This repository contains two Lean 4 (v4.35.0-rc4) formalization libraries for small Turing machines whose halting behavior encodes deep mathematical problems or complex dynamical invariants:

1. Conjecture-Checking Turing Machines (Cryptids/) — Certified state reductions (goldbach_21, zf_380, riemann_95, erdos_ternary_14) and end-to-end Lean 4 mathematical semantics proofs (Halts M ↔ ...) for Goldbach's Conjecture, ZF Set Theory Consistency, the Riemann Hypothesis, Erdős's ternary conjecture, and Brocard's problem (brocard_28, a new 28-state machine).
2. Busy Beaver Holdouts (BusyBeaver/) — Kernel-verified behavioral proofs (IsHalt, IsNotHalt, HasCollatzReduction, HasStackedCollatzReduction, IsValidAcceleration) for Busy Beaver 6 (BB(6)) and Busy Beaver (3, 3) (BB(3, 3)) holdouts—covering both community-documented machines and novel Collatz-like accelerations discovered via the autonomous agentic pipeline of At the Limits of Computability: Steps Towards Autonomous AI Discovery on the Busy Beaver Frontier (some are potential Cryptids, subject to community acceptance)—connected to ConjectureMachines.Machine via BusyBeaver/Bridge.lean.

All Lean 4 proofs are checked by the kernel without sorry and depend only on standard Lean axioms:

• Constructive (["propext", "Quot.sound"], no Classical.choice): All ZF machine-to-proof-search equivalences (halts_zf393_iff_zfInconsistent, Zf380.Semantics.halts_candidate_iff_zfInconsistent, halts_zf432_iff_wadeZFInconsistent), first-order Peano Arithmetic (PAProvable) consistency and $\Sigma_1^0$ standard-model adequacy (not_paProvable_false, paHalts_holds_iff_halts, paZFInconsistent_holds_iff_zfInconsistent in Cryptids/Semantics/PA/), the direct simulation state reductions (Goldbach21.lean, Zf380.lean, Riemann104.lean), the erdos_ternary_14 reduction (ErdosTernary14.lean), the brocard_28 equivalence Halts brocard28 ↔ ∃ n, 7 < n ∧ IsSquare (n! + 1) (Brocard28.lean, Semantics/Brocard28.lean), and all five candidate_fewer_states theorems, audited in CI via comparator/constructive.json. Because classical Lean (ZFSet.{0} in Soundness.lean) proves ¬ ZFInconsistent—so any non-halting machine would vacuously satisfy Halts M ↔ ZFInconsistent under Classical.choice—excluding Classical.choice blocks classical Con(ZF) shortcuts, while deeply embedded PAProvable provides a weak first-order metatheory strictly below ZF (see Foundational Note on Con(ZF) and Constructive Equivalence).
• Classical (["propext", "Classical.choice", "Quot.sound"]): First-order PA standard-model soundness and object-level PAProvable proofs that zf_380 and zf_393 halt iff ZF is inconsistent (paProvable_sound_nat, paProvable_halts_zf380_iff_zf393, paProvable_halts_zf393_iff_zfInconsistent, paProvable_halts_zf380_iff_zfInconsistent; proved semantically in arbitrary PAModels via first-order induction and reflected to PAProvable via internal Gödel–Henkin completeness paProvable_iff_holdsIn_all_models, which uses Classical.choice), set-theoretic soundness and non-halting in ZFSet.{0} (zfProvable_sound_zfSet, not_zfInconsistent, not_halts_zf393, not_halts_candidate), negated-$\Pi_1^0$ Goldbach equivalence (¬ GoldbachConjecture, which uses Markov's Principle), the arithmetic-stage riemann_95 reduction (Nat.Prime), complex-analytic RiemannHypothesis equivalences, the erdos_ternary_15 and erdos_ternary_14 equivalences with ErdosTernaryConjecture (Nat.digits, and Markov's Principle for ¬ ErdosTernaryConjecture), the brocard_28 equivalence with ¬ BrocardConjecture (Markov's Principle), and the BB(6) and BB(3, 3) holdout reductions, audited via comparator/all.json, comparator/pa_semantics.json, and comparator/busy_beaver.json.

The 95-state Riemann machine has a kernel-checked blank-tape halting equivalence to the 104-state machine and to ¬ RiemannHypothesis. Its transition table, complete 3,634-step initializer, and proof guide are included.

Fetch the precompiled Mathlib cache:

lake exe cache get

Build the conjecture-checking library (Cryptids and Challenge, default targets) and audit with leanprover/comparator:

lake build
lake comparator --config comparator/constructive.json
lake comparator --config comparator/all.json
python3 verify.py

Build the Busy Beaver library (BusyBeaver and BusyBeaverChallenge) and audit with leanprover/comparator:

lake build BusyBeaver BusyBeaverChallenge
lake comparator --config comparator/busy_beaver.json

• Cryptids/ — Conjecture-checking Turing machines (Machines.lean, Propositions.lean), state-reduction proofs (Goldbach21.lean, Zf380.lean, Riemann95.lean), and end-to-end mathematical semantics (Cryptids/Semantics/).
• BusyBeaver/ — Busy Beaver holdout specifications (Defs.lean), foundational lemmas (Basic.lean), RLE loader and bridge to ConjectureMachines (Bridge.lean), and machine behavioral proofs.
• Challenge.lean, BusyBeaverChallenge.lean & comparator/ — Self-contained target theorem specifications and configuration files for leanprover/comparator.
• artifacts/ — Canonical machine.txt transition tables and reduction write-ups for goldbach_21, zf_380, riemann_104, riemann_95, erdos_ternary_14, and brocard_28.
• baselines/ — Verbatim upstream source files, canonical 5-tuple tables, and PROVENANCE.md.
• RELATED_WORK.md — Literature lineage and (Q, 2) Turing machine conventions.
• verify.py — Standalone Python 3 verification script for the conjecture-checking machines.

Pull requests and issues are welcome, especially for smaller Turing machines for these problems, machines for other conjectures, Busy Beaver holdout proofs, or streamlined Lean proofs. Please also open an issue if you notice anything amiss, such as a missing reference or provenance detail. See CONTRIBUTING.md for general guidelines.

We thank Eric Wieser for careful review, as well as the Busy Beaver Challenge community, int-y1 (whose BusyLean project pioneered Lean 4 Busy Beaver formalizations and directed-head configuration representations), Yijun Leng, Stefan O'Rear, Yuri Matiyasevich, Scott Aaronson, Adam Yedidia, Andrew Wade, Joseph Shunia, Tristan Stérin, and Damien Woods for the baseline Turing machines and compilers, and the Mathlib community.

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