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In mathematics, the Hasse invariant of a quadratic form Q over a field K takes values in the Brauer group Br. The name "Hasse–Witt" comes from Helmut Hasse and Ernst Witt.

The quadratic form Q may be taken as a diagonal form

Its invariant is then defined as the product of the classes in the Brauer group of all the quaternion algebras

This is independent of the diagonal form chosen to compute it.

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