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We introduce the notion of a partial corepresentation of a given Hopf algebra H over a coalgebra C and the closely related concept of a partial H-comodule. We prove that there exists a universal coalgebra Hpar, associated to the original Hopf algebra H, such that the category of regular partial H-comodules is isomorphic to the category of Hpar-comodules. We introduce the notion of a Hopf coalgebroid and show that the universal coalgebra Hpar has the structure of a Hopf coalgebroid over a suitable coalgebra., SCOPUS: ar.j, info:eu-repo/semantics/published

We study dualities between Lie algebras and Lie coalgebras, and their respective (co)representations. To allow a study of dualities in an infinite-dimensional setting, we introduce the notions of Lie monads and Lie comonads, as special cases of YB-Lie algebras and YB-Lie coalgebras in additive monoidal categories. We show that (strong) dualities between Lie algebras and Lie coalgebras are closely related to (iso)morphisms between associated Lie monads and Lie comonads. In the case of a duality between two Hopf algebras - in the sense of Takeuchi - we recover a duality between a Lie algebra and a Lie coalgebra - in the sense defined in this note - by computing the primitive and the indecomposable elements, respectively. © 2014 Elsevier Inc., SCOPUS: ar.j, info:eu-repo/semantics/published

The results in our paper heavily rely on the journal version of [T. Brzezinski, A note on coring extensions, Ann. Univ. Ferrara Sez. VII (N.S.) 51 (2005) 15–27; a corrected version is available at http://arxiv.org/abs/math/0410020v3 , Theorem 2.6]. Since it turned out recently that in the proof of the quoted theorem there are some assumptions missing, our derived results are not expected to hold at the stated level of generality either. Here we supplement the constructions in our article with the missing assumptions and show that they hold in most of our examples. In order to handle also the non-fitting case of cleft extensions by arbitrary Hopf algebroids, Morita contexts are constructed that do not necessarily correspond to coring extensions. They are used to prove a Strong Structure Theorem for cleft extensions by arbitrary Hopf algebroids. In this way we obtain in particular a corrected form of the journal version of [G. Bohm, Integral theory for Hopf algebroids, Algebr. Represent. Theory 8 (4) (2005) 563–599; Corrigendum, to be published; see also http://arxiv.org/abs/math/0403195v4 , Theorem 4.2], whose original proof contains a similar error.

A Morita context is constructed for any comodule of a coring and, more generally, for an $L$-$\cC$ bicomodule $\Sigma$ for a pure coring extension $(\cD:L)$ of $(\cC:A)$. It is related to a 2-object subcategory of the category of $k$-linear functors $\Mm^\Cc\to\Mm^\Dd$. Strictness of the Morita context is shown to imply the Galois property of $\Sigma$ as a $\cC$-comodule and a Weak Structure Theorem. Sufficient conditions are found also for a Strong Structure Theorem to hold. Cleft property of an $L$-$\cC$ bicomodule $\Sigma$ -- implying strictness of the associated Morita context -- is introduced. It is shown to be equivalent to being a Galois $\cC$-comodule and isomorphic to $\End^\cC(\Sigma)\otimes_{L} \cD$, in the category of left modules for the ring $\End^\cC(\Sigma)$ and right comodules for the coring $\cD$, i.e. satisfying the normal basis property. Algebra extensions, that are cleft extensions by a Hopf algebra, a coalgebra or a pure Hopf algebroid, as well as cleft entwining structures (over commutative or non-commutative base rings) and cleft weak entwining structures, are shown to provide examples of cleft bicomodules. Cleft extensions by arbitrary Hopf algebroids are described in terms of Morita contexts that do not necessarily correspond to coring extensions., Comment: 34 pages LaTeX. v2:A missing purity assumption is added throughout Sections 3, 4 and 5