Small values and forbidden values for the Fourier antidiagonal constant of a finite group

Abstract

For a finite group $G$, let ${\rm AD}(G)$ denote the Fourier norm of the antidiagonal in $G\times G$. It was shown recently by the author (IMRN, 2023) that ${\rm AD}(G)$ coincides with the amenability constant of the Fourier algebra of $G$, and is equal to the normalized sum of the cubes of character degrees of $G$. Motivated by a gap result for amenability constants due to Johnson (JLMS, 1994), we determine exactly which numbers in the interval $[1,2]$ arise as values of ${\rm AD}(G)$. As a by-product, we show that the set of values of ${\rm AD}(G)$ does not contain all its limit points. Some other calculations or bounds for ${\rm AD}(G)$ are given for familiar classes of finite groups. We also indicate a connection between ${\rm AD}(G)$ and the commuting probability of $G$, and use this to show that every finite group $G$ satisfying ${\rm AD}(G)< \frac{61}{15}$ must be solvable; here the value $\frac{61}{15}$ is best possible.