Computable analysis for verified exact real computation

Michal Konečný, Florian Steinberg, Holger Thies

    Research output: Chapter in Book/Published conference outputConference publication


    We use ideas from computable analysis to formalize exact real number computation in the Coq proof assistant. Our formalization is built on top of the Incone library, a Coq library for computable analysis. We use the theoretical framework that computable analysis provides to systematically generate target specifications for real number algorithms. First we give very simple algorithms that fulfill these specifications based on rational approximations. To provide more efficient algorithms, we develop alternate representations that utilize an existing formalization of floating-point algorithms and interval arithmetic in combination with methods used by software packages for exact real arithmetic that focus on execution speed. We also define a general framework to define real number algorithms independently of their concrete encoding and to prove them correct. Algorithms verified in our framework can be extracted to Haskell programs for efficient computation. The performance of the extracted code is comparable to programs produced using non-verified software packages. This is without the need to optimize the extracted code by hand. As an example, we formalize an algorithm for the square root function based on the Heron method. The algorithm is parametric in the implementation of the real number datatype, not referring to any details of its implementation. Thus the same verified algorithm can be used with different real number representations. Since Boolean valued comparisons of real numbers are not decidable, our algorithms use basic operations that take values in the Kleeneans and Sierpinski space. We develop some of the theory of these spaces. To capture the semantics of non-sequential operations, such as the “parallel or”, we use multivalued functions.

    Original languageEnglish
    Title of host publication40th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science, FSTTCS 2020
    EditorsNitin Saxena, Sunil Simon
    ISBN (Electronic)9783959771740
    Publication statusPublished - Dec 2020
    Event40th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science, FSTTCS 2020 - Virtual, Goa, India
    Duration: 14 Dec 202018 Dec 2020

    Publication series

    NameLeibniz International Proceedings in Informatics, LIPIcs
    ISSN (Print)1868-8969


    Conference40th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science, FSTTCS 2020
    CityVirtual, Goa

    Bibliographical note

    Funding Information:
    Funding Michal Konečný: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 731143. Holger Thies: Supported by JSPS KAKENHI Grant Numbers JP18J10407 and JP20K19744 and by the Japan Society for the Promotion of Science (JSPS), Core-to-Core Program (A. Advanced Research Networks).

    Publisher Copyright:
    © Michal Konečný, Florian Steinberg, and Holger Thies; licensed under Creative Commons License CC-BY.


    • Computable Analysis
    • Coq
    • Exact real computation
    • Formal proofs
    • Proof assistant


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