Elliptic curves of high rank over function fields

By the Mordell-Weil theorem the group of Q(z)-rational points of an elliptic curve is finitely generated. It is not known whether the rank of this group can get arbitrary large as the curve varies. Mestre and Nagao have constructed examples of elliptic curves E with rank at least 13. In this paper a method is explained for finding a 14th independent point on E, which is defined over k(z), with [k:Q]=2. The method is applied to Nagao's curve. For this curve one has k=Q(sqrt{-3}). The curves E and 13 of the 14 independent points are already defined over a smaller field k(t), with [k(z):k(t)]=2. Again for Nagao's curve it is proved that the rank of E(\bar Q(t)) is exactly 13, and that rank E(Q(t)) is exactly 12.