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On the generation of Positivstellensatz witnesses in degenerate cases. (English) Zbl 1342.68296

Van Eekelen, Marko (ed.) et al., Interactive theorem proving. Second international conference, ITP 2011, Berg en Dal, The Netherlands, August 22–25, 2011. Proceedings. Berlin: Springer (ISBN 978-3-642-22862-9/pbk). Lecture Notes in Computer Science 6898, 249-264 (2011).
Summary: 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 (Positivstellensatz). 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.
For the entire collection see [Zbl 1219.68024].

MSC:

68T15 Theorem proving (deduction, resolution, etc.) (MSC2010)
03C10 Quantifier elimination, model completeness, and related topics
12D15 Fields related with sums of squares (formally real fields, Pythagorean fields, etc.)

Software:

DSDP5; Coq; RAGlib

References:

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