Overlap functions for measures in conformal iterated function systems

TL;DR

This paper introduces overlap functions and numbers for measures in conformal iterated function systems with overlaps, relating them to entropy and Hausdorff dimension, and applies these concepts to Bernoulli convolutions.

Contribution

It develops new notions of overlap functions and numbers for measures in overlapping IFS, connecting them to entropy and fractal dimension estimates, and applies these to Bernoulli convolutions.

Findings

Overlap number relates to conditional entropy of the measure.

Upper bounds for Hausdorff dimension are derived using pressure and overlap numbers.

Overlap numbers for Bernoulli convolutions are estimated and related to the dimension.

Abstract

We study conformal iterated function systems (IFS) $\mathcal S = \{\phi_i\}_{i \in I}$ with arbitrary overlaps, and measures $\mu$ on limit sets $\Lambda$, which are projections of equilibrium measures $\hat \mu$ with respect to a certain lift map $\Phi$ on $\Sigma_I^+ \times \Lambda$. No type of Open Set Condition is assumed. We introduce a notion of overlap function and overlap number for such a measure $\hat \mu$ with respect to $\mathcal S$; and, in particular a notion of (topological) overlap number $o(\mathcal S)$. These notions take in consideration the $n$-chains between points in the limit set. We prove that $o(\mathcal S, \hat \mu)$ is related to a conditional entropy of $\hat \mu$ with respect to the lift $\Phi$. Various types of projections to $\Lambda$ of invariant measures are studied. We obtain upper estimates for the Hausdorff dimension $HD(\mu)$ of $\mu$ on $\Lambda$, by using pressure functions and $o(\mathcal S, \hat \mu)$. In particular, this applies to projections of Bernoulli measures on $\Sigma_I^+$. Next, we apply the results to Bernoulli convolutions $\nu_\lambda$ for $\lambda \in (\frac 12, 1)$, which correspond to self-similar measures determined by composing, with equal probabilities, the contractions of an IFS with overlaps $\mathcal S_\lambda$. We prove that for all $\lambda \in (\frac 12, 1)$, there exists a relation between $HD(\nu_\lambda)$ and the overlap number $o(\mathcal S_\lambda)$. The number $o(\mathcal S_\lambda)$ is approximated with integrals on $\Sigma_2^+$ with respect to the uniform Bernoulli measure $\nu_{(\frac 12, \frac 12)}$. We also estimate $o(\mathcal S_\lambda)$ for certain values of $\lambda$.