Vietoris--Rips Coindex Thresholds for Round Spheres: Spherical Joins and Chromatic Obstructions for Gromov--Hausdorff Distances

Abstract

For $0\le m\le n$, let $c_{m,n}$ be the infimum of scales at which the Vietoris--Rips filtration of the round sphere $\mathbb{S}^m$ admits a continuous odd map from $\mathbb{S}^n$. A quantitative Borsuk--Ulam theorem gives $c_{m,n}/2\le d_{\mathrm{GH}}(\mathbb{S}^m,\mathbb{S}^n)$, and it was asked whether equality always holds. Using a synchronized product-measure lift of the spherical join, we construct continuous odd maps between Vietoris--Rips metric thickenings with target scale equal to the maximum input scale. Iteration gives $c_{m+d,n+d}\le c_{m,n}$ for every integer $d\ge0$. More generally, the finite join law adds $m_i+1$ and $n_i+1$ separately and bounds the resulting $c$-value by $\max_i c_{m_i,n_i}$. For $0\le r<π$, the pairs $(k,\ell)$ with $c_{k-1,\ell-1}\le r$ are closed under addition, so Fekete's lemma gives a limit for the maximal admissible $\ell/k$ as $k\to\infty$. This structure, exact values, and projective-code estimates give finite and asymptotic bounds. If $1\le m_j c_{m_j,n_j}/2$ for all sufficiently large $j$, producing infinitely many counterexamples.