2.11. Selmer and Sha 67 Finally, the Shafarevich-Tate group is the quotient of the Selmer group by its subgroup E(Q)/2E(Q). Thus, each element of the Shafarevich- Tate group is represented by C(1, 1) or by a homogeneous space that is solvable everywhere locally but does not have a rational point: X2(E/Q) = {C(1, 1)} {C(δ1,δ2) Sel2(E/Q) without points over Q}. These three groups, Selmer, X (or “Sha”) and E/2E, fit in a short exact sequence 0 −→ E(Q)/2E(Q) −→ Sel2(E/Q) −→ X2(E/Q) −→ 0. In other words, the map ψ : E(Q)/2E(Q) Sel2(E/Q) is injective, the map φ : Sel2(E/Q) X2(E/Q) is surjective, and the kernel of φ is the image of ψ. Remark 2.11.1. Here for simplicity we have defined what number theorists would usually refer to as the 2-part of the Selmer group (Sel2(E/Q) above) and the 2-torsion of X (the group X2 as de- fined above). For the definition of the full Selmer and X groups, see [Sil86], Ch. X, §4. Example 2.11.2. Let E : y2 = x3 1156x, as in Example 2.10.4. The full group of homogeneous spaces H has 64 elements: H = {C(δ1,δ2) : δi = ±1, ±2, ±17, ±34}. The spaces in H with δ2 0 do not have points over R, so they do not belong to Sel2(E/Q). Moreover, we showed that the spaces (δ1,δ2) = (2,2), (17,2), (34,2), (−1,1), (−2,1), (−17,1), (−34,1), (−1,17), (−2,17), (−17,17), (−34,17), (1,34), (2,34), (17,34), and (34,34) do not have points over Q2. Therefore, they do not belong to Sel2(E/Q) either. All other spaces have rational points therefore, they are everywhere locally solvable, so they all belong to Sel2(E/Q). Hence, Sel2(E/Q) = {C(δ1,δ2) : (δ1,δ2) = (1,1),(−1,34),(−34,2),(34,17), (1,17),(34,1),(−34,34),(−2,2), (17,1),(−17,34),(2,1),(−2,34), (−17,2),(17,17),(−1,2),(2,17)}.
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