Motivation
A linear filling inequality captures a form of large-scale negative curvature. The source asks whether a finite aspherical complex with that property must admit a geometric model of local nonpositive curvature. The source is OpenAI's September 2026 manuscript.
Setting
A finite complex has its usual barycentric realization. Asphericity means higher homotopy groups vanish. A linear disk-filling inequality bounds triangular face moves needed to fill an edge loop by a constant times its length.
Formalization targets
∃K:K finite, connected, aspherical, with linear disk filling,π1K hyperbolic with no geometric CAT(0) action.
The defined proposition MainStatement holds, namely that there exist a finite abstract simplicial complex K on vertices Fin n, with its standard barycentric realization |K| (nonnegative coordinate vectors summing to 1 supported on a face), a basepoint x in |K|, and a real constant C ≥ 0 such that all of the following hold. |K| is connected and aspherical at x, meaning every homotopy group π_{m}(|K|,x) with m ≥ 2 is trivial. K satisfies a linear disk-filling inequality: every closed edge path p that bounds some disk, where a disk is a chain of elementary moves (collapsing a repeated vertex, removing a spur a,b,a, or replacing a,b,c by a,c across a nondegenerate triangular face, which costs one) reducing p to a single vertex, admits such a disk using at most C times (length of p minus 1) triangular faces. The fundamental group G = π₁(|K|,x) is word hyperbolic in the four-point sense: for some finite generating set S and some δ ≥ 0, every a,b,c,d in G satisfy d(a,c)+d(b,d) ≤ max(d(a,b)+d(c,d), d(a,d)+d(b,c)) + 2δ, where d is word distance. G admits no geometric CAT(0) action: there is no proper cocompact isometric action of G on any nonempty proper complete metric space X (in universe u) that is CAT(0), in the sense that segments exist with the squared-distance comparison inequality. Finally, for every finite complex L whose realization is homotopy equivalent to |K|, there is no metric on |L|, inducing its given topology, that is geodesic and locally CAT(−1), meaning each point has a ball that satisfies the hyperbolic-cosine midpoint inequality cosh d(z,m) ≤ (cosh d(z,x)+cosh d(z,y))/(2cosh(d(x,y)/2)) for segment midpoints m.
The goal is OAI.HyperbolicObstruction.main.
Significance
The goal also excludes a finite homotopy-equivalent complex with a compatible geodesic locally CAT(-1) metric. It joins a finite topological model, a group-theoretic obstruction, and a metric-model consequence. The manuscript presents a mathematical argument; the attached goal currently has status Open, so the requested contribution is a Lean proof of the displayed formal target.
Difficulty
A large-scale hyperbolicity inequality does not directly construct a local curvature metric. Excluding all geometric actions requires more than finding a defect in the original complex’s chosen metric.
Formalization scope
Word hyperbolicity uses the four-point word-distance condition. Excluded CAT(0) actions are on nonempty proper complete spaces. The final metric must induce the finite realization’s existing topology; changing that topology is not permitted.
The shared definitions are supplied by HyperbolicObstruction. The target uses Lean 4.33.1 with the pinned Mathlib environment. Contributions should preserve the existing statement and develop reusable supporting lemmas in its definitions.
Selected references
- OpenAI, A hyperbolic group with no geometric CAT(0) action, preprint, September 2026. Manuscript.
- OpenAI, accompanying Lean statement. Pinned source.