One of the main missions of the Abdus Salam International Centre for Theoretical Physics in Trieste, Italy, founded in 1964, is to foster the growth of advanced studies and scientific research in developing countries. To this end, the Centre organizes a number of schools and workshops in a variety of physical and mathematical disciplines.

Since unpublished material presented at the meetings might prove to be of interest also to scientists who did not take part in the schools and work­shops, the Centre has decided to make it available through a new publication series entitled ICTP Lecture Notes. It is hoped that this formally structured pedagogical material on advanced topics will be helpful to young students and seasoned researchers alike.

The Centre is grateful to all lecturers and editors who kindly authorize the ICTP to publish their notes in this series.

Since the initiative is new, comments and suggestions are most welcome and greatly appreciated. Information regarding this series can be obtained from the Publications Section or by e-mail to “pub_off@ictp. it”. The series is published in-house and is also made available on-line via the ICTP web site: “http://www. ictp. it/~pub_off/lectures/”.

Katepalli R. Sreenivasan, Director Abdus Salam Honorary Professor