This is an expanded version of the lectures that I delivered to the Department of Pure Mathematics at the University of Calcutta in the period 27 January -6 February 1998. I am most grateful to that University for inviting me to be its Rani and Asutosh Ganguli Visiting Professor for 1998; to Professor Mihir Chakroborty, Head of its Department of Pure Mathematics, for generating so much interest in my visit and for his many kindnesses to me during my memorable stay in his city; and to the graduate students in his Department, for making me so welcome and for showing such enthusiasm for my subject matter.