a1 Address From 1st January 1999: Department of Mathematics, The Pennsylvania State University, University Park, PA 16802
This is an expanded version of two lectures given at the conference held at Sydney University in December 1997 on the 50th anniversary of the death of G. H. Hardy.
(Received December 19 1997)
(Revised July 11 1998)
1991 Mathematics subject classification (Amer. Math. Soc.)
p1 Current address: Department of Mathematics Huxley Building Imperial College 180 Queen's Gate London, SW7 2BZ