Morita theory for coring extensions and cleft bicomodules

Gabriella Böhm, Joost Vercruysse

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15 Citations (Scopus)


A Morita context is constructed for any comodule of a coring and, more generally, for an L - C bicomodule Σ for a coring extension (D : L) of (C : A). It is related to a 2-object subcategory of the category of k-linear functors MC → MD. Strictness of the Morita context is shown to imply the Galois property of Σ as a C-comodule and a Weak Structure Theorem. Sufficient conditions are found also for a Strong Structure Theorem to hold. Cleft property of an L - C bicomodule Σ-implying strictness of the associated Morita context-is introduced. It is shown to be equivalent to being a GaloisC -comodule and isomorphic to EndC (Σ) ⊗L D, in the category of left modules for the ring EndC (Σ) and right comodules for the coring D, i.e. satisfying the normal basis property. Algebra extensions, that are cleft extensions by a Hopf algebra, a coalgebra or a 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.

Original languageEnglish
Pages (from-to)611-648
Number of pages38
JournalAdvances in Mathematics
Issue number2
Publication statusPublished - Mar 1 2007



  • Cleft bicomodule
  • Coring extension
  • Morita theory
  • Weak and strong structure theorems

ASJC Scopus subject areas

  • Mathematics(all)

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