Counting Problems for Elliptic Curves over a Fixed Finite Field

Let E be an elliptic curve defined over a finite field with q elements. Hasse’s theorem says that #E(F_q) = q + 1 - t_E where |t_E| is at most twice the square root of q. Deuring uses the theory of complex multiplication to express the number of isomorphism classes of curves with a fixed value of t_E in terms of sums of ideal class numbers of orders in quadratic imaginary fields. Birch shows that as q goes to infinity the normalized values of these point counts converge to the Sato-Tate distribution by applying the Selberg Trace Formula.

In this talk we discuss finer counting questions for elliptic curves over a fixed finite field. We study the distribution of rational point counts for elliptic curves containing a specified subgroup, giving exact formulas for moments in terms of traces of Hecke operators. This leads to formulas for the expected value of the exponent of the group of rational points of an elliptic curve over F_q and for the probability that this group is cyclic. This is joint with work Ian Petrow (ETH Zurich).

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