Compositio Mathematica

Special Points on the Product of Two Modular Curves

a1 Irmar, Campus de Beaulieu, 35042 Rennes cedex, France, e-mail:

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edixhoven b   [Google Scholar] 


We prove, assuming the generalized Riemann hypothesis for imaginary quadratic fields, the following special case of a conjecture of Oort, concerning Zarsiski closures of sets of CM points in Shimura varieties. Let X be an irreducible algebraic curve in C$^2$, containing infinitely many points of which both coordinates are j-invariants of CM elliptic curves. Suppose that both projections from X to C are not constant. Then there is an integer m [gt-or-equal, slanted] 1such that X is the image, under the usual map, of the modular curve Y$_0$(m). The proof uses some number theory and some topological arguments.

Key Words: Shimura varieties; complex multiplication; subvarieties; elliptic curves; modular curves..