Subexponential Bohnenblust-Hille Inequalities on Finite Cyclic Groups

Slote, Volberg and Zhang proved a dimension-free Bohnenblust--Hille inequality on products of finite cyclic groups for functions of bounded total degree. Later, Becker, Klein, Slote, Volberg and Zhang proved $\BHdeg{d}{q}\leq(C\log q)^{2d}$ and asked whether the optimal constants are subexponential in the degree. More recently, Defant, Galicer, Mansilla, Masty\l o and Muro established an exponential Bohnenblust--Hille estimate for the larger support-sensitive class governed by the number of active coordinates. We prove that the optimal constants for this larger class grow subexponentially. More precisely, if $\BHint{d}{q}$ denotes the optimal constant for functions on $C_q^N$ whose Fourier characters involve at most $d$ coordinates, with no restriction on the nonzero local frequencies, then, for every fixed $q\geq2$, $$\BHint{d}{q}\leq \exp\left(c_q\sqrt{d\log d} +O_q\left(\sqrt{\frac d{\log d}}\log\log d\right)\right) \qquad(d\to\infty),$$ where $c_2=2$ and $c_q=\sqrt{2q\log(q-1)/(q-2)}$ for $q\geq3$. Since bounded total degree implies bounded interaction order, this also answers the question of Becker et al. The proof uses Potts hypercontractivity, Blei's mixed-norm inequality, orbit counting, and a mixed polarization estimate.