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Hokkaido Mathematical Journal
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Note on Miyashita-Ulbrich action and H-separable extension

Note on Miyashita-Ulbrich action and \(H\)-separable extension
Authors: KADISON, Lars;

Note on Miyashita-Ulbrich action and H-separable extension

Abstract

Consider algebras over a given commutative ring \(R\), and let \(A\) be a Hopf algebra which is a finitely generated and projective module over \(R\). Suppose \(B\) is an \(A\)-Galois extension of a subalgebra \(C\) and there is a homomorphism \(\alpha\) of \(B\) into an algebra \(E\). Then \(E\) is a bimodule over \(B\), a bimodule over \(C\) by restriction of \(\alpha\), and the opposite \(\Omega^{\text{op}}\) of the ring \(\Omega\) of endomorphisms of the right \(C\)-module \(E\) is a left \(A\)-module algebra. Right multiplication by an element of the centralizer \(V\) of \(\alpha(C)\) in \(E\) is an endomorphism of the right \(C\)-module \(E\); and thus \(V\) can be identified with a subring of \(\Omega^{\text{op}}\) which is a left \(A\)-module subalgebra, and the action of \(A\) on \(V\) is the Miyashita-Ulbrich action. On the other hand, let \(E\) be a ring extension of an algebra \(C\) such that the right \(C\)-module \(E\) is a progenerator. If \(E\) is an Azumaya algebra; then the ring \(\Omega\) of endomorphisms of the right \(C\)-module \(E\) is isomorphic to \(E\otimes V^{\text{op}}\) where \(V\) is the centralizer of \(C\) in \(E\), and \textit{Y. Doi} and \textit{M. Takeuchi} [J. Algebra 121, No. 2, 488-516 (1989; Zbl 0675.16004)] used Morita duality to show that any action of \(A\) on \(V\) by which \(V\) becomes a left \(A\)-module algebra arises from an \(A\)-Galois extension of \(C\). In this paper, the author shows that \(\Omega\) is isomorphic to \(E\otimes_ZV^{\text{op}}\), where \(Z\) is the center of \(E\), whenever \(E\) is an \(H\)-separable extension of \(C\) and uses the same methods of Morita duality to obtain a very satisfactory generalization of the results of Doi and Takeuchi.

Keywords

Separable algebras (e.g., quaternion algebras, Azumaya algebras, etc.), endomorphisms, Hopf algebras, Morita dualities, \(H\)-separable extensions, Hopf-Galois extensions, Miyashita-Ulbrich actions, Azumaya algebras, progenerators, Hopf algebras (associative rings and algebras)

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
3
Average
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Average
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