Several people have offered invaluable help during the preparation of this third edition. Thanks are due to my agent Michael Coombs for his encouragement and hard work on my behalf. My colleagues at Exeter University, Dave Earle and Alan Slater, were, as always, generous with their time: their expertise has been an important factor in deciding what to include in this new edition. Dr Peter van der Helm of the University of Nijmegen stimulated my interest in algorithmic information theory and sent me a pre-print of his important 2002 paper.
Thanks are due to Tessa and Harry Gordon for all their help and to my friend Chrissie Morris for commenting on many parts of the manuscript and for her constant encouragement.
Bettina Newman prepared the illustrations with her usual skill and efficiency.
Exeter University, February 2004