A uniformly discrete counterexample to bounded approximation in Lipschitz-free spacesOpen Problem
Motivation
Uniform discreteness places a lower bound on distances between points. The target tests whether that metric simplicity forces uniformly bounded finite-rank approximation in the associated Lipschitz-free space. The pinned manuscript supplies the research context.
Setting
The free space is the closed real span of point evaluations in the dual of Lipschitz functions vanishing at a base point. Approximation and bounded approximation retain their distinct operator-norm quantifiers.
Formalization target
The selected goal is OAI.DiscreteFree.source_endpoints. Its central assertion is
The theorem states that two propositions hold together, both about the Lipschitz-free space FreeSpace(o) of a pointed metric space (M,o), which is the closed linear span, inside the dual of the Banach space Lip0(o) of real Lipschitz functions vanishing at o (normed by the supremum of the slopes |f(x)-f(y)|/d(x,y)), of the point evaluations; delta(o,a) denotes the evaluation at a. The first, BlockStatement, says that for every natural number p ≥ 1 there exist a countable metric space M and a base point o with all distinct points at distance at least 1 and all distances at most 300p²+150p+5, together with a finite set A of M containing o, such that every bounded linear operator T on FreeSpace(o) of finite rank and norm at most p moves some delta(o,a) with a in A by at least 1/2, that is ‖T(delta(o,a))-delta(o,a)‖ ≥ 1/2. The second, MainStatement, says there exist a countable metric space M and a point o such that distinct points are at distance at least 1, M is unbounded, M is not a proper space, FreeSpace(o) has the approximation property (finite-rank bounded operators approximate the identity uniformly on every compact set to within any ε>0), and yet for every Λ ≥ 1 FreeSpace(o) fails the bounded approximation property with constant Λ, meaning the approximating finite-rank operators cannot all be chosen with norm at most Λ. The theorem is stated with sorry, so no proof is asserted here.
Significance and status
The selected conjunction includes quantitative block obstructions and an unbounded, nonproper, uniformly discrete example. The quantitative real-ℓ1 renorming target is retained as an additional reference. The target is currently Open on Prove2Me. The manuscript's mathematical argument and a machine-checked proof of the selected statement are separate deliverables.
Difficulty
Compact-set approximation must remain possible while every candidate uniform norm bound fails. The finite block obstruction must survive in one countable space.
Formalization scope
The exact published goal, its hypotheses and its referenced definition blocks specify the requested formalization. The explanatory formula above is a summary; all quantifiers and additional clauses in the linked statement remain required.
Additional published targets are included as separate references:
Selected references
- OpenAI, A uniformly discrete counterexample to bounded approximation in Lipschitz-free spaces, preprint, 2026. Manuscript.
- OpenAI, accompanying formal statements, commit
adc7f1241b42. Selected goal source.