Usable Computer-Checked Proofs and Computations for Number Theorists
Proof assistants (also called interactive theorem provers) are
increasingly used in academia and industry to verify the correctness
of hardware, software, and protocols. However, despite the
trustworthiness guarantees they offer, most mathematicians find them
too laborious to use.
The goal of the Lean Forward project is to collaborate with number theorists to formally prove theorems about research mathematics and to address the main usability issues hampering the adoption of proof assistants in mathematical circles. The theorems will be selected together with our collaborators to guide the development of formal libraries and verified tools.
Our vehicle will be Lean, a disruptive proof assistant developed at Microsoft Research and Carnegie Mellon University. Lean draws on decades of experience in interactive and automatic theorem provers (e.g., Coq, Isabelle/HOL, and Z3). Its logic is very expressive, and emphasis is placed on strong proof automation. The system is easy to extend via metaprograms that rely on the same logical language that is used to express specifications and proofs.
To support the formalization of theorems, we will develop formal libraries of fundamental number theory and explore how to automate proof search in these. Among many other things, we will create libraries for p-adic numbers and integers and design reasoning procedures for these, exploring how to best exploit Lean's metaprogramming framework. Moreover, we will develop techniques and tools that help mathematicians perform accurate computations and reasoning using proof assistants, integrating procedures from computer algebra systems in a foundational, verified fashion. Finally, we will contribute to Lean's development with ideas and features designed to benefit all users, notably an efficient built-in procedure for first-order reasoning and an integration with automatic theorem provers. The ultimate aim is to develop a proof assistant that actually helps mathematicians, by making them more productive and more confident in their results.
Proof assistants (also called interactive theorem provers) are
interactive tools that make it possible to develop computer-checked,
formal proofs of theorems. Two early systems, Automath and Mizar,
were designed to mechanize mathematics. Landmark achievements
include the formal proofs of the Kepler conjecture by Hales et al.
and of the odd-order theorem by Gonthier et al.
The main strength of proof assistants is their trustworthiness: all definitions and lemmas are checked for well-formedness, and even the most trivial proof steps are verified by an inference kernel with respect to a logical system. It took a formal proof to dispel doubts about Hales's proof.
Beyond trustworthiness, formal proofs can also clarify arguments, by exposing and explaining difficult steps. Even without developing any proofs, making theorem statements (including the definitions and hypotheses they rely on) precise can be a substantial gain for communicating results and for making the vast body of mathematics analyzable by computers.
Moreover, by keeping track of changes across large developments, proof assistants facilitate experiments with variants and extensions. When changing a definition, the user is alerted to the inferences that need repair and to redundant definitions, lemmas, and hypotheses.
Finally, modern proof assistants provide automatic proof search that can find long deduction chains quickly, easily beating pen-and-paper methods. Automation is especially useful for highly computational proofs with hundreds or thousands of cases. Despite the success stories, proof assistants can be tedious to use. Most mathematicians find them more of a hindrance than a help. Today, the main users are in hardware verification and programming language research.
Despite the many obstacles, there is a strong feeling in various parts
of the research community that mathematics deserves to be formalized.
Fields medalist Vladimir Voevodsky has been one of the strongest
advocates of this view. His research area was plagued by flawed
theorems, to the point where he stopped believing pen-and-paper
With the emergence of huge computational proofs, the issue of peer reviewing is becoming more pressing. There are many disputed theorems, including Mochizuki's claimed proof of the abc conjecture. It sometimes happens that articles pass peer review, and get published in prestigious journals such as the Annals of Mathematics, before it emerges that a lemma is flawed and the entire proof collapses. More applied branches of research, such as cryptography, are also subject to controversies. By relying to a greater extent on computers to develop and check proofs, researchers can raise the reviewing standards, even for computational proofs, thereby increasing confidence in the results' validity.
Interactive theorem proving is steadily gaining ground. In some
areas of computer science, it is common for research papers to be
accompanied by formal proofs. Proof assistants are even deployed in
the classroom, replacing pen and paper. These circumstances point to
a future where these tools will be routinely used, resulting in more
reliable science. But to have a substantial impact on mathematical
practice, we must narrow the usability gap.
The difficulties are as much social as technical. Proof assistants are developed primarily by computer scientists. Much of the formalized mathematics is motivated by hardware or software verification. Mathematicians largely dismiss proof assistants as impractical, so the technology improves only slowly. Sadly, even routine operations such as factorization of polynomials can become small challenges when moving to a formal-logic environment. We want to break the vicious circle by working closely with mathematicians. We aim to bring a proof assistant, its automation, and its libraries further, guided by the needs of mathematicians who understand the value of proof assistants.
Specifically, we will collaborate with Sander Dahmen and his team at the VU Amsterdam to formalize parts of the team's research in number theory and recent results in related areas, addressing usability issues as they arise. Cooperation with research mathematicians will benefit Lean Forward substantially: not only will they guide us through their field and help us carry out the formalization, they will provide frank feedback on our technology and act as advocates for formal verification in their community.
Our vehicle will be the Lean proof assistant. Lean is a new open source system developed by Leonardo de Moura (Microsoft Research), Jeremy Avigad (Carnegie Mellon), and their colleagues. The system's design and engineering is unusually clean and efficient. Lean attempts to combine the best from two leading proof assistants:
Lean's logical foundation is a variant of Coq's calculus of inductive constructions, a dependent type theory. Lean distinguishes itself with its small inference kernel and strong automation. Independent proof checkers provide additional guarantees. Lean's support for dependent types is smoother than Coq's, thanks to flexible pattern matching and a generalized congruence closure algorithm. A mechanism for introducing quotient types and a transfer tool facilitate reasoning up to isomorphism without resorting to setoids or homotopy type theory.
For the design of basic algebraic libraries, Lean's developers turned to Isabelle/HOL for inspiration. The libraries rely on type classes, a mechanism to categorize types and their operations (e.g., "(Z, +) forms a group"). Type classes interact well with Lean's dependent types. By contrast, in Isabelle/HOL, there is no way to use type classes to reason about the integers modulo n as a ring.
Our overall aim will be met by pursuing four scientific objectives, presented below. Our starting point is that there is tremendous value in simultaneously using and developing a proof assistant. Lean Forward combines these two activities.
Formally prove carefully selected theorems about research number theory
From the mathematicians' point of view, formalization can increase
the trustworthiness of their results and the lucidity of their
proofs, while raising awareness of formal verification in the
mathematics community. From a computer science perspective,
formalization will guide the development of proving
We will formalize theorems emerging from Dahmen's research, notably in the context of his Vidi project New Diophantine Directions, as well as other theorems that interest him and his team.
Moreover, we will participate in Hales's Formal Abstracts initiative, which aims at enticing mathematicians to state their theorems formally in Lean, including any definitions they rely on. Published articles will be complemented by these formal abstracts. Even such lightweight computerization can be a challenge, which is indicative of the inherent difficulty of making mathematics rigorous. Formal abstracts can serve as a gateway to formal proofs and can be used by powerful automatic search mechanisms.
Russell quipped that "the method of 'postulating' what we want has many advantages; they are the same as the advantages of theft over honest toil." However, for most mathematicians, this fundamentalism is unrealistic. Modern research on Diophantine equations often depends on the modularity of elliptic curves, which constitutes the final step in the proof of Fermat's last theorem and relies on an overwhelming amount of theory. Full formalization being unrealistic, we will embrace flexible levels of formality.
Develop formal libraries of fundamental number theory and explore how to automate proof search
This objective is linked to the first objective: we will develop
libraries guided by the theorems we aim at formalizing, but we
will also choose the theorems to produce useful reusable libraries
as a side effect. We will also help develop missing basic
libraries, contributing to the
Lean mathlib. The design of formal mathematical libraries poses many
- Usable design: The experience with building large formal libraries is that textbook lemmas and theorems do not suffice. A lot of scaffolding—mathematically trivial but practically crucial aspects, such as naming conventions, idioms, and syntax—is necessary for usability.
- Proof readability: Many formal proofs are "write only." In our proofs, the important steps will be clearly identified, using Lean's Mizar-like declarative style. Declarative proofs can be used to communicate with mathematicians and are easier to port across systems.
- Strong automation: A good formal library includes heuristics and algorithms—tactics. Lean's revolutionary metaprogramming framework exposes the system's data structures and procedures within the language of Lean itself. Users can comfortably develop tactics in Lean via a monadic interface. Automation can be developed together with the libraries, rather than as an afterthought.
The libraries will be developed with maintainability in mind. To ensure their longevity, we will initiate a repository of third-party Lean formalizations and tools. Our goal is not only to prove lemmas and theorems but also to verify algorithms and use verified algorithms to check entries in online databases (e.g., LMFDB). Beyond their intrinsic appeal, the algorithms often arise naturally in proofs. Moreover, verified algorithms can be used to prove lemmas using reflection.
Design techniques and develop tools that help mathematicians carry out accurate computations and reasoning
Many vital tools are missing. Chief among these is a practical
integration of popular computer algebra systems into Lean. These
systems offer a vast collection of algorithms. The main difficulty
is to obtain and reconstruct certificates produced by the algebra
systems, to validate the results in the proof assistant.
Fortunately, for many algorithms, the answers are their own
certificates. We plan to expand the experimental Lean–Mathematica
bridge to cover more algorithms and algebra systems. Some
algorithms are provided by open-source libraries and could be
included in Lean.
Moreover, we plan to integrate our Nunchaku counterexample generator and complete the integration of the Polya inequality prover. We will also exploit this opportunity to investigate how to extend Nunchaku—which relies on satisfiability modulo theories (SMT) and model finding—to reason about p-adic numbers and other mathematical entities.
Contribute directly to Lean's development with ideas and features designed to increase user productivity
One of the primary objectives with Lean is to bridge the gap
between automatic and interactive theorem proving. A cornerstone
for this vision is white-box automation: the key techniques
underlying SMT solvers, such as the congruence closure for
reasoning about equality and the quantifier instantiation
heuristics are available to users via the metaprogramming
framework. Despite this, several reasoning components, as well as
some important metaprogramming "tape and glue," are missing.
We will design a cohesive set of reasoning components.
We also want to contribute to the Lean simplifier's support for arithmetic and the envisioned algebraic normalizer. The main difficulty is to balance raw deductive power, extensibility, and predictability. Furthermore, we will design a relevance filter that selects lemmas from background libraries, for use with tactics. One of the challenges is to perform the heuristic selection in a robust way, so that filtering can be used both for proof search and for rechecking the proof.
Finally, it will be useful to integrate external automatic theorem provers in Lean. A new generation of higher-order provers is emerging—including Leo-III, Satallax, and higher-order versions of CVC4, E, Vampire, veriT, and Zipperposition. The Matryoshka project funds work on E and veriT. Integrating these provers will dramatically increase automation. The challenge is to design a sound, complete, and yet efficient encoding of Lean's rich type system, and to reconstruct proofs in Lean.
The Lean Forward Team
Funded by the project
expected to start in October 2019
expected to start in September 2019
expected to start in January 2020
École Polytechnique Paris
Formalizing the solution to the cap set problem
Sander R. Dahmen, Johannes Hölzl, and Robert Y. Lewis. Accepted at 10th International Conference on Interactive Theorem Proving (ITP 2019).Preprint (PDF) ⋅ Proof sketch (PDF)
Superposition with lambdas
Alexander Bentkamp, Jasmin Blanchette, Sophie Tourret, Petar Vukmirović, and Uwe Waldmann. Accepted at 27th International Conference on Automated Deduction (CADE-27).Preprint (PDF) ⋅ Report (PDF)
Extending a brainiac prover to lambda-free higher-order logic
Petar Vukmirović, Jasmin Christian Blanchette, Simon Cruanes, and Stephan Schulz. In Vojnar, T., Zhang, L. (eds.) 25th International Conference on Tools and Algorithms for the Construction and Analysis of Systems (TACAS 2019), LNCS, Springer, 2019.Preprint (PDF) ⋅ Report (PDF)
A formal proof of Hensel's lemma over the p-adic integers
Robert Y. Lewis. In Mahboubi, A., Myreen, M. O. (eds.) 8th ACM SIGPLAN International Conference on Certified Programs and Proofs (CPP 2019), pp. 15–26, ACM, 2019.Postprint (PDF)
Meta-programming with the Lean Proof Assistant
Pablo Le Hénaff. MSc internship report, École Polytechnique Paris, 2018.Thesis (PDF)
A Formally Verified Proof of the Mason–Stothers Theorem in Lean
Jens Wagemaker. BSc thesis, Vrije Universiteit Amsterdam, 2018.Thesis (PDF)
Logical Verification 2019–2020
Period 2 of 2019–2020, Vrije Universiteit Amsterdam, Amsterdam, the Netherlands. Lecturers: Alexander Bentkamp and Jasmin Blanchette.
Logic and Modeling 2018–2019
Period 5 of 2018–2019, Vrije Universiteit Amsterdam, Amsterdam, the Netherlands. Lecturer: Robert Y. Lewis.
Lean Together 2019
7–11 January 2019, Amsterdam, the Netherlands.
Logical Verification 2018–2019
Period 2 of 2018–2019, Vrije Universiteit Amsterdam, Amsterdam, the Netherlands. Lecturers: Jasmin Blanchette and Johannes Hölzl.
The project is thankful for the support and advice of the following friends and colleagues: Alessio d'Arielli, Peter Boven, Wan Fokkink, Cristiano Giuffrida, Mojca Lovrenčak, Roy Overbeek, Anja Palatzke, Lawrence Paulson, Thomas Sturm, Rikkert Stuwe, Mark Summerfield, Susanti Tang-Budiwandojo, and Caroline Waij.