Two-Parameter Martingales and Their Quadratic Variation

This book has two-fold aims. In a first part it gives an introductory, thorough and essentially self-contained treatment of the general theory of two-parameter processes that has developed since around 1975. Apart from two survey papers by Merzbach and Me

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1308 Peter lmkeller

Two-Parameter Martingales and Their Quadratic Variation

Springer-Verlag Berlin Heidelberg New York London Paris Tokyo

Author

Peter lmkeller Mathematisches lnstitut der Ludwig-Maximilians-Universitat Munchen Theresienstr. 39, 8000 Munchen, Federal Republic of Germany

Mathematics Subject Classification (1980): 60G07, 60G44, 60E 15, 60G42, 60G48, 60G55, 60G60, 60H05 ISBN 978-3-540-19233-6 Springer-Verlag Berlin Heidelberg New York ISBN 978-0-387-19233-8 Springer-Verlag New York Berlin Heidelberg

This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. Duplication of this publication or parts thereof is only permitted under the provisions of the German Copyright Law of September 9, 1965, in its version of June 24, 1985, and a copyright fee must always be paid. Violations fall under the prosecution act of the German Copyright Law. ©Springer-Verlag Berlin Heidelberg 1988 Printing and binding: Druckhaus Beltz, Hemsbach/Bergstr. 2146/3140-543210

Contents

Introduction 0. Notations and conventions

23

I. Basics; processes depending on a parameter

28

1. Regular functions and their discontinuities

29

2. Measurability and basic processes

34

3. One-directional stopping; a section theorem

42

4. One-directional projection theorems

47

5. One-directional decomposition theorems

52

6. Regularity of one-directional projections

59

II. Two-parameter processes

66

7. Two-directional stopping; a predictable section theorem 8. Martingale inequalities and applications

67 72

9. A stochastic integral and the regularity of martingales

80

10. Two-directional projection theorems

86

11. Two-directional decomposition theorems

92

III. Jumps of martingales and their compensation

99

12. Simple sets and increasing processes

101

13. The decomposition of simple sets

109

14. The jumps of regular martingales

116

15. The compensation of martingale jumps

124

IV. Quadratic variation and structure of martingales

131

16. Some general results on quadratic variation

132

17. Quadratic variations of compensated components

139

18. Orthogonality of compensated components

147

19. The structure of square integrable martingales

151

JV

20. The quadratic variation of square and L log+Lintegrable martingales

157

21. Inequalities for the quadratic variation1

a counterexample

162

References

169

Index of definitions

174

Index of special symbols

176

Introduction

There are many fields of stochastics where multi-parameter processes can be encountered. For example, to register the positions of the spins of a ferromagnetic substance at a fixed instant of time, one has to attach an appropriate state space to every point of a three-dimensional lattice. Mathematically this leads to a family of random variables indexed by a subset of

m3 ,

a

special case of a so-cal