analysis / Banach space theory; Lipschitz-free spaces

Kalton's question: does the Lipschitz-free space of every uniformly discrete metric space have the bounded approximation property?

Theorem 1.1: there is a countable, unbounded, nonproper pointed metric space with d(x,y)≥1d(x,y)\ge1 for distinct points whose real Lipschitz-free space has AP but fails λ\lambda-BAP for every finite λ\lambda. It is built as a wedge of bounded blocks (Theorem 1.2) in which a finite anchor set defeats every finite-rank operator of norm at most pp. Consequences stated in the paper: for each pp, real ℓ1\ell_1 has an equivalent norm with 2Bp2B_p-BAP failing pp-BAP (for p=1p=1 failing MAP), and F(M)\mathcal F(M) is a separable Banach space that is not approximable in Kalton's nonlinear sense (Kalton 2012, Problem 1). Real scalars only; the proper countable case is excluded by Dalet's theorem.

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analysisSep 26, 2026Significance 24/100Registry: lean checked

Kalton's question: does the Lipschitz-free space of every uniformly discrete metric space have the bounded approximation property?

Prior state unknown→disproved

Theorem 1.1: there is a countable, unbounded, nonproper pointed metric space with d(x,y)≥1d(x,y)\ge1 for distinct points whose real Lipschitz-free space has AP but fails λ\lambda-BAP for every finite λ\lambda. It is built as a wedge of bounded blocks (Theorem 1.2) in which a finite anchor set defeats every finite-rank operator of norm at most pp. Consequences stated in the paper: for each pp, real ℓ1\ell_1 has an equi…

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Theorem 1.1: there is a countable, unbounded, nonproper pointed metric space with d(x,y)≥1d(x,y)\ge1 for distinct points whose real Lipschitz-free space has AP but fails λ\lambda-BAP for every finite λ\lambda. It is built as a wedge of bounded blocks (Theorem 1.2) in which a finite anchor set defeats every finite-rank operator of norm at most pp. Consequences stated in the paper: for each pp, real ℓ1\ell_1 has an equivalent norm with 2Bp2B_p-BAP failing pp-BAP (for p=1p=1 failing MAP), and F(M)\mathcal F(M) is a separable Banach space that is not approximable in Kalton's nonlinear sense (Kalton 2012, Problem 1). Real scalars only; the proper countable case is excluded by Dalet's theorem.

Theorem 1.1: there is a countable, unbounded, nonproper pointed metric space with d(x,y)≥1d(x,y)\ge1 for distinct points whose real Lipschitz-free space has AP but fails λ\lambda-BAP for every finite λ\lambda. It is built as a wedge of bounded blocks (Theorem 1.2) in which a finite anchor set defeats every finite-rank operator of norm at most pp. Consequences stated in the paper: for each pp, real ℓ1\ell_1 has an equivalent norm with 2Bp2B_p-BAP failing pp-BAP (for p=1p=1 failing MAP), and F(M)\mathcal F(M) is a separable Banach space that is not approximable in Kalton's nonlinear sense (Kalton 2012, Problem 1). Real scalars only; the proper countable case is excluded by Dalet's theorem.

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