Abstract
For every action φ ∈ Hom(G, Autk(K)) of a group G on a commutative ring K we introduce two abelian monoids. The monoid Cliffk(φ) consists of equivalence classes of strongly G-graded algebras of type φ up to G-graded Clifford system extensions of K-central algebras. The monoid \({{\cal C}_k}(\phi )\) consists of equivariance classes of homomorphisms of type φ from G to the Picard groups of K-central algebras (generalized collective characters). Furthermore, for every such φ there is an exact sequence of abelian monoids
This sequence describes the obstruction to realizing a generalized collective character of type φ, that is it determines if such a character is associated to some strongly G-graded k-algebra. The rightmost homomorphism is often surjective, terminating the above sequence. When φ is a Galois action, then the well-known restriction-obstruction sequence of Brauer groups is an image of an exact sequence of sub-monoids appearing in the above sequence.
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Acknowledgement
The author is indebted to E. Aljadeff for suggesting Theorem 5.5, and to A. Antony, D. Blanc and O. Schnabel for valuable discussions, and to the referee for useful comments.
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Ginosar, Y. Realization-obstruction exact sequences for Clifford system extensions. Isr. J. Math. 247, 955–985 (2022). https://doi.org/10.1007/s11856-022-2300-z
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DOI: https://doi.org/10.1007/s11856-022-2300-z