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An Elementary Recursive Bound for Effective Positivstellensatz and Hilbert’s 17th problem

About this Title

Henri Lombardi, Laboratoire of Mathématiques (UMR CNRS 6623) UFR des Sciences et Techniques, Université of Franche-Comté 25030 Besançon cedex France, Daniel Perrucci, Departamento de Matemática, FCEN, Universidad de Buenos Aires and IMAS CONICET-UBA, Argentina and Marie-Françoise Roy, IRMAR (UMR CNRS 6625), Université de Rennes 1, Campus de Beaulieu 35042 Rennes cedex France

Publication: Memoirs of the American Mathematical Society
Publication Year: 2020; Volume 263, Number 1277
ISBNs: 978-1-4704-4108-1 (print); 978-1-4704-5662-7 (online)
DOI: https://doi.org/10.1090/memo/1277
Published electronically: March 2, 2020
Keywords: Hilbert’s 17th problem, Positivstellensatz, Real Nullstellensatz, degree bounds, elementary recursive functions
MSC: Primary 12D15, 14P99, 13J30

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Table of Contents

Chapters

  • 1. Introduction
  • 2. Weak inference and weak existence
  • 3. Intermediate value theorem
  • 4. Fundamental theorem of algebra
  • 5. Hermite’s Theory
  • 6. Elimination of one variable
  • 7. Proof of the main theorems
  • 8. Annex

Abstract

We prove an elementary recursive bound on the degrees for Hilbert’s 17th problem. More precisely we express a nonnegative polynomial as a sum of squares of rational functions, and we obtain as degree estimates for the numerators and denominators the following tower of five exponentials \begin{equation*} 2^{ 2^{ 2^{d^{4^{k}}} } } \end{equation*} where $d$ is the degree and $k$ is the number of variables of the input polynomial. Our method is based on the proof of an elementary recursive bound on the degrees for Stengle’s Positivstellensatz. More precisely we give an algebraic certificate of the emptyness of the realization of a system of sign conditions and we obtain as degree bounds for this certificate a tower of five exponentials, namely \begin{equation*} 2^{ 2^{\left (2^{\max \{2,d\}^{4^{k}}}+ s^{2^{k}}\max \{2, d\}^{16^{k}\mathrm {bit}(d)} \right )} } \end{equation*} where $d$ is a bound on the degrees, $s$ is the number of polynomials and $k$ is the number of variables of the input polynomials.

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