Sharp Metric Cotype Inequalities for $L_1$ via Nonlinear Cut Smoothing

Abstract

For every $1\le p<\infty$ and even integer $m\ge 4$, we determine the optimal order of the $p$-moment torus inequality for $L_1$: it is $m^p{n}^{(1-p/2{)}_{+}}+n$, with comparison constants independent of $p,m,n$ after taking $p$-th roots. Consequently, for every $2\le q<\infty$ and $1\le p\le q$, the corresponding metric cotype inequality holds at the sharp scale $m=O(n^{1/q})$. In particular, the quadratic inequality holds with $m=O(\sqrt{n})$, answering the $L_1$ sharp metric cotype question of Mendel and Naor. The proof combines finite cut representations with a nonlinear smoothing estimate and an exact ternary rounding identity.

13 pages, no figures