Two-point correlations of multiplicative functions with dense orbits

Abstract

Let $f,g:\mathbb{N}\to \mathbb{T}$ be completely multiplicative functions with dense images in the complex unit circle $\mathbb{T}$. We prove that, for every non-empty open set $U\subset {\mathbb{T}}^2$, the set of integers $n$ such that $(f(n),g(n+1))\in U$ has positive lower logarithmic density, unless the pair $(f,g)$ is of a special form. This result strengthens earlier theorems of Klurman and Mangerel, as well as of Charamaras, Mountakis, and Tsinas, and yields substantially simpler proofs.