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**On the Julia set of a typical quadratic polynomial with a Siegel disk.**
*(English)*
Zbl 1069.37038

The paper studies quadratic polynomials of the form \(P(z) = z^2 + rz\), where \(r=e^{2\pi i \theta}\) for some irrational number \(0<\theta < 1\). A growth condition is assumed on the integers \(a_n\) making up the continued fraction expansion of \(\theta\), namely that \(\log (a_n) = {\mathcal O}(\sqrt{n})\). The set of \(\theta\)’s satisfying this condition has full measure.

The main result is that for such a \(\theta\), the Julia set of \(P\) is locally connected and has Lebesgue measure 0. It follows that there is a set of \(\theta\)’s of full measure for which the corresponding polynomials \(P\) have Siegel disks which are Jordan domains whose boundary contains the critical point of \(P\). A major part of the proof involves area and length estimates which enables one to apply a theorem of David concerning the integrability of Beltrami differentials whose dilatation is unbounded. This allows one to reduce the problem to a somewhat simpler one, which the authors solve, using an approach due to Yoccoz.

The main result is that for such a \(\theta\), the Julia set of \(P\) is locally connected and has Lebesgue measure 0. It follows that there is a set of \(\theta\)’s of full measure for which the corresponding polynomials \(P\) have Siegel disks which are Jordan domains whose boundary contains the critical point of \(P\). A major part of the proof involves area and length estimates which enables one to apply a theorem of David concerning the integrability of Beltrami differentials whose dilatation is unbounded. This allows one to reduce the problem to a somewhat simpler one, which the authors solve, using an approach due to Yoccoz.

Reviewer: Mike Hurley (Cleveland)