In mathematics, the Hasse invariant (or Hasse–Witt invariant) of a quadratic form Q over a field K takes values in the Brauer group Br(K). The name "Hasse–Witt" comes from Helmut Hasse and Ernst Witt.
The quadratic form Q may be taken as a diagonal form
Σ aixi2.
Its invariant is then defined as the product of the classes in the Brauer group of all the quaternion algebras
(ai, aj) for i < j.
This is independent of the diagonal form chosen to compute it.[1]
It may also be viewed as the second Stiefel–Whitney class of Q.
The invariant may be computed for a specific symbol φ taking values in the group C2 = .[2]
In the context of quadratic forms over a local field, the Hasse invariant may be defined using the Hilbert symbol, the unique symbol taking values in C2.[3] The invariants of a quadratic forms over a local field are precisely the dimension, discriminant and Hasse invariant.[4]
For quadratic forms over a number field, there is a Hasse invariant ±1 for every finite place. The invariants of a form over a number field are precisely the dimension, discriminant, all local Hasse invariants and the signatures coming from real embeddings.[5]
. Introduction to Quadratic Forms over Fields . 67 . . Tsit Yuen Lam . . 2005 . 0-8218-1095-2 . 1068.11023 . 2104929 .
. Jean-Pierre Serre . A Course in Arithmetic . . 7 . . 1973 . 0-387-90040-3 . 0256.12001 . registration .