Boolean Functions with Small Approximate Spectral Norm

The sum of the absolute values of the Fourier coefficients of a function $f:{\mathbb{F}}_2^n\to \mathbb{R}$ is called the spectral norm of $f$. Green and Sanders’ quantitative version of Cohen’s idempotent theorem states that if the spectral norm of $f:{\mathbb{F}}_2^n\to \{0,1\}$ is at most $M$, then the support of $f$ belongs to the ring of sets generated by at most $\mathrm{\ell }(M)$ cosets, where $\mathrm{\ell }(M)$ is a constant that only depends on $M$. We prove that the above statement can be generalized to \emph{approximate} spectral norms if and only if the support of $f$ and its complement satisfy a certain arithmetic connectivity condition. In particular, our theorem provides a new proof of the quantitative Cohen’s theorem for ${\mathbb{F}}_2^n$.