-1}} such that {\sum R^{-1}(h_1 \otimes g_1) R(h_2 \otimes g_2) = \epsilon(h)\epsilon(g) = \sum R(h_1 \otimes g_1) R^{-1}(h_2\otimes g_2)}. We also require it to obey three other relations:
\displaystyle \sum g_1h_1 R(h_2\otimes g_2) = \sum R(h_1 \otimes g_1) h_2g_2
\displaystyle R (hf \otimes f) = \sum R(h\otimes f_1)R(g\otimes f_2)
\displaystyle R(h\otimes gf) = \sum R(h_1 \otimes f)R(h_2 \otimes g)
for {g,h,f \in H}.
Let {V} and {W} be {H}-co-modules with structure maps {\beta_V: V \rightarrow H \otimes V} by {v \mapsto \sum g_1 \otimes v_1} and {\beta_W: W \rightarrow H \otimes W} by {w \mapsto \sum h_1 \otimes w_1} The tensor functor works like such:
\displaystyle \beta_{V\otimes W}(v\otimes w) = \sum g_1 h_1 \otimes v_1 \otimes w_1
What does the braiding on the category of co-modules {^H\mathcal{M}} look like? I propose that the braiding {\Psi} is then:
\displaystyle \Psi_{V,W}(v\otimes w) = \left(\sum R(g_1 \otimes h_1) \right) w_1 \otimes v_1
I've not proved it yet.
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