Sphere version of Banach--Kadec--Paley theorem

Abstract

We introduce the notion of almost Lipschitz embeddings between metric spaces and apply it to the quantitative geometry of unit spheres of classical \(L_p\)-spaces. Our main result is a sphere version of the classical Banach--Kadec--Paley theorem: for \(1\le p,q<\infty\), the unit sphere \(S_{L_q}\) admits an almost Lipschitz embedding into \(L_p\) if and only if \(L_q\) admits a linear embedding into \(L_p\). Consequently, the almost Lipschitz geometry of \(L_p\)-spheres completely determines the underlying linear structure of \(L_p\)-spaces. The proof relies on quantitative estimates for moduli of continuity of sphere mappings. These estimates are derived from Kalton--Randrianarivony's concentration inequalities, Mendel--Naor's metric cotype inequality, and Naor's sharp metric \(X_p\) inequality. They also yield several sharp results on the Hölder geometry of \(L_p\)-spheres.