Let H be a Hopf algebra. Today we're going to talk about its actions (and its coactions, time permitting). A Hopf algebra is just an algebra, which is just a ring, and ring's have actions (equiv. modules), and we're comfortable with that already. For this to make sense on a Hopf algebra, we merely require that the action agrees with all the necessary units, products, coproducts, et cetera.
More precisely, we say that H acts on an algebra A (or that A is an H-module algebra) if we have a linear map \triangleright : H \otimes A \rightarrow A, written h \triangleright a such that
\displaystyle h \triangleright (ab) = m(\Delta(h) \triangleright (a \otimes b))
with \triangleright extended to tensor products and m the product of A, and
\displaystyle h\triangleright 1_A = \epsilon(h) 1_A
Some weirdness emerges here, for instance, if H=kG, the group hopf algebra of a finite group G (the vector space having elements of G as a basis, the product being the group operation, and the coproduct defined by \Delta(g) = g \otimes g, antipode, units, et cetera are the obvious choice), then a hopf action collapses to a typical group action:
\displaystyle g\triangleright(ab) = (g\triangleright a) (g\triangleright b)
But if we forget the Hopf algebra structure for a second, we know that kG is exactly the same as k(G), that is, as functions defined on G with values in k, as any ``vector'' in kG defines a function and vice versa. Hence, as vector spaces kG and k(G) are naturally isomorphic. This is not the case with the Hopf algebra structure, however (though we have seen how the two are dually paired already), as the coproduct on k(G) is \Delta(f)(x,y) = f(xy), where we identify k(G \times G) with k(G) \otimes k(G) (this is exactly what a tensor product is for, actually.) In this case, the hopf algebra action on an algebra A is the same as a G-grading on A. Let me explain this (with help from a kind person on math.stackexchange):
Say A is G-graded and for each a \in A let |a| denote the element in G such that a \in A_{|a|}. We can get a k(G) action from this by f \triangleright a = f(|a|) a. To go the other way, start from a Hopf algebra action \triangleright: k(G) \otimes A \rightarrow A. Then for each g \in G, let A_g be the set of all elements of A on which every f\in k(G) acts via scalar multiplication by f(g). One then has to prove that these sets are nonempty, that they are subspaces, that A decomposes into direct sums of them, and that they obey the axioms of a grading. I've been assured by various sources that it can be done!
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