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A zero–infinity law for well-approximable points in Julia sets

Published online by Cambridge University Press:  06 November 2002

RICHARD HILL
Affiliation:
Department of Mathematics, University College London, Gower Street, London WC1E 6BT, UK (e-mail: the.richard.hill@bigfoot.com)
SANJU L. VELANI
Affiliation:
Department of Mathematics, Queen Mary, University of London, Mile End Road, London E1 4NS, UK (e-mail: s.velani@qmw.ac.uk)

Abstract

Let T:J \to J be an expanding rational map of the Riemann sphere acting on its Julia set J and f:J\to \mathbb{R} denote a Hölder continuous function satisfying f(x) > \log|T^\prime(x)| for all x in J. Then for any point z_0 in J define the set D_{z_0}(f) of ‘well-approximable’ points to be the set of points in J which lie in the Euclidean ball

B\bigg(y,\exp\bigg(-\sum_{i=0}^{n-1} f(T^iy)\bigg)\bigg)

for infinitely many pairs (y,n) satisfying T^n(y)=z_0. In our 1997 paper, we calculated the Hausdorff dimension of D_{z_0} (f). In the present paper, we shall show that the Hausdorff measure \mathcal{H}^s of this set is either zero or infinite. This is in line with the general philosophy that all ‘naturally’ occurring sets of well-approximable points should have zero or infinite Hausdorff measure.

Type
Research Article
Copyright
2002 Cambridge University Press

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