`ComparatorChallenges/ThompsonNonamenability.lean`

Explanation legend Lean4 Domain Explanation Language (L4DEL)

L4DEL explanations are a separate layer. They do not change the Lean source. Intuition is explanatory prose; numbered items are a reading path; sources link the wording back to evidence.

L4DEL: Lean4 Domain Explanation Language

Annotation provenance. This overlay contains 120 curated AI-written annotations and 1273 mechanical source-derived annotations. They have not been human-reviewed as explanations; treat them as unverified and not mathematical authority. Retained generation history: The 9 original curated annotations were produced in a Claude Sonnet 5.5 session at medium effort; Claude Opus 5.5 is recorded in that session as the advisor model, not the direct author. Those 9 annotations were revised on 2026-10-14 to bind all of them to exact page-one quotations from 6 pinned OpenAI manuscripts, the matching pinned formalization notes, and named historical primary sources; no human review is claimed, and the exact runtime model identifier was not exposed to the artifact generator. The 3 manuscript-grounded matrix-multiplication annotations were composed from pinned OpenAI manuscripts and named historical sources in a Codex session on 2026-10-14; no human review is claimed, and the exact runtime model identifier was not exposed to the artifact generator. Six additional target-specific annotations for Catalan's constant, the irrationality exponent of pi, dyadic Erdos similarity, Unique Games, quintic Lienard systems, and Yau nodal growth were composed from pinned OpenAI manuscripts, exact page-one quotations, exact Lean statements, and named historical sources in a Codex session on 2026-10-14; no human review is claimed, and the exact runtime model identifier was not exposed to the artifact generator. Four additional target-specific annotations for Seymour's second-neighborhood conjecture, Cannon's conjecture, Koebe circle-domain uniformization, and circle-domain rigidity were composed from 4 pinned OpenAI manuscripts, exact page-one quotations, exact Lean statements, and named historical primary sources in a Codex session on 2026-10-14; no human review is claimed, and the exact runtime model identifier was not exposed to the artifact generator. Four additional target-specific annotations for simple Lebesgue spectrum on the three-torus, Rokhlin multiple mixing, the nine-dimensional Borsuk counterexample, and Tingley's sphere-isometry problem were composed from 4 pinned OpenAI manuscripts, exact page-one quotations, exact Lean statements, and named historical primary sources in a Codex session on 2026-10-14; no human review is claimed, and the exact runtime model identifier was not exposed to the artifact generator. Four additional target-specific annotations for the Abhyankar-Sathaye counterexample, the circulant Hadamard conjecture, the Eilenberg-Ganea counterexample, and short Egyptian fractions were composed from 4 pinned OpenAI manuscripts, exact page-one quotations, exact Lean statements, and quotations from Sathaye, Turyn with Storer, Eilenberg with Ganea, and Erdos in a Codex session on 2026-10-14; no human review is claimed, and the exact runtime model identifier was not exposed to the artifact generator. Four additional target-specific annotations for the complete Crouzeix inequality, unrestricted Euclidean five-coloring, the general Mahler conjecture, and Thompson F nonamenability were composed on 2026-10-14 from exact Lean statements, 4 pinned OpenAI manuscripts, the matching formalization notes, and linked historical sources by Crouzeix, de Grey, Mahler, Cannon, Floyd, and Parry; no human review is claimed. Four additional target-specific annotations for algorithmic strong thin trees, Barnette's Hamiltonian-cycle conjecture, the explicit Artin CAT(0) counterexample, and generalized amplitude-damping capacity were composed on 2026-10-14 from exact Lean statements, 4 pinned OpenAI manuscripts, matching formalization notes, and linked historical sources by Anari with Goddyn, Arts with Hopman and Timmermans, Charney, and Khatri with Sharma and Wilde; no human review is claimed. Fourteen solution-level annotations for Seymour's second-neighborhood theorem were added on 2026-10-14 from the pinned 15-page manuscript, covering the exact-neighborhood model, minimal-counterexample reduction, subset deficit, transitive blowup, pruning, extremal contradiction, and proof-bearing final theorem. The subset and blowup annotations also bind exact quotations from Tyler Seacrest (2019) and Brantner, Brockman, Kay, and Snively (2009); no human review is claimed. Thirty-seven declaration-level annotations were added on 2026-10-14 for the rest of ComparatorChallenges/AlgorithmicThinTrees.lean, covering its multigraph semantics, stack-machine cost model, explicit and binary-multiplicity encodings, output certificate, and complete algorithmic proposition. They are bound to the pinned OpenAI manuscript and linked earlier thin-tree sources by Luis Goddyn, Nima Anari, and Shayan Oveis Gharan; the annotation explicitly preserves that the concluding comparator theorem still has body by sorry, and no human review is claimed. Thirty-one supporting declarations in ComparatorChallenges/UniqueGamesTheorem.lean were upgraded or added on 2026-10-14, completing curated coverage of all thirty-two declarations in that file. The annotations cover the finite game model, Boolean formula and parser, binary-vector translations, explicit output serialization, finite-alphabet machine contract, and gap theorem; they distinguish manuscript-backed mathematics from Lean-specific canonical parsing and unary game output encoding, bind exact OpenAI manuscript quotations, and credit Subhash Khot through his 2010 survey. The comparator theorem remains explicitly labeled as having body by sorry; no human review is claimed. The source-derived generator was hardened on 2026-10-14 so a formalization note covering multiple comparator results no longer copies its combined Scope prose onto each declaration; those targets now use declaration-specific source comments when available and otherwise remain explicitly bounded to the exact Lean statement. The remaining source-derived annotations were generated mechanically from pinned formalization notes and source comments by tooling developed in a GPT-5.6 Sol session at ultra reasoning; GPT-5.6 Sol did not originate the source prose.

Credit: OpenAI's pinned repository says an unreleased internal model produced the manuscripts and supporting proof artifacts. The collection builds on the mathematics community's accumulated problems, results, language, and formal libraries; each manuscript's BibTeX and bibliography are the authority for individual scholarly credit.

Project sources: Pinned openai/math documentation, Pinned formalization catalogue

13 declarations, 1 contain sorry, 0 declared as axioms

abbrev OAI.ThompsonNonamenability.UnitInterval line 16
Also declared: OAI/GroupTheory/Thompson/Model.lean:16 (same declaration text).
def OAI.ThompsonNonamenability.unitIntervalZero line 18
Also declared: OAI/GroupTheory/Thompson/Model.lean:18 (same declaration text).
def OAI.ThompsonNonamenability.unitIntervalOne line 19
Also declared: OAI/GroupTheory/Thompson/Model.lean:19 (same declaration text).
structure OAI.ThompsonNonamenability.IntervalHomeomorph line 21
Also declared: OAI/GroupTheory/Thompson/Model.lean:21 (same declaration text).
instance OAI.ThompsonNonamenability.IntervalHomeomorph.(instance at line 26) line 26
def OAI.ThompsonNonamenability.IsDyadic line 31
Also declared: OAI/GroupTheory/Thompson/Model.lean:31 (same declaration text).
structure OAI.ThompsonNonamenability.DyadicPLWitness line 34
Also declared: OAI/GroupTheory/Thompson/Model.lean:34 (same declaration text).
def OAI.ThompsonNonamenability.HasDyadicPLPieces line 50
Also declared: OAI/GroupTheory/Thompson/Model.lean:50 (same declaration text).
def OAI.ThompsonNonamenability.F line 52
Also declared: OAI/GroupTheory/Thompson/Model.lean:52 (same declaration text).
abbrev OAI.ThompsonNonamenability.BoundedReal line 54
Also declared: OAI/GroupTheory/Thompson/Model.lean:54 (same declaration text).
def OAI.ThompsonNonamenability.leftPull line 58
Also declared: OAI/GroupTheory/Thompson/Model.lean:58 (same declaration text).
structure OAI.ThompsonNonamenability.InvariantMean line 71
Also declared: OAI/GroupTheory/Thompson/Model.lean:71 (same declaration text).
theorem OAI.ThompsonNonamenability.thompson_F_nonamenable_composition Challenge statement: Thompson's group F has no invariant mean sorry line 78
L4DEL explanation
This confirms Geoghegan’s conjecture and resolves the amenability problem for F
— OpenAI Thompson-F manuscript (abstract)
if F is not amenable, then F is a finitely presented group in NF ∖ AG
— Cannon, Floyd, and Parry (1996), p. 228
Thompson's group $F$ is modeled by increasing, dyadic piecewise-linear homeomorphisms of the unit interval. The theorem supplies the composition group law and rules out every positive normalized left-invariant mean on its bounded real-valued functions; that is the asserted nonamenability.
1. The existentially supplied Group F must multiply homeomorphisms by composition: (h * g)(x) = h(g(x)) for every pair of elements and every point of the unit interval.
2. InvariantMean F packages a real-linear functional on bounded functions F → ℝ, positivity on nonnegative functions, normalization at the constant one function, and invariance under every left translation.
3. The second conjunct is ¬ Nonempty (InvariantMean F). The Lean statement gives no explicit Følner-boundary constant and fixes no prescribed generating set.
4. Cannon, Floyd, and Parry explained why either answer would place this finitely presented group in an exceptional structural class. The linked OpenAI manuscript claims Geoghegan's 1979 nonamenability conjecture. This comparator body is sorry, paired with OAI.GroupTheory.Thompson.Main.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f), OpenAI Thompson-F manuscript (abstract), Pinned OpenAI formalization note 248, Cannon, Floyd, and Parry (1996), p. 228
Distinct definitions of this name: OAI/GroupTheory/Thompson/Main.lean:69 (same statement text; different proof or body). Qualify references as OAI.ThompsonNonamenability.thompson_F_nonamenable_composition@path.

amenable group: a group admitting a positive normalized mean on bounded real functions that is unchanged by left translation
dyadic piecewise-linear homeomorphism: an increasing interval homeomorphism with finitely many dyadic breakpoints and slopes that are integer powers of two
```
theorem thompson_F_nonamenable_composition :
    ∃ group : Group F, letI := group;
…
```
Mentions: OAI.ThompsonNonamenability.F