The Uniform Gromov Hausdorff Gap Problem for Approximating Spheres by Finite Homogeneous Spaces

Abstract

Let $S^n$ be the unit round sphere with its intrinsic angular metric, normalized so that $\mathrm{diam}{S}^n=\pi$. For finite homogeneous metric spaces $X$, put \[ δ_n=\inf_X d_{GH}(X,S^n). \] The main open problem is whether ${\inf }_{n\ge 2}{\delta }_n>0$. Gelander's theorem gives ${\delta }_n>0$ in each fixed dimension, but not uniformly. An abstract cross-polytope construction gives the universal upper bound ${\delta }_n\le \pi /4$. In the opposite direction, ChatGPT combines the passage from small Gromov--Hausdorff error to an approximate finite action on the sphere, logarithmic stability of approximate inner-product-preserving maps due to Cuesta, operator-norm stability of almost representations, and Green's width theorem for finite transitive sets. This gives the quantitative bound \[ δ_n\ge \frac{c}{(1+\log(n+1))^2} \] for all sufficiently large $n$.