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arXiv · 2611.09226

A Fermat--Torricelli convex-hull characterization of inner-product spaces via Birkhoff--James orthogonality

Zhiyao Fang·Junxiang Qi·Qi Liu
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Abstract

The classical location problem posed by Weber [Über den Standort der Industrien,Tübingen: J. C. B. Mohr, 1909] has motivated various geometric characterizations of inner-product spaces. Benítez, Fernández and Soriano [Trans. Amer. Math. Soc. \textbf{354} (2002), 5027--5038] characterized real inner-product spaces through the convex-hull property of Fermat--Torricelli medians.Pakhrou [J. Funct. Anal. \textbf{285} (2023), 110078] further established a characterization using $γ$-centers and equilateral triangles. Building on these results, we show that the Fermat--Torricelli convex-hull criterion remains valid when restricted to triples satisfying a one-sided Birkhoff--James orthogonality condition.The proof combines norming functionals, radial rescaling,Alexandrov differentiability, and the area formula.No completeness, smoothness, or strict convexity assumptions are required.

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Zhiyao Fang, Junxiang Qi, Qi Liu. 2026-11-06. A Fermat--Torricelli convex-hull characterization of inner-product spaces via Birkhoff--James orthogonality. https://arxiv.org/abs/2611.09226

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