**Quadratic Forms Over F_{2}**

Over *F*_{2} the Arf invariant is 0 if the quadratic form is equivalent to a direct sum of copies of the binary form, and it is 1 if the form is a direct sum of with a number of copies of .

William Browder has called the Arf invariant the *democratic invariant* because it is the value which is assumed most often by the quadratic form. Another characterization: *q* has Arf invariant 0 if and only if the underlying 2*k*-dimensional vector space over the field **F**_{2} has a *k*-dimensional subspace on which *q* is identically 0 – that is, a totally isotropic subspace of half the dimension; its isotropy index is *k* (this is the maximum dimension of a totally isotropic subspace of a nonsingular form).

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