We compute the ,-Selmer group for a family of elliptic curves, where , is an isogeny of degree 5, then find a practical formula for the Cassels Tate pairing on the ,-Selmer groups and use it to show that a particular family of elliptic curves have non-trivial 5-torsion in their Shafarevich Tate group.2000 Academic Press
INTRODUCTIONLet K be a number field, EÂK an elliptic curve defined over K, and E(K) the Mordell Weil group of points on E with coordinates in K. Denote by (EÂK) the Shafarevich Tate group of EÂK. The Shafarevich Tate group arises frequently in the study of elliptic curves although much about its size and structure remains mysterious. The group is believed to be finite and the conjecture of Birch and Swinnerton-Dyer asserts that its cardinality is a factor in the leading term of the L-series of the elliptic curve at s=1. Kolyvagin [8] proved that E(Q) and (EÂQ) are both finite for a certain class of modular curves. Rubin [16] showed that if E is an elliptic curve defined over an imaginary quadratic field, K, with complex multiplication by K, then if L(E ÂK , 1){0, (EÂK) is finite. Although finiteness of the Shafarevich Tate group has been proven in some cases, a general result is far from being achieved and we instead try to gather information about its cardinality. For example how large is the m-torsion subgroup? In the past, the 2-and 3-torsion in (EÂK) have been studied with the most success. Kramer [10] used a descent procedure to produce a family of elliptic curves with arbitrarily large 2-torsion in (EÂK); and Cassels [2] showed that the 3-torsion can be arbitrarily large by constructing certain quadratic twists of E. Here we will give a method for finding non-trivial 5-torsion