2.6. Elliptic curves over finite fields 41 where all the coordinates should be regarded as congruences modulo 5. Thus, N5 = 6, which is in the range given by Hasse’s bound: 1.5278 . . . = 5 + 1 2 5 N5 5 + 1 + 2 5 = 10.4721 . . . . Similarly, one can verify that N7 = 13. The connections between the numbers Np and the group E(Q) are numerous and of great interest. The most surprising relationship is captured by the Birch and Swinnerton-Dyer conjecture (Conjecture 5.2.1) that relates the growth of Np (as p varies) with the rank of the elliptic curve E/Q. We shall discuss this conjecture in Section 5.2 in more detail. In the next proposition we describe a different connection between Np and E(Q). We shall use the following notation: if G is an abelian group and m 2, then the points of G of order dividing m will be denoted by G[m]. Proposition 2.6.15 ([Sil86], Ch. VII, Prop. 3.1). Let E/Q be an elliptic curve, p a prime number and m a natural number not divisible by p. Suppose that E/Q has good reduction at p. Then the reduction map modulo p, E(Q)[m] −→ E(Fp), is an injective homomorphism of abelian groups. In particular, the number of elements of E(Q)[m] divides the number of elements of E(Fp). The previous proposition can be very useful when calculating the torsion subgroup of an elliptic curve. Let’s see an application: Example 2.6.16. Let E/Q: y2 = x3 +3. In Example 2.6.14 we have seen that N5 = 6 and N7 = 13, and E/Q has bad reduction only at 2 and 3. If q = 5,7 is a prime number, then E(Q)[q] is trivial. Indeed, Proposition 2.6.15 implies that |E(Q)[q]| divides N5 = 6 and also N7 = 13. Thus, |E(Q)[q]| must divide gcd(6,13) = 1. In the case of q = 5, we know that |E(Q)[5]| divides N7 = 13. Moreover, by Lagrange’s theorem from group theory, if E(Q)[p] is non-trivial, then p divides |E(Q)[p]| (later on we will see that E(Q)[p] is always a subgroup of Z/pZ × Z/pZ see Exercise 3.7.5). Since 5
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