Exponential convergence can happen in weighted Birkhoff averages via quasi‐periodicity with arbitrary nonresonance and low regularity

Abstract Since Krengel's work [Monatsh. Math. 86 (1978/79), 3–6] in 1978, it has been widely known that no effective rate of convergence exists in the ergodic theorem. For toral translations, however, by choosing appropriate weights one can accelerate the convergence of ergodic averages to an exponential rate, but this intuitively requires both highly nonresonant frequencies and very regular observables. In this paper, we uncover a new phenomenon: Even for any given nonresonant frequency, there exists a nontrivial family of weights and observables of low regularity such that the weighted Birkhoff averages along quasi‐periodic orbits converge at a quantitative, uniform, and exponential rate. This not only yields a finer understanding of the deep interaction between nonresonance and regularity in ergodic theory, but also stands as a weighted counterpart to a Yoccoz‐type result [C. R. Acad. Sci. Paris Sér. A‐B 291 (1980), A655–A658; Astérisque (1995), 89–242].