The formal theory of Hopf algebras part II: the case of Hopf algebras
The category HopfR of Hopf algebras over a commutative unital ring R is analyzed with respect to its categorical properties. The main results are: (1) For every ring R the category HopfR is locally presentable, it is coreflective in the category of bialgebras over R, over every R-algebra there exists a cofree Hopf algebra. (2) If, in addition, R is absoluty flat, then HopfR is reflective in the category of bialgebras as well, and there exists a free Hopf algebra over every R-coalgebra. Similar results are obtained for relevant subcategories of HopfR. Moreover it is shown that, for every commutative unital ring R, the so-called "dual algebra functor" has a left adjoint and that, more generally, universal measuring coalgebras exist.
Keywords: Hopf algebras, *bialgebras, limits, colimits, free Hopf algebras, cofree Hopf algebras, Hopf envelope, universal measuring coalgebra.