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On trace forms of Hilbertian fields. (English) Zbl 0662.10015
A quadratic form over a field K is called positive if its signature is nonnegative for every possible ordering of the field K; and it is called algebraic if it is Witt equivalent to the trace form of an algebraic extension L of K. The authors consider the question of when positive forms are algebraic.
Suppose K has the property that whenever a polynomial f(T,X) over K in indeterminates T and X is irreducible as a polynomial in X over K(T), then there exists an element $$\beta$$ of K such that f($$\beta$$,X) is an irreducible polynomial in X over K. It is proved that the form $$\delta X^ 2$$ is algebraic for any element $$\delta$$ of K which is positive for every possible ordering of K. This theorem is combined with a theorem by W. C. Waterhouse [Arch. Math. 47, 229-231 (1986; Zbl 0607.10013)] to give many special cases in which positive forms are algebraic.
Reviewer: H.F.Kreimer

MSC:
 11E04 Quadratic forms over general fields 11E16 General binary quadratic forms 12F10 Separable extensions, Galois theory
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