Witt group

Named after German mathematician Ernst Witt (1911–1991), who introduced the concept in 1937.

Witt group (plural Witt groups)

1. (algebra) Given a field k of characteristic ≠ 2, the abelian group of equivalence classes of nondegenerate symmetric bilinear forms over k (where the equivalence relation is such that two forms are equivalent if each is obtainable from the other by adding a metabolic quadratic space), with the group operation corresponding to that of orthogonal direct sum of forms;
   (algebraic geometry, by extension) given a variety X, the quotient of the Grothendieck group of isometry classes of quadratic spaces on X, with respect to orthogonal sum, modulo the subgroup generated by metabolic spaces
   (category theory, by extension) given a category with duality, the quotient of isometry classes of symmetric spaces, modulo metabolic spaces.
   • 1995, Raman Parimala, “Study of quadratic forms - Some connections with geometry”, in S. D. Chatterji, editor, Proceedings of the International Congress of Mathematicians, Zürich m94, Birkhäuser, page 328:

     A general method of studying the Witt group of a smooth variety is through the graded group associated to the filtration induced by the filtration of the Witt group of the function field by powers of the fundamental ideal of even rank forms.

   • 2011, Marco Schlichting, Higher Algebraic K-theory, Guillermo Cortiñas (editor), Topics in Algebraic and Topological K-Theory, Springer, Lecture Notes in Mathematics 2008, page 167,

     The second reason for this emphasis is that an analog of the Thomason-Waldhausen Localization Theorem also holds for many other (co-) homology theories besides K-theory, among which Hochschild homology, (negative, periodic, ordinary) cyclic homology , topological Hochschild (and cyclic) homology , triangular Witt groups and higher Grothendieck–Witt groups .

   • 2020, Matthias Wendt, “Oriented Schubert calculus in Chow-Witt rings of Grassmannians”, in Federico Binda, Marc Levine, Manh Toan Nguyen, Oliver Röndigs, editors, Motivic Homotopy Theory and Refined Enumerative Geometry, American Mathematical Society, pages 239–240:

     The Witt group of a category with duality is given as the quotient of the isometry classes of symmetric spaces modulo metabolic spaces. […] For coherent and derived Witt groups, the derived tensor product of complexes gives rise to duality-preserving functions and consequently to pairings in triangular Witt groups, cf. [GN03].

• A Witt group of a field can be made into a commutative ring by equipping it with a ring product defined as the tensor product of quadratic forms.
  • This construction is sometimes called a Witt ring; the term is ambiguous, however, being also often used for a completely unrelated ring of Witt vectors.

• Witt decomposition
• Witt index
• Witt ring
• Witt vector

• Witt's theorem on Wikipedia.Wikipedia
• Witt ring on Encyclopedia of Mathematics