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Skew Hopf algebras, irreducible extensions and the pi-method

Lars Kadison
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Abstract

To a depth two extension A | B, we associate the dual bialgebroids S := \End {}_BA_B and T := (A ø_B A)^B over the centralizer R=C_A(B). In the set-up where R is a subalgebra of B, which is quite common, two nondegenerate pairings of S and T will define an anti-automorphism τof the algebra S. Making use of a two-sided depth two structure, we prove that τis an antipode and S is a Hopf algebroid of a type we call skew Hopf algebra. A final section discusses how τand the nondegenerate pairings generalize to modules via the pi-method for depth two, and a certain derived mapping of cochain complexes is nullhomotopic.

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Lars Kadison. 2007-12-28. Skew Hopf algebras, irreducible extensions and the pi-method. https://arxiv.org/abs/math/0701427

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