Equivalence of Positive Hausdorff Measure and the Open Set Condition for Self-conformal Sets

A compact set K is self-conformal if it is a finite union of its images by conformal contractions. It is well known that if the conformal contractions satisfy the “open set condition” (OSC), then K has positive sdimensional Hausdorff measure, where s is the solution of Bowen’s pressure equation. We prove that the OSC, the strong OSC, and positivity of the s-dimensional Hausdorff measure are equivalent for conformal contractions; this answers a question of R. D. Mauldin. In the self-similar case, when the contractions are linear, this equivalence was proved by Schief (1994), who used a result of Bandt and Graf (1992), but the proofs in these papers do not extend to the nonlinear setting.