Fatou Type Theorems Maximal Functions and Approach Regions

A basic principle governing the boundary behaviour of holomorphic func­ tions (and harmonic functions) is this: Under certain growth conditions, for almost every point in the boundary of the domain, these functions ad­ mit a boundary limit, if we approach

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Series Editors Hyman Bass Joseph Oesterle Alan Weinstein

Fausto Di Biase

Fatou Type Theorems Maximal Functions and Approach Regions

Birkhtiuser Boston • Basel • Berlin

Department of Mathematics Princeton University Princeton, NJ 08540

University Roma-Tor Vergata Dip. Matematica 00133 Rome, Italy

Library of Congress Cataloging-in-Publication Data Di Biase, Fausto, 1962Fatou type theorems: maximal functions and approach regions / Fausto Di Biase. p. cm. -- (Progress in mathematics ; v. 147) Includes bibliographical references (p. - ) and index. ISBN-13: 978-1-4612-7496-4 DOl: 10.1007/978-1-4612-2310-8

e-ISBN-13: 978-1-4612-2310-8

1. Holomorphic functions. 2. Fatou theorems. 3. Functions of several complex variables. I. Title. II. Series: Progress in mathematics (Boston, Mass.) ; vol. 147 QA331.D5581997 95-30694 S15--dc21 CIP

AMS Classification Codes: 3IB25 and 32A40.

Printed on acid-free paper

© 1998 Birkhiiuser

Birkhiiuser

~

Softcover reprint of the hardcover 1st edition 1998 Copyright is not claimed for works of U.S. Government employees. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior permission of the copyright owner. Permission to photocopy for internal or personal use of specific clients is granted by Birkhiiuser Boston for libraries and other users registered with the Copyright Clearance Center (CCC), provided that the base fee of $6.00 per copy, plus $0.20 per page is paid directly to CCC, 222 Rosewood Drive, Danvers, MA 01923, U.S.A. Special requests should be addressed directly to Birkhiiuser Boston, 675 Massachusetts Avenue, Cambridge, MA 02139, U.S.A.

Typesetting by the author in LATEX

9 8 765 432 1

Contents Preface

I

vii

Background

1

1 Prelude 1.1 1.2 1.3 1.4

The Unit Disc . Spaces of Homogeneous Type Euclidean Half-Spaces . . . . Maximal Operators and Convergence

2 Preliminary Results 2.1 2.2 2.3

Approach Regions . . . . . . . . . . The Nagel-Stein Approach Regions Goals, Problems and Results .

3 The Geometric Contexts 3.1 3.2 3.3

II

NTA Domains in ]Rn Domains in Trees . . . . . . . . .

en

Exotic Approach Regions

4 Approach Regions for Trees 4.1 4.2

The Dyadic Tree The General Tree

5 Embedded Trees 5.1 5.2 5.3

The Unit Disc Quasi-Dyadic Decompositions . . . . . The Maximal Decomposition of a Ball

3 3 16

18 24

27 27

43 51

55 55

59 73

85 87 87 88 99

99 104

108

CONTENTS

vi 5.4

Admissible Embeddings

6 Applications 6.1 Euclidean Half-Spaces 6.2 NTA Domains in IRn . 6.3 Finite-Type Domains in 1(:2 6.4 Strongly Pseudo convex Domains in

. 110

123

en

124 124 125 128

Notes

129

List of Figures

131

Guide to Notation

133

Bibliography

135

Index

149

Preface A basic principle governing the boundary behaviour of holomorphic functions (and harmonic functions) is this: Under certain growth conditions, for almost every point in the boundary of the domain, these functions admit a boundary