Thurston algorithm for real entire maps: existence and uniqueness

Abstract

This paper deals with the real Thurston pullback algorithm for entire maps with finitely many finite singular values, all real. Given a finite forward-invariant marking containing the singular values and satisfying an interval no-Levy-cycle condition, the pullback is shown to have a unique fixed point and it is shown that every orbit converges to it. The resulting entire map uniquely realises the marked dynamics up to positive affine conjugacy. Thus this paper established Thurston realisation and uniqueness in the setting of real entire maps with a finite postsingular set. As usual, the realisation problem reduces to the existence of a fixed point of the Thurston map $T$ which in the real setting acts on a simplex $W$ of dimension $s-2$ if the required postsingular set consists of $s$ points. The key estimate concerns a finite real preimage interval: when its critical multiplicity is bounded and its endpoint values stay separated from the singular values unless they equal to one, its hyperbolic diameter grows at most double-logarithmically as the separation tends to zero. An expansive marking (absence of Levy cycles) then rules out configurations near the boundary of the simplex whose displacement under the Thurston map is at most a fixed multiple of their distance to the boundary. Using a version of the Brouwer fixed point theorem, this immediately implies that the Thurston map $T$ has a fixed point. The argument permits infinitely many real and non-real critical points and different behaviours as the real variable tends to $+\infty$ and $-\infty$. Additional estimates for fixed labelled inverse branches, which are not strictly necessary for the proof of the Main Theorem, are given in the appendices.