On the generation of positivstellensatz witnesses in degenerate cases

  • Authors:
  • David Monniaux;Pierre Corbineau

  • Affiliations:
  • CNRS, VERIMAG;Université Joseph Fourier, VERIMAG

  • Venue:
  • ITP'11 Proceedings of the Second international conference on Interactive theorem proving
  • Year:
  • 2011

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Abstract

One can reduce the problem of proving that a polynomial is nonnegative, or more generally of proving that a system of polynomial inequalities has no solutions, to finding polynomials that are sums of squares of polynomials and satisfy some linear equality (Positivstellen-satz). This produces a witness for the desired property, from which it is reasonably easy to obtain a formal proof of the property suitable for a proof assistant such as Coq. The problem of finding a witness reduces to a feasibility problem in semidefinite programming, for which there exist numerical solvers. Unfortunately, this problem is in general not strictly feasible, meaning the solution can be a convex set with empty interior, in which case the numerical optimization method fails. Previously published methods thus assumed strict feasibility; we propose a workaround for this difficulty. We implemented our method and illustrate its use with examples, including extractions of proofs to Coq.