Density combinatorics theorems in fractal dimension theory of continued fractions

Abstract

We build a bridge from density combinatorics to dimension theory of continued fractions. We establish a fractal transference principle that transfers common properties of subsets of $\mathbb N$ with positive upper density to properties of subsets of irrationals in $(0,1)$ for which the set $\{a_n(x)\colon n\in\mathbb N\}$ of partial quotients induces an injection $n\in\mathbb N\mapsto a_n(x)\in\mathbb N$. Let $(*)$ be a certain property that holds for any subset of $\mathbb N$ with positive upper density. The principle asserts that for any subset $S$ of $\mathbb N$ with positive upper density, there exists a set $E_S$ of Hausdorff dimension $1/2$ such that the set $\bigcup_{n\in\mathbb N}\bigcap_{x\in E_S}\{a_n(x)\}\cap S$ has the same upper density as that of $S$, and thus inherits property $(*)$. Examples of $(*)$ include the existence of arithmetic progressions of arbitrary lengths and the existence of arbitrary polynomial progressions, known as Szemer\'edi's and Bergelson-Leibman's theorems respectively. In the same spirit, we establish a relativized version of the principle applicable to the primes, to the primes of the form $y^2+z^2+1$, to the sets given by the Piatetski-Shapiro sequences.