 In this case, the subgroup "U" above is zero, and hence the Arf invariant is an element of the base field F2; it is either 0 or 1.If the field "K" of characteristic 2 is not perfect (that is, "K" is different from its subfield "K"2 of squares), then the Clifford algebra is another important invariant of a quadratic form.A corrected version of Arf's original statement is that if the degree ["K"