On Sárközy's Local Extrema Conjectures

Abstract

Let $f: \mathbb{N} \rightarrow (0,\infty)$ be a multiplicative function, and let $ω> 0$. We consider the frequency with which patterns of the form $$ f(n) > ω\max\{f(n-1),f(n+1)\}, \quad f(n) < \frac{1}ω\min\{f(n-1),f(n+1)\} $$ occur in the sequence $(f(n))_n$, and give a sharp classification of all positive-valued multiplicative functions for which at least one of these patterns only occurs with logarithmic density $0$. As a particular application, we resolve, in a strong form, two conjectures of A. Sárközy from 2001, classifying all multiplicative functions $f: \mathbb{N} \rightarrow \mathbb{N}$ that have either finitely many local maxima, or finitely many local minima. The proof is based on a natural ``pretentiousness'' dichotomy for additive functions. Key tools in the analysis include an analogue of the Matomäki-Radziwiłłtheorem for additive functions, due to the author, as well as a new bound for the concentration of the gaps $g(n+1)-g(n)$ of an additive function $g$, which may be of independent interest.