J. reine angew. Math. 775 (2021), 117–143 Journal für die reine und angewandte Mathematik DOI 10.1515/crelle-2021-0004 © De Gruyter 2021 Generalized Birch lemma and the 2-part of the Birch and Swinnerton-Dyer conjecture for certain elliptic curves By Jie Shu at Shanghai and Shuai Zhai at Cambridge Abstract. In the present paper, we generalize the celebrated classical lemma of Birch and Heegner on quadratic twists of elliptic curves over Q. We prove the existence of explicit infinite families of quadratic twists with analytic ranks 0 and 1 for a large class of elliptic curves, and use Heegner points to explicitly construct rational points of infinite order on the twists of rank 1. In addition, we show that these families of quadratic twists satisfy the 2-part of the Birch and Swinnerton-Dyer conjecture when the original curve does. We also prove a new result in the direction of the Goldfeld conjecture. 1. Introduction Let E be an elliptic curve defined over Q, and let N be the conductor of E. Then E is modular by the theorem of Wiles et al. [3, 31], and we let f X0.N / E be an optimal W ! modular parametrization sending the cusp at infinity, which is denoted by Œ , to the zero 1 1 element of E. We write Œ0 for the cusp of X0.N / arising from the zero point in P .Q/ under the complex uniformization of X0.N /. By the theorem of Manin–Drinfeld, f .Œ0/ is a torsion point in E.Q/. For any square-free integer D 1, we write E.D/ for the twist of E by the ¤ extension Q.pD/=Q, and L.E.D/; s/ for its complex L-series. We assume throughout this paper that the group EŒ2.Q/ of rational 2-division points on E is cyclic of order 2, and we define the elliptic curve E =Q to be the quotient curve E E=EŒ2.Q/. In all that follows, 0 0 D we will make the following assumption: (Tor) EŒ2.Q/ E Œ2.Q/ Z=2Z: D 0 D Thus Q.EŒ2/ and Q.E0Œ2/ are quadratic extensions of Q. We also remark that it is proven in Proposition 2.3 that, for any elliptic curve E over Q with f .Œ0/ 2E.Q/, condition (Tor) 62 The corresponding author is Jie Shu. Jie Shu is supported by the National Natural Science Foundation of China (Grant No. 11701092). 118 Shu and Zhai, Generalized Birch lemma holds for E if and only if there exists an odd prime q of good reduction for E such that  1à (1.1) aq 1 mod 4; Á q where aq is the trace of Frobenius on the reduction of E modulo q. We will use the follow- ing explicit set of primes, which, by Chebotarev’s theorem, has positive density in the set of all primes. Definition 1.1. A prime q is admissible for E if .q; 2N / 1 and q is inert in both of D the quadratic fields Q.EŒ2/ and Q.E0Œ2/. Note also that, assuming that our elliptic curve E satisfies condition (Tor), we prove in Proposition 2.2 that an odd prime q of good reduction is admissible for E in the above sense if and only if it satisfies (1.1). In what follows, p will always denote a prime > 3 such that p 3 mod 4, and we Á will write K Q.p p/. We say that p satisfies the Heegner hypothesis for .E; K/ if every D prime ` dividing N splits in the field K. For any odd prime q, put q . 1 /q, where . / D q denotes the Legendre symbol. We shall prove the following result, which generalizes an old lemma of Birch [2]. Theorem 1.2. Let E be an elliptic curve over Q satisfying f .Œ0/ 2E.Q/ and condi- 62 tion (Tor). Let p > 3 be any prime 3 mod 4 satisfying the Heegner hypothesis for .E; K/, Á where K Q.p p/. For any integer r 0, let q1; : : : ; qr be distinct admissible primes which D are not equal to p, and put M q q q . Then we have D 1 2 r .M / .M / ords 1L.E ; s/ rank E .Q/ 0; D D D . pM / . pM / ords 1L.E ; s/ rank E .Q/ 1: D D D .M / . pM / Moreover, the Shafarevich–Tate groups X.E / and X.E / are both finite. In view of the above remarks, we immediately obtain the following corollary. Corollary 1.3. Let E be any elliptic curve over Q such that f .Œ0/ 2E.Q/, and there 62 exists an odd prime q of good reduction for E satisfying (1.1). Then, for any integer r 1, there exist infinitely many square-free integers M and M 0, having exactly r prime factors, such that L.E.M /; s/ does not vanish at s 1, and L.E.M 0/; s/ has a zero of order 1 at s 1. D D Theorem 1.2 and Corollary 1.3 improve the results in [8, Theorem 1.1] and [4, Theo- rem 1.1], which were motivated by the remarkable progress on the congruent number problem made by Tian [29]. Note, however, that, in these two papers, much stronger conditions are im- posed on the prime factors of M and M 0. In particular, the generalizations of the Birch lemma in [8, Corollary 2.6] and [4, Theorem 1.1] require that the prime factors q of M and M 0 satisfy r 1 q 1 mod 4 and aq 0 mod 2 , where, as above, r denotes the number of prime factors Á Á C of M or M 0. Theorem 1.2 and Corollary 1.3, can be applied to many elliptic curves, espe- cially to elliptic curves without complex multiplication. We present a wide range of examples in Section5. Note also that in Theorem 1.2, M will be negative precisely when the num- ber of admissible primes q 3 mod 4 dividing M is odd, whence pM will be positive. In Á some cases, this enables us to use Heegner points to construct rational points of infinite order Shu and Zhai, Generalized Birch lemma 119 on real quadratic twists of our elliptic curve E, as for example in Section 5.1, where E is a Neumann–Setzer curve. For the 2-part of the Birch and Swinnerton-Dyer conjecture for the quadratic twists of E occurring in Theorem 1.2, we prove the following result (see Theorem 4.10). Theorem 1.4. Let E and M be as in Theorem 1.2. Assume that (i) the Manin constant of E is odd, and (ii) every prime ` dividing 2N splits in both K Q.p p/ and Q.pM/. D Then, if the 2-part of the Birch and Swinnerton-Dyer conjecture holds for E, it also holds for .M / . pM / both E and E . In Section 5.3, we give infinitely many examples of elliptic curves over Q, without com- plex multiplication, which satisfy the full Birch and Swinnerton-Dyer conjecture, by combining this result with a recent theorem of Wan [30] on the p-part of the Birch and Swinnerton-Dyer conjecture for primes p > 2, whose proof uses deep methods from Iwasawa theory. We now present another application of Theorem 1.2. In 1979, Goldfeld [10] conjectured that, for every elliptic curve defined over Q, amongst the set of all its quadratic twists with root number 1, there is a subset of density one where the central L-value of the twist is non-zero, C and amongst the set of all its quadratic twists with root number 1, there is a subset of density one where the central L-value of the twist has a zero of order equal to 1. For an elliptic curve E over Q, define .D/ Nr .E; X/ # square free D Z D X; ords 1L.E ; s/ r ; D ¹ 2 W j j Ä D D º where r 0; 1. We prove the following result. D Theorem 1.5. Let E be an elliptic curve over Q satisfying f .Œ0/ 2E.Q/, and con- 62 dition (Tor). Then, as X , we have ! 1 X Nr .E; X/ : log3=4 X Previously, for any given elliptic curve E over Q, Ono and Skinner [21] showed that X N0.E; X/ ; log X and later Ono [20] proved that, in particular for E without a rational 2-torsion point, X N0.E; X/ 1 ˛ .log X/ 1 " for some 0 < ˛ < 1. Perelli and Pomykała [23] proved that N1.E; X/ X for every " > 0. Much progress for various families of elliptic curves has been made towards Goldfeld’s conjecture. For example, see the early survey article by Silverberg [25], and very recent results in [15,28]. However, Theorem 1.5 improves previous results when E satisfies f .Œ0/ 2E.Q/ 62 and condition (Tor). The proof of Theorem 1.5 is a straightforward consequence of Theo- rem 1.2. Indeed, we just count the number of integers M appearing in Theorem 1.2, say X S0.X/ 1; WD M X j jÄ 120 Shu and Zhai, Generalized Birch lemma where M q q as in Theorem 1.2. In view of Proposition 2.2, the Chebotarev density D 1 r theorem shows that the admissible primes have natural density 1=4 in the set of all primes. Applying the Ikehara Tauberian theorem, it follows that c X c X S .X/ 0 0 ; 0 1 1=4 3=4 .log X/ .log X/ where c0 is a constant.