`ComparatorChallenges/CoboundaryExpanders.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

16 declarations, 1 contain sorry, 0 declared as axioms

Bounded-degree coboundary expanders in every dimension

Finite connected pure complexes with uniformly bounded top-face incidence and uniform weighted coboundary expansion over F₂ in all degrees below the dimension.

structure OAI.CoboundaryExpanders.Complex A finite abstract simplicial complex with every labelled vertex present line 19
Source documentation
A finite abstract simplicial complex with every labelled vertex present.
L4DEL explanation
A finite abstract simplicial complex with every labelled vertex present.
1. This explanation is a condensed rendering of the declaration's own doc comment at the pinned source commit; consult the exact Lean statement below for authority.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:19 (same declaration text).
```
structure Complex where
…
```
def OAI.CoboundaryExpanders.Complex.facesOf Faces of dimension i have exactly i + 1 vertices line 29
Source documentation
Faces of dimension i have exactly i + 1 vertices.
L4DEL explanation
Faces of dimension i have exactly i + 1 vertices.
1. This explanation is a condensed rendering of the declaration's own doc comment at the pinned source commit; consult the exact Lean statement below for authority.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:29 (same declaration text).
```
def facesOf (X : Complex) (i : ℕ) : Finset (Finset (Fin X.vertexCount)) :=
…
```
Mentions: OAI.CoboundaryExpanders.Complex
def OAI.CoboundaryExpanders.Complex.Face line 32
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:32 (same declaration text).
instance OAI.CoboundaryExpanders.Complex.(instance at line 34) line 34
instance OAI.CoboundaryExpanders.Complex.(instance at line 35) line 35
def OAI.CoboundaryExpanders.Complex.Pure Exact purity: top faces exist and every face lies in a top face line 38
Source documentation
Exact purity: top faces exist and every face lies in a top face.
L4DEL explanation
Exact purity: top faces exist and every face lies in a top face.
1. This explanation is a condensed rendering of the declaration's own doc comment at the pinned source commit; consult the exact Lean statement below for authority.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:38 (same declaration text).
```
def Pure (X : Complex) (s : ℕ) : Prop :=
…
```
Mentions: OAI.CoboundaryExpanders.Complex
def OAI.CoboundaryExpanders.Complex.Connected Connectedness of the nonempty one-skeleton line 43
Source documentation
Connectedness of the nonempty one-skeleton.
L4DEL explanation
Connectedness of the nonempty one-skeleton.
1. This explanation is a condensed rendering of the declaration's own doc comment at the pinned source commit; consult the exact Lean statement below for authority.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:43 (same declaration text).
```
def Connected (X : Complex) : Prop :=
…
```
Mentions: OAI.CoboundaryExpanders.Complex
abbrev OAI.CoboundaryExpanders.Complex.Cochain line 48
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:48 (same declaration text).
def OAI.CoboundaryExpanders.Complex.weight Weights are normalized counts of incident top-dimensional faces line 51
Source documentation
Weights are normalized counts of incident top-dimensional faces.
L4DEL explanation
Weights are normalized counts of incident top-dimensional faces.
1. This explanation is a condensed rendering of the declaration's own doc comment at the pinned source commit; consult the exact Lean statement below for authority.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:51 (same declaration text).
```
def weight (X : Complex) (s i : ℕ) (σ : X.Face i) : ℝ :=
…
```
Mentions: OAI.CoboundaryExpanders.Complex
def OAI.CoboundaryExpanders.Complex.norm Weighted support norm line 56
Source documentation
Weighted support norm.
L4DEL explanation
Weighted support norm.
1. This explanation is a condensed rendering of the declaration's own doc comment at the pinned source commit; consult the exact Lean statement below for authority.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:56 (same declaration text).
```
def norm (X : Complex) (s i : ℕ) (f : X.Cochain i) : ℝ :=
…
```
Mentions: OAI.CoboundaryExpanders.Complex
def OAI.CoboundaryExpanders.Complex.coboundary Simplicial coboundary over F₂ line 60
Source documentation
Simplicial coboundary over F₂.
L4DEL explanation
Simplicial coboundary over F₂.
1. This explanation is a condensed rendering of the declaration's own doc comment at the pinned source commit; consult the exact Lean statement below for authority.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:60 (same declaration text).
```
def coboundary (X : Complex) (i : ℕ) (f : X.Cochain i) : X.Cochain (i + 1) :=
…
```
Mentions: OAI.CoboundaryExpanders.Complex
def OAI.CoboundaryExpanders.Complex.coboundaries Actual coboundaries, with constant cochains in degree zero line 64
Source documentation
Actual coboundaries, with constant cochains in degree zero.
L4DEL explanation
Actual coboundaries, with constant cochains in degree zero.
1. This explanation is a condensed rendering of the declaration's own doc comment at the pinned source commit; consult the exact Lean statement below for authority.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:64 (same declaration text).
```
def coboundaries (X : Complex) : (i : ℕ) → Set (X.Cochain i)
  | 0 => {f | ∃ c : ZMod 2, f = fun _ => c}
  | i + 1 => Set.range (X.coboundary i)

/-- Distance in the weighted support norm; on a nonempty finite set it is a minimum. -/
```
Mentions: OAI.CoboundaryExpanders.Complex, OAI.CoboundaryExpanders.Complex.norm
def OAI.CoboundaryExpanders.Complex.distance Distance in the weighted support norm; on a nonempty finite set it is a minimum line 69
Source documentation
Distance in the weighted support norm; on a nonempty finite set it is a minimum.
L4DEL explanation
Distance in the weighted support norm; on a nonempty finite set it is a minimum.
1. This explanation is a condensed rendering of the declaration's own doc comment at the pinned source commit; consult the exact Lean statement below for authority.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:73 (same declaration text).
```
def distance (X : Complex) (s i : ℕ) (f : X.Cochain i)
    (A : Set (X.Cochain i)) : ℝ :=
…
```
Mentions: OAI.CoboundaryExpanders.Complex
def OAI.CoboundaryExpanders.Complex.topDegree Number of top-dimensional faces incident to a vertex line 74
Source documentation
Number of top-dimensional faces incident to a vertex.
L4DEL explanation
Number of top-dimensional faces incident to a vertex.
1. This explanation is a condensed rendering of the declaration's own doc comment at the pinned source commit; consult the exact Lean statement below for authority.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:78 (same declaration text).
```
def topDegree (X : Complex) (s : ℕ) (v : Fin X.vertexCount) : ℕ :=
…
```
Mentions: OAI.CoboundaryExpanders.Complex
def OAI.CoboundaryExpanders.MainStatement Both constants are uniform over the sequence, all degrees below d, and all cochains line 80
Source documentation
Both constants are uniform over the sequence, all degrees below d, and all cochains.
L4DEL explanation
Both constants are uniform over the sequence, all degrees below d, and all cochains.
1. This explanation is a condensed rendering of the declaration's own doc comment at the pinned source commit; consult the exact Lean statement below for authority.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Also declared: OAI/Combinatorics/Coboundary/Complex.lean:86 (same declaration text).
```
def MainStatement : Prop :=
…
```
theorem OAI.CoboundaryExpanders.main Challenge statement: Bounded-degree coboundary expanders sorry line 91
L4DEL explanation
The formalization constructs bounded-degree $\mathbb F_2$ coboundary expanders in every dimension $d\ge3$. For each such $d$, it gives finite pure connected $d$-dimensional simplicial complexes with vertex counts tending to infinity, one uniform bound on top-dimensional degree at each vertex, and one positive coboundary-expansion constant in every degree below $d$. The constants may depend on $d$. The graph and two-dimensional cases used for the paper's all-positive-dimensions conclusion are separate.
1. This is an exact comparator target linked by the pinned repository's formalization note for this result.
2. The repository's comparator configuration selects OAI.Combinatorics.Coboundary.Main as the released solution module checked against this declaration.
Sources and attribution: Pinned Lean source (fd4aeeb2ee4f)
Distinct definitions of this name: OAI/Combinatorics/Coboundary/Main.lean:88 (same statement text; different proof or body). Qualify references as OAI.CoboundaryExpanders.main@path.
```
theorem main : MainStatement := by
…
```
Mentions: OAI.CoboundaryExpanders.MainStatement