A Verified Decision Procedure for Univariate Real Arithmetic with the BKR Algorithm

Authors Katherine Cordwell , Yong Kiam Tan , André Platzer

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Author Details

Katherine Cordwell
  • Computer Science Department, Carnegie Mellon University, Pittsburgh, PA, USA
Yong Kiam Tan
  • Computer Science Department, Carnegie Mellon University, Pittsburgh, PA, USA
André Platzer
  • Computer Science Department, Carnegie Mellon University, Pittsburgh, PA, USA


We thank Brandon Bohrer, Fabian Immler, and Wenda Li for useful discussions about Isabelle/HOL and its libraries. We also thank the ITP'21 anonymous reviewers and Magnus Myreen for helpful feedback on earlier drafts of this paper.

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Katherine Cordwell, Yong Kiam Tan, and André Platzer. A Verified Decision Procedure for Univariate Real Arithmetic with the BKR Algorithm. In 12th International Conference on Interactive Theorem Proving (ITP 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 193, pp. 14:1-14:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


We formalize the univariate fragment of Ben-Or, Kozen, and Reif’s (BKR) decision procedure for first-order real arithmetic in Isabelle/HOL. BKR’s algorithm has good potential for parallelism and was designed to be used in practice. Its key insight is a clever recursive procedure that computes the set of all consistent sign assignments for an input set of univariate polynomials while carefully managing intermediate steps to avoid exponential blowup from naively enumerating all possible sign assignments (this insight is fundamental for both the univariate case and the general case). Our proof combines ideas from BKR and a follow-up work by Renegar that are well-suited for formalization. The resulting proof outline allows us to build substantially on Isabelle/HOL’s libraries for algebra, analysis, and matrices. Our main extensions to existing libraries are also detailed.

Subject Classification

ACM Subject Classification
  • Theory of computation → Logic and verification
  • quantifier elimination
  • matrix
  • theorem proving
  • real arithmetic


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