Discriminants and nonnegative polynomials

  • Authors:
  • Jiawang Nie

  • Affiliations:
  • -

  • Venue:
  • Journal of Symbolic Computation
  • Year:
  • 2012

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Abstract

For a semialgebraic set K in R^n, let P"d(K)={f@?R[x]"@?"d:f(u)=0@?u@?K} be the cone of polynomials in x@?R^n of degrees at most d that are nonnegative on K. This paper studies the geometry of its boundary @?P"d(K). We show that when K=R^n and d is even, its boundary @?P"d(K) lies on the irreducible hypersurface defined by the discriminant @D(f) of f. We show that when K={x@?R^n:g"1(x)=...=g"m(x)=0} is a real algebraic variety, @?P"d(K) lies on the hypersurface defined by the discriminant @D(f,g"1,...,g"m) of f,g"1,...,g"m. We show that when K is a general semialgebraic set, @?P"d(K) lies on a union of hypersurfaces defined by the discriminantal equations. Explicit formulae for the degrees of these hypersurfaces and discriminants are given. We also prove that typically P"d(K) does not have a barrier of type -log@f(f) when @f(f) is required to be a polynomial, but such a barrier exists if @f(f) is allowed to be semialgebraic. Some illustrating examples are shown.