Optimal Hypercontractivity and Log--Sobolev inequalities on Cyclic Groups $\mathbb{Z}_{m\cdot 2^k}$

TL;DR

This paper establishes optimal hypercontractivity and Log--Sobolev inequalities for Poisson-like semigroups on certain cyclic groups, identifying precise conditions and constants, and introduces novel analytical techniques for these inequalities.

Contribution

It proves sharp hypercontractivity bounds and Log--Sobolev inequalities on cyclic groups of specific sizes, using a novel KKT analysis and Dirichlet form comparison.

Findings

Hypercontractivity holds iff t ≥ (1/2) log((q-1)/(p-1))

Sharp Log--Sobolev inequalities with constant 2 are established

Method involves KKT analysis and Dirichlet form comparison for specific cyclic groups

Abstract

For $1<p\le q<\infty$ and $n\in\{3\cdot 2^{k},2^{k}\}$ with $k\ge 1$, we prove that the Poisson-like semigroup $(P_t)_{t\in \mathbb{R}_+}$ on $\mathbb{Z}_n$, associated with the word length $\psi_n(k)=\min(k,n-k)$, is hypercontractive from $L_p$ to $L_q$ if and only if $t\ge \tfrac{1}{2}\log\big(\tfrac{q-1}{p-1}\big)$. We establish sharp Log--Sobolev inequalities with the optimal constant $2$, by performing a KKT analysis, and lifting from the base cases $\mathbb{Z}_6$ and $\mathbb{Z}_4$ via a Cooley--Tukey $n\mapsto 2n$ comparison of Dirichlet forms. The general case for arbitrary $n$ remains open.