(1) Department of Mathematics, University of Toronto, M5S 1A1 Toronto, Ontario, Canada;(2) Department of Mathematics, Duke University, 27706 Durham, NC, USA;(3) Cornell University, 14853 Ithaca, NY, USA
Abstract:
The Manin—Drinfeld theorem asserts the finiteness of the cuspidal divisor class group of a modular curve corresponding to
a congruence subgroup. The purpose of the note is to draw attention to the connection between this theorem and Ramanujan sums,
and to the question of what happens for non-congruence subgroups.
Research partially supported by an NSERC grant.
Alfred P Sloan Fellow. Research partially supported by the NSF.