Transversality Family of Expanding Rational Semigroups

TL;DR

This paper investigates the Hausdorff dimension and measure of Julia sets in expanding rational semigroups, establishing conditions under which these sets have predictable geometric properties and providing new examples with specific dimensional characteristics.

Contribution

It introduces the transversality condition for rational semigroup families, linking it to Hausdorff dimension results and providing explicit examples with controlled fractal dimensions.

Findings

For almost every parameter, the Hausdorff dimension equals the zero of the pressure function.

The Hausdorff dimension of the exceptional parameter set is estimated.

Under certain conditions, the Julia set has positive Lebesgue measure.

Abstract

This paper deals with both complex dynamical systems and conformal iterated function systems. We study finitely generated expanding semigroups of rational maps with overlaps on the Riemann sphere. We show that if a $d$-parameter family of such semigroups satisfies the transversality condition, then for almost every parameter value the Hausdorff dimension of the Julia set is the minimum of 2 and the zero of the pressure function. Moreover, the Hausdorff dimension of the exceptional set of parameters is estimated. We also show that if the zero of the pressure function is greater than 2, then typically the 2-dimensional Lebesgue measure of the Julia set is positive. Some sufficient conditions for a family to satisfy the transversality conditions are given. We give non-trivial examples of families of semigroups of non-linear polynomials with transversality condition for which the Hausdorff dimension of the Julia set is typically equal to the zero of the pressure function and is less than 2. We also show that a family of small perturbations of Sierpi\'nski gasket system satisfies that for a typical parameter value, the Hausdorff dimension of the Julia set (limit set) is equal to the zero of the pressure function, which is equal to the similarity dimension. Combining the arguments on the transversality condition, thermodynamical formalisms and potential theory, we show that for each complex number $a$ with $|a|\neq 0,1$, the family of small perturbations of the semigroup generated by ${z^{2}, az^2} $ satisfies that for a typical parameter value, the 2-dimensional Lebesgue measure of the Julia set is positive.