Nicolas Grenier-Boley, Detlev Hoffmann with an appendix by Claus Scheiderer: Isomorphism criteria for Witt rings of real fields

Submission: 2009, Jun 26

We prove isomorphism criteria for Witt rings and reduced Witt rings of certain types of real fields. Refined criteria are obtained under the additional assumption that the field be SAP. This leads to a generalization of a result by Koprowski on Witt equivalence of function fields of transcendence degree 1 over a real closed field. Isomorphism criteria are also obtained for Witt rings of hermitian forms over a quadratic extension of a real base field and for Witt groups of hermitian forms over a quaternion algebra with a real field as center. All these criteria are expressed in terms of properties involving topological subspaces of the space of orderings of the base field.

2000 Mathematics Subject Classification: Primary 11E10 ; Secondary 11E04, 11E81, 12D15

Keywords and Phrases: Witt ring, reduced Witt ring, Witt group, real field, ordering, preordering, Witt equivalence, hermitian form, quadratic form, quaternion algebra, function field.

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