Counting Elliptic Surfaces over Finite Fields

We count the number of isomorphism classes of elliptic curves of given height d over the field of rational functions in one variable over the finite field of q elements. We also estimate the number of isomorphism classes of elliptic surfaces over the projective line, which have a polarization of relative degree 3. This leads to an upper bound for the average 3-Selmer rank of the aforementionned curves. Finally, we deduce a new upper bound for the average rank of elliptic curves in the large d limit, namely the average rank is asymptotically bounded by 1.5 +O(1/q). 2000 Math. Subj. Class. 14G, 11G, 14H25, 1452.