The subject of my lecture is not a large-scale and general theory but rather a special problem in conformai mapping. By way of justification for bringing it to the attention of a broader audience I may cite, first of all, its deep nature and honorable origin-the problem originated in Riemann's memoir [46] of 1857 on algebraic functions, and after the passage of a century some definitive insight into it is only now emerging as a result of the collective efforts of a group of mathematicians. Secondly, it is intimately bound up with a variety of topics of more general interest: conformai mapping with particular reference to extremal problems, several complex variables, and partial differential equations, on the one hand; topology and algebraic geometry, on the other and, in particular aspects, even group representations and arithmetic.
Introduction and history.Briefly put, and in modern paraphrase, Riemann observed: (a) that for compact (algebraic) Riemann surfaces of genus g = 0, i.e., simply connected, one has the analogue of the Riemann mapping theorem, that is, all such surfaces not only admit topological maps on one another but also conformai maps, in particular, on the Riemann sphere (all maps of surfaces in this paper are assumed to be homeomorphisms without further mention) ; (b) for g>0 it is no longer true that the existence of a map of one surface on another (equivalent to their having the same genus) implies the existence of a conformai map; and (c) as a partial compensation for (b) and substitute for (a) in case g>0, if one considers the conformai equivalence classes obtained by identifying surfaces which admit mutual conformai maps, these classes form not an isolated point as in (a) but in some sense or other a continuum of finite dimension, in fact, of dimension one for g= 1 and 3g -3 for g^2. Riemann called a set of parameters for the continuum moduli (Riemann and his immediate successors often spoke cryptically of the moduli, as though some particular set of moduli were implicitly distinguished-this has not been borne out by the modern research to be described below).