Reformalization Study of Jordan Curve Theorem in Proof Assistants

Simon Guilloud, Sankalp Gambhir, Samuel Chassot· July 3, 2026 View original

Summary

This paper presents a case study in reformalization, where formal proofs from one proof assistant are translated into another. It reports three reformalizations of the Jordan Curve Theorem from Mizar to Lean, HOL Light to Lean, and HOL Light to Agda, analyzing pipeline design choices.

This paper delves into the concept of reformalization, a specialized form of autoformalization where the input is not natural language but an existing formal proof developed in a different proof assistant. The study focuses on the Jordan Curve Theorem, a foundational result in topology, as a complex case study for this process. The researchers detail three specific reformalizations: translating a proof from Mizar into Lean, and two translations from HOL Light into Lean and Agda, respectively. This multi-faceted approach allows for a comparative analysis of the challenges and successes involved in porting highly intricate formal mathematical developments across different formal verification systems. The analysis identifies and discusses various pipeline design choices that are critical for the practical success of reformalization tasks. This includes aspects like parsing, semantic mapping, and handling differences in logical foundations and library structures between the source and target proof assistants. The findings contribute to the broader goal of making formal mathematics more interoperable and accessible across different verification tools.

Why it matters

Professionals in software engineering, formal verification, and AI for mathematics can gain insights into the challenges and methodologies for translating and verifying complex proofs across different formal systems, improving reliability and interoperability.

How to implement this in your domain

  1. 1Explore tools and techniques for autoformalization or reformalization to translate mathematical specifications or proofs between different formal systems.
  2. 2Investigate the use of proof assistants (e.g., Lean, Agda) for formal verification of critical software components or algorithms.
  3. 3Contribute to or utilize open-source efforts aimed at building bridges between different formal verification ecosystems.
  4. 4Consider the implications of formal proof interoperability for developing highly reliable AI systems that require mathematical guarantees.

Who benefits

Software EngineeringCybersecurityAI DevelopmentAcademiaAerospace

Key takeaways

  • Reformalization involves translating formal proofs between different proof assistants.
  • The Jordan Curve Theorem serves as a complex case study for this process.
  • Pipeline design choices are critical for practical reformalization tasks.
  • This research contributes to improving interoperability and accessibility in formal mathematics.

Original post by Simon Guilloud, Sankalp Gambhir, Samuel Chassot

"arXiv:2607.01734v1 Announce Type: new Abstract: We present a case study in reformalization, a variant of autoformalization in which the input proof is not natural language but a formal development in a different proof assistant. Concretely, we report three reformalizations of the…"

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Originally posted by Simon Guilloud, Sankalp Gambhir, Samuel Chassot on X · view source

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