Synthetic theories such as homotopy type theory axiomatize classical mathematical objects such as spaces up to homotopy. Although theorems in synthetic theories translate to theorems about the axiomatized structures on paper, this fact has not yet been exploited in proof assistants. This makes it challenging to formalize results in classical mathematics using synthetic methods. For example, Cubical Agda supports reasoning about cubical types, but cubical proofs have not been translated to proofs about cubical set models, let alone their topological realizations. To bridge this gap, we present SynthLean: a proof assistant that combines reasoning using synthetic theories with reasoning about their models. SynthLean embeds Martin-Löf type theory as a domain-specific language in Lean, supporting a bidirectional workflow: constructions can be made internally in Martin-Löf type theory as well as externally in a model of the theory. A certifying normalization-by-evaluation typechecker automatically proves that internal definitions have sound interpretations in any model; conversely, semantic entities can be axiomatized in the syntax. Our implementation handles universes, Σ, Π, and identity types, as well as arbitrary axiomatized constants. To provide a familiar experience for Lean users, we reuse Lean’s tactic language and syntax in the internal mode, and base our formalization of natural model semantics on Mathlib. By taking a generic approach, SynthLean can be used to mechanize various interpretations of internal languages such as the groupoid, cubical, or simplicial models of homotopy type theory in HoTTLean.

Conway's Game of Life (GOL) is a cellular automaton that has captured the interest of hobbyists and mathematicians alike for more than 50 years. The Game of Life is Turing complete, and people have been building increasingly sophisticated constructions within GOL, such as 8-bit displays, Turing machines, and even an implementation of GOL itself. In this paper, we report on a project to build an implementation of GOL within GOL, via logic circuits, fully formally verified within the HOL4 theorem prover. This required a combination of interactive tactic proving, symbolic simulation, and semi-automated forward proof to assemble the components into an infinite circuit which can calculate the next step of the simulation while respecting signal propagation delays. The result is a verified "GOL in GOL compiler" which takes an initial GOL state and returns a mega-cell version of it that can be passed to off-the-shelf GOL simulators, such as Golly. We believe these techniques are also applicable to other cellular automata, as well as for hardware verification which takes into account both the physical configuration of components and wire delays.

A recent breakthrough in computer-assisted mathematics showed that every set of 30 points in the plane in general position (i.e., no three points on a common line) contains an empty convex hexagon. Heule and Scheucher solved this problem with a combination of geometric insights and automated reasoning techniques by constructing CNF formulas ϕ_n, with O(n⁴) clauses, such that if ϕ_n is unsatisfiable then every set of n points in general position must contain an empty convex hexagon. An unsatisfiability proof for n = 30 was then found with a SAT solver using 17 300 CPU hours of parallel computation. In this paper, we formalize and verify this result in the Lean theorem prover. Our formalization covers ideas in discrete computational geometry and SAT encoding techniques by introducing a framework that connects geometric objects to propositional assignments. We see this as a key step towards the formal verification of other SAT-based results in geometry, since the abstractions we use have been successfully applied to similar problems. Overall, we hope that our work sets a new standard for the verification of geometry problems relying on extensive computation, and that it increases the trust the mathematical community places in computer-assisted proofs.

This paper describes a new open-source proof processing tool, mizar-rs, a wholesale reimplementation of core parts of the Mizar proof system, written in Rust. In particular, the "checker" and "analyzer" of Mizar are implemented, which together form the trusted core of Mizar. This is to our knowledge the first and only external implementation of these components. Thanks to the loose coupling of Mizar's passes, it is possible to use the checker as a drop-in replacement for the original, and we have used this to verify the entire MML in 11.8 minutes on 8 cores, a 4.8x speedup over the original Pascal implementation. Since Mizar is not designed to have a small trusted core, checking Mizar proofs entails following Mizar closely, so our ability to detect bugs is limited. Nevertheless, we were able to find multiple memory errors, four soundness bugs in the original (which were not being exploited in MML), in addition to one non-critical bug which was being exploited in 46 different MML articles. We hope to use this checker as a base for proof export tooling, as well as revitalizing development of the language.

We introduce FRAT, a new proof format for unsatisfiable SAT problems, and its associated toolchain. Compared to DRAT, the FRAT format allows solvers to include more information in proofs to reduce the computational cost of subsequent elaboration to LRAT. The format is easy to parse forward and backward, and it is extensible to future proof methods. The provision of optional proof steps allows SAT solver developers to balance implementation effort against elaboration time, with little to no overhead on solver time. We benchmark our FRAT toolchain against a comparable DRAT toolchain and confirm >84% median reduction in elaboration time and >94% median decrease in peak memory usage.

Abstract We introduce , a new proof format for unsatisfiable SAT problems, and its associated toolchain. Compared to , the format allows solvers to include more information in proofs to reduce the computational cost of subsequent elaboration to . The format is easy to parse forward and backward, and it is extensible to future proof methods. The provision of optional proof steps allows SAT solver developers to balance implementation effort against elaboration time, with little to no overhead on solver time. We benchmark our toolchain against a comparable toolchain and confirm >84% median reduction in elaboration time and >94% median decrease in peak memory usage.

A broad class of data types, including arbitrary nestings of inductive types, coinductive types, and quotients, can be represented as quotients of polynomial functors. This provides perspicuous ways of constructing them and reasoning about them in an interactive theorem prover.

We present an extension to the $\mathtt{mathlib}$ library of the Lean theorem prover formalizing the foundations of computability theory. We use primitive recursive functions and partial recursive functions as the main objects of study, and we use a constructive encoding of partial functions such that they are executable when the programs in question provably halt. Main theorems include the construction of a universal partial recursive function and a proof of the undecidability of the halting problem. Type class inference provides a transparent way to supply Gödel numberings where needed and encapsulate the encoding details.