Elliptic curves with large analytic order of the Tate-Shafarevich group

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1 Elliptic curves with large analytic order of the Tate-Shafarevich group Andrzej Dabrowski and Mariusz Wodzicki 0. Introduction Let E be an elliptic curve over Q of conductor N = N(E) and let L(E, s) = n=1 a nn s denote the corresponding L-series. Let X(E) be the Tate-Shafarevich group of E (conjecturally finite), E(Q) be the group of global points, and R be the regulator (with respect to the Néron-Tate height pairing). Finally, let ω be the real period, and c = ω or 2ω (according to whether E(R) is connected or not), and let c fin denote the fudge factor (the Tamagawa number) of E. The L-series L(E, s) converges for Re s > 3/2. The modularity conjecture (completely settled by Wiles-Taylor-Diamond-Breuil-Conrad [BCDT]) implies that L(E, s) has analytic continuation to an entire function. Conjecture 1 (Birch and Swinnerton-Dyer). L-function L(E, s) has a zero of order r = rank E(Q) at s = 1, and L(E, s) (1) lim s 1 (s 1) r = c (E)c fin (E)R(E) X(E) E(Q) tors 2. It has been known for a long time that there are elliptic curves with arbitrarily large values of X(E) (Cassels [Ca] was the first one to show this by considering quadratic twists by many different primes). Kramer [Kr] finds semistable elliptic curves with discriminant m(16m + 1) and X(E) 2 2k 2, where k is the number of prime factors of 16m + 1. Their methods give at best families with X(E) N c log log N for some positive constant. Assuming the Birch and Swinnerton-Dyer conjecture, Mai and Murty [MM] have shown that for any elliptic curve E, and infinitely many square-free d, lim d N(E d ) 1 4 ɛ X(E d ) = 0.

2 It was proved only in 1987 that there is any elliptic curve over Q for which group X(E) is finite (Rubin [Ru], Kolyvagin [Ko], Kato). A few years later, Goldfeld and Szpiro [GS], and Mai and Murty [Ra], proposed the following general conjecture: (2) X(E) N(E) 1/2+ɛ, N. The Goldfeld-Szpiro-Mai-Murty conjecture, (2), holds if the Birch and Swinnerton- Dyer conjecture holds true for all rank zero quadratic twists E d of E. Assuming additionally the Lindelöf conjecture, L(E d, 1) N(E d ) ɛ, one then easily obtains that X(E d ) N(E d ) 1/4+ɛ. In general, for elliptic curves that satisfy the Birch and Swinnerton-Dyer conjecture, Goldfeld and Szpiro [GS] show that the Goldfeld-Szpiro-Mai-Murty conjecture is equivalent to the Szpiro conjecture: (E) N(E) 6+ɛ where (E) denotes the discriminant of the minimal model of E. It is conjectured [dw] that 6 in the exponent of the conductor is the smallest possible number for which the above asymptotic bound holds. Conjecture 2 (de Weger). For any ɛ > 0 there exist infinitely many elliptic curves over Q with X(E) N(E) 1/2 ɛ. De Weger demonstrates [dw] that the above conjecture follows from the following three conjectures: the Birch and Swinnerton-Dyer conjecture in the rank zero case, the Szpiro conjecture, and the Riemann hypothesis for certain Rankin-Selberg zeta functions associated to certain modular forms of weight 3 2. He gives eleven examples of elliptic curves with the Goldfeld-Szpiro ratio GS(E) := X(E) N(E) 1, the largest value being Nitaj [Ni] produces 47 other examples with GS(E) 1, the largest value of GS(E) being Note that curves with GS(E) > 1 were already known from Cremona s tables [Cr]. As far as the order of the Tate-Shafarevich group is concerned, De Weger [dw] gives an example of an elliptic curve with X(E) = 224 2, Rose [Ro] gives another example of an elliptic curve with X(E) = 635 2, and the curve with the largest known order of that group: X(E) =

3 was produced by Nitaj [Ni]. For the family of cubic twists considered by Zagier and Kramarz [ZK] E d : x 3 + y 3 = d (d cubic-free) the largest value of X(E d ) does not exceed 21 2 when d (In this case, standard conjectures imply that X(E d ) N(E d ) 1/3+ɛ ). Under assumption of the Birch and Swinnerton-Dyer conjecture, they compute X(E) for a rank zero elliptic curve E by evaluating L(E, 1) with sufficient accuracy. Subsequently, we shall refer to this number the analytic order of the Tate- Shafarevich group of E, and we will continue denoting it X(E). This is unlikely to cause confusion. For quadratic twists of a given curve one can calculate the analytic order of Tate- Shafarevich group using a well known theorem due to Waldspurger [Wa] combined with purely combinatorial methods. Consider, for example, the family E d : y 2 = x 3 d 2 x (d 1 an odd square-free integer) of congruent number elliptic curves. Let and η(z) = q 1/24 n=1 (1 q n ), Θ(z) = η(8z)η(16z)θ(2z) =: n= a(n)q n. When curve E d is of rank zero then, assuming as usual the Birch and Swinnerton- Dyer conjecture, we have (see [Tu]): (3) X(E d) = n=1 ( ) 2 a(d) τ(d) where τ(d) denotes the number of divisors of d. (Coefficients a(d) can also be calculated using the formula in [On].) Conjecturally X(E d) N(E d) 1/4+ɛ, hence the sequence of curves E d (and, more generally, families of quadratic twists) is not a likely candidate to produce curves with large Goldfeld-Szpiro ratios. The primary aim of this article is to present results of our search for curves with exceptionally large analytic orders of the Tate-Shafarevich group. We exhibit 134 q n2,

4 examples of rank zero elliptic curves with X(E) > which was the largest known value for any explicit curve. Our record is a curve with We consider the family X(E) = 63, E(n, p) : y 2 = x(x + p)(x + p 4 3 2n+1 ) and its isogenous curves for p (Z \ 0) [ 1000, 1000] and n 19. Compared to previously published results, we faced working with curves of very big conductor. Big conductor means a very slow convergence of the approximation to L(E, 1). The main difficulty was to design a successful search strategy for curves with exceptionally large Goldfeld-Szpiro ratios which translated into large analytic orders of the Tate-Shafarevich group. Our explorations brought also a number of unplanned discoveries: examples of curves of rank zero with the value of L(E, 1) much smaller, or much bigger, than in any previously known example (see Tables 6 and 5 below). One particularly remarkable example involves a pair of non-isogeneous curves whose values of L(E, 1) coincide in the first 11 digits after the point! (see formula (23)). Details of the computations, tables and related remarks are contained in section 3. Section 2 contains some heuristics concerning an (explicit) approach to conjecture 2 via our families of elliptic curves. The actual calculations were carried by the second author in the Summer and the Fall 2002 on a variety of computers, almost all of them located in the Department of Mathematics in Berkeley. Supplemental computations were conducted also in 2003 and the Summer The results were reported by M.W. at the conference Geometric Methods in Algebra and Number Theory which took place in December 2003 in Miami. 1. The Shimura correspondence and a theorem of Waldspurger The Shimura correspondence is a map relating certain half-integral weight cusp forms to modular forms of even weight. Theorem. [Sh] Let k 3 be an odd integer, N a positive integer divisible by 4, and χ a character modulo N. Suppose that f S k/2 (N, χ) is an eigenform for all T (p 2 ) with corresponding eigenvalues λ p. Define a function g(z) = b n q n by the identity (4) n=1 b n q n = p [1 λ p p s + χ(p) 2 p k 2 2s ] 1.

5 Then g M k 1 (N/2, χ 2 ). In particular, g is an eigenform for all T (p), also having the eigenvalues λ p. The connection between modular forms and L-series is provided by a theorem of Waldspurger (below we state a special case of his result). Theorem. [Wa; Theorem 1] Let Φ be a cusp form of weight 2, level divisible by 16, and trivial character which is the image of a form f of weight 3/2 and quadratic character χ under the Shimura correspondence. Then there is a function A(t) from squarefree integers to C such that (i) A(t) 2 = L(Φ χ 1 χ t, 1)/π, and (ii) for each positive integer N, there is a finite set C of functions c(n) such that { A(n sf )c(n)q n c(n) C} spans the space of cusp forms of weight 3/2, level N, and character χ which correspond to Φ by the Shimura map. In constructing weight 3/2 modular forms one can use two methods. The first is to construct weight 1 cusp forms (using representation theory, say) and multiply these by theta series. The second is to use ternary quadratic forms to construct modular forms directly (Shimura). Let E be an elliptic curve over Q, of conductor N; let f denote the corresponding modular form of weight 2. Below (Examples) we give examples, when modular form F of weight 3/2 and level N (N N ) with respect to Γ 0 (N ) which is an eigenform of the Hecke operators T p 2 and is mapped to f under the Shimura map, can be constructed explicitly. In general case one can show, using [Ko1], that such F exists for E with odd conductor. We have the following result of Waldspurger (Corollary 2 of Theorem 1 in [Wa]). Corollary. If d and d are square-free natural numbers prime to N such that dd p N Q p/q 2 p, and if a(d) resp. a(d ) are d-th resp. d -th Fourier coefficients of F, then (5) dl(ed, 1) a(d) 2 = d L(E d, 1) a(d ) 2. Fix d 0 such that L(E 0, 1) a(d 0 ) 0. Take d such that dd 0 p N Q p/q 2 p. Then Corollary combined with the Birch and Swinnerton-Dyer conjecture implies (6) a(d) X(E d ) c p. p 6Nd The work of Goldfeld and Viola [GV] implies (conjectural) average value of X(E d ) in the rank zero case. For example, take E = X 0 (11). Then for d = p, primes satisfying p 3(mod 4), we obtain (7) the average value of X(E d ) A E d 1/2,

6 where A E is an explicit constant depending only on E. Kohnen and Zagier presented in [KZ],[Ko2] elementary proof of a version of Waldspurger s theorem for the case of modular forms on congruence subgroups. Their result gives explicitly the constant of proportionality between a(d) 2 s and central critical values of the quadratic twists of modular L-series. This allows, in particular, to deduce some results about the distribution of L(E d, 1) and partially confirm the above conjecture of Goldfeld and Viola. The average value of L(E d, 1) is established in the work of Murty and Murty [MuM], and a good error term is derived in a paper of Iwaniec [Iw]. Examples Notations. If q := e 2πiz, η(z) = q 1 24 n=1 (1 qn ), Θ(z) = n Z qn2, Θ t (z) = Θ(tz) = n Z qtn2. χ t := ( t ) shall denote Kronecker s character for Q( t). Example 1. [Tu] Let E : y 2 = x 3 x; let Ω = Ω E = denote the real period. For any square-free integer d consider the quadratic twist E d : y 2 = x 3 d 2 x. Let f i (z) := η(8z)η(16z)θ(2 i z) (i = 1, 2). It turns out that f 1 (z) = a 1 (n)q n S 3/2 (128, χ 0 ) and f 2 (z) = a 2 (n)q n S 3/2 (128, χ 2 ) are mapped to Φ(z) = η 2 (4z)η 2 (8z) (the cusp form associated to E) by the Shimura correspondence. If d 1 is an odd square-free integer, then Tunnell [Tu] proved that (8) L(E id, 1) = 2i 1 a i (d) 2 Ω 4 2 i 1 d. Therefore assuming the Birch and Swinnerton-Dyer conjecture, E id has Q-rank zero iff a i (d) 0. In addition, if a i (d) 0, then (9) X(E id ) = ( ) 2 ai (d), τ(d) where τ(d) denotes the number of divisors of d. One can check that η(8z)η(16z) = (Θ(z) Θ(4z))(Θ(32z) 1 2 Θ(8z)). Hence and f 1 (z) = x,y,z Z f 2 (z) = x,y,z Z q 2x2 +y 2 +32z q 4x2 +y 2 +32z x,y,z Z q 2x2 +y 2 +8z 2 q 4x2 +y 2 +8z 2. x,y,z Z

7 Consequently, for odd d, a i (d) = {(x, y, z) Z 3 : d = 2ix 2 +y 2 +32z 2 } 1 2 {(x, y, z) Z3 : d = 2ix 2 +y 2 +8z 2 }. Assuming the Birch and Swinnerton-Dyer conjecture, if E d has rank zero, then Ono [On] proves that X(E d ) is a simple explicit finite linear combination of X(Ed ), where 1 d d. Example 2. [Fr1],[Fr2],[Fe] Let E : y 2 = x 3 1. For any square-free positive integer d 1(mod 4, (d, 6) = 1, consider the quadratic twist E d : y 2 = x 3 d 3. Let a(d) denote the d-th Fourier coefficient of η 2 (12z)Θ(z). Frey proves the following result (10) L(E d, 1) = 0 if d 3(mod 4) a(d) 2 L(E,1) ( a(d) a(13) ( a(d) a(5) ( a(d) a(17) d if d 1(mod 24) ) 2 13 d L(E, 1) if d 13(mod 24) ) 2 5 dl(e, 1) if d 5(mod 24) ) 2 17 d L(E, 1) if d 17(mod 24) One can show that a(d) = 1 2 ( 1) n, where the sum is taken over all m, n, k Z satisfying m 2 + n 2 + k 2 = d, 3 m, 3 n and 2 m + n. Let l = l 1 + [ l ], where l i = {p d : p i(mod 3)}. Assuming the Birch and Swinnerton-Dyer conjecture, we obtain ( ) 2 a(d) (11) X(E d ) =, if a(d) 0. Example 3. [Le] Let E : y 2 = x 3 +21x x be the elliptic curve with complex multiplication by the ring of integers in Q( 7). Let Ω be the real period of E; let Ω d be the real period of E d. Let φ be the cusp form associated to E. Then Φ = φ χ 2 S 2 ( , χ 1 ). Let g = g g 6 and h = h h 6, where 2 l g 1 = [q (14m+1)2 +(14n) 2 q (14m+7)2 +(14n+6) 2 ] g 2 = [q (14m+3)2 +(14n) 2 q (14m+7)2 +(14n+4) 2 ]

8 g 3 = [q (14m+5)2 +(14n) 2 q (14m+7)2 +(14n+2) 2 ] g 4 = [q (14m+1)2 +(14n+2) 2 q (14m+5)2 +(14n+6) 2 ] g 5 = [q (14m+3)2 +(14n+6) 2 q (14m+1)2 +(14n+4) 2 ] g 6 = [q (14m+5)2 +(14n+4) 2 q (14m+3)2 +(14n+2) 2 ] h 1 = [q (7m+1)2 +(2(7n) 2 q (7m)2 +2(7n+2) 2 ] h 2 = [q (7m+3)2 +2(7n) 2 q (7m)2 +2(7n+1) 2 ] h 3 = [q 7m+1)2 +2(7n+1) 2 q (7m+3)2 +2(7n+2) 2 ] h 4 = [q (7m+2)2 +2(7n) 2 q (7m)2 +2(7n+3) 2 ] h 5 = [q (7m+2)2 +2(7n+2) 2 q (7m+1)2 +2(7n+3) 2 ] h 6 = [q (7m+3)2 +2(7n+3) 2 q (7m+2)2 +2(7n+1) 2 ], and all sums are taken over all m, n Z. One checks that gθ 28 S 3/2 (784, χ 7 ), hθ 14 S 3/2 (392, χ 7 ), hθ 7 S 3/2 (392, χ 14 ). Let χ 2 be the Dirichlet character of conductor 16 which is defined by χ 2 (3) = i = χ 2 (5). It turns out that the cusp form φ χ 2 is the image under the Shimura correspondence of each of the forms gθ 28 χ 2, hθ 14 χ 2, and hθ 7 χ 2. Let gθ 28 = a n q n and hθ 14 = b n q n. Let d be a positive square-free integer, prime to 7. Lehman proves the following results: (1) If d 1(mod 4), then { 1 2 (12) L(E d, 1) = Ω da 2 d if (d/7) = 1 Ω d a 2 d if (d/7) = 1 (2) If d 2, 3(mod 4), then { 2Ωd b 2 d if (d/7) = 1 (13) L(E d, 1) = 4Ω d b 2 d if (d/7) = 1 Therefore, assuming the Birch and Swinnerton-Dyer conjecture, we obtain (for d positive, square-free and prime to 7): if d 1(mod 4) and a d 0, then X(E d ) = a2 d 4 l,

9 if d 2, 3(mod 4) and b d 0, then X(E d ) = b2 d 4 l. Here l = l(d) is defined as follows. Let l 1 be the number of odd prime divisors p d such that (p/7) = 1, and let l 2 be the number of prime divisors p d such that (p/7) = 1. Define l to be l l 2 if l 2 is even, and to be l (l 2 1) if l 2 is odd. Example 4. [Ne] Let E : x 3 + y 3 = 1 denote the Fermat s curve; let Ω = Γ(1/3)3 2π denote the real period. L-series of E equals L(E, s) = α(nα) s, α 1(3) where α runs through all elements of the Eisenstein ring Z[ρ] congruent to 1 modulo 3. It is well known that L(E, s) = L(f, s) for f S 2 (Γ 0 (27), χ 0 ). Let F (z) = η(6z)η(18z)θ(3z) and F + (z) = η(6z)η(18z)θ(9z). Then F S 3/2 (108, χ 0 ) and F + S 3/2 (108, χ 3 ) are mapped, under Shimura s correspondence, to f. Let F ± (z) = n=1 c(±n)qn. Put { e(n), for n < 0 c (n) = c(n) e( 3n), for n > 0, where { 1, for n 5(mod 8) e(n) = 1/3, for n 5(mod 8). Let d be a square-free integer prime to 3. Nekovár proves, that { 1, for d < 0 (14) L(E d, 1) = Ω 1/2 c (d) 2 3, for d > 0, where is the conductor of ( ) d. Therefore, assuming the Birch and Swinnerton- Dyer conjecture, if c(d) 0, then ( t(d)c ) 2 (d) (15) X(E d ) =, where t(d) = E d (Q) tors, and 2 β(d) β(d) = #{p d : p = x y 2 } + 1 #{p d : p 2(mod 3)}. 2 Example 5. Let E : y 2 +y = x 3 x be the curve of conductor 11. The corresponding cusp form f equals qη(z) 2 η(11z) 2. The weight 3/2 form F = n=1 a(n)qn = 1 2 (θ 1(q) θ 2 (q)) is mapped to f by the Shimura correspondence, where θ 1 (q) = q x2 +11y 2 +11z 2,

10 (summation over (x, y, z) Z 3 satisfying x y(mod 2)) and θ 2 (q) = q (x2 +11y 2 +33z 2 )/3. (summation over (x, y, z) Z 3 satisfying x y(mod 3) and y z(mod 2)) One proves, that (16) L(E d, 1) = Ka( d ) d, d < 0, d 2, 6, 7, 8, 10(mod 11) where K = It turns out that K = 3Ω, where Ω is the period of the elliptic curve y 2 + y = x 3 3x 5. Therefore, assuming the Birch and Swinnerton-Dyer conjecture, if a( d ) 0, then (17) X(E d ) = E d(q) tors 2 a( d ) 2. c(e d ) Remarks concerning representations of integers by ternary quadratic forms Let r(n; a, b, c) denote the number of representations on n by diagonal ternary quadratic form ax 2 + by 2 + cz 2. ( Let) L(s, n) denote the Dirichlet L-series attached to the quadratic character. Let h( n) denote the class number of the n imaginary quadratic field Q( n). According to Gauß, we have r(n; 1, 1, 1) = 12h( n), for positive n 1(mod 4). By using Lomadze s method [Lo] one can prove, for example, the following results (i) Assume n is odd, square-free positive integer. Then (18) r(n; 1, 2, 8) = 8 n L(1, n); π (ii) For any square-free n 1, 3(mod 8), such that L(E n, 1) 0, (here E : y 2 = x 3 x) we have (19) r(n; 1, 2, 32) 4h( n) as n tends to infinity. It is a classical problem to find an asymptotic formula for the number of integral points (in the region) on the ellipsoid q(x 1,..., x k ) = n as n, where q is a positive definite integral quadratic form. It is well known, that (20) r(n; 1, 1, 1) = 4 3 πx O(x). n x

11 The general asymptotic formula (including positive ternary quadratic forms) is discussed in [DS-P]. 2. An (explicit) approach to Conjecture 2 Fix a prime r of the type 4k + 3. Let E(r, n; p, q) denote the elliptic curve y 2 = x(x + p)(x q), with p + q = 4r 2n+1, where p is an odd prime and q is a positive square-free integer. (Chen [Ch] showed that every sufficiently large even number is the sum of a prime and a natural number which has at most two prime factors.) Standard descent methods lead to the following result. Proposition 0. Assume p < q are odd primes, with p 5(mod 8). Then E(r, n; p, q) have Q-rank zero. Proposition 0 strongly supports the following expectation. Conjecture A. Fix a prime r of the type 4k+3. For any ɛ > 0 there exist infinitely many n N such that E(r, n; p, q) has Q-rank zero, where 4r 2n+1 = p + q with p a prime and q a positive square-free integer satisfying p q ɛ. Proposition 1. Assume p < q, with q having at most two prime factors. Then we have π (i) c (E(r, n; p, q)) = r n+1/2 AGM(1, q/(p + q)) and (ii) where and (iii ) c 2 = c fin (E(r, n; p, q)) = 2c 2 c q c r, { 2 if p 1(mod 4) 4 if p 3(mod 4),, c r = { ( 2(2n + 1) if p ) r = 1 4 if ( p r { 2 if q is a prime c q = 4 if q is a product of two primes ; The conductor is given by the formula where N(E(r, n; p, q)) = 2 f 2 pr rad(q), { 3 if p 1(mod 4) f 2 = 4 if p 3(mod 4). Proof. Use [Ni], Propositions 2.1, 3.1 and 3.2. ) = 1,

12 Proposition 2. Fix a prime r of the type 4k+3. Assume the Birch and Swinnerton -Dyer conjecture holds true for the family E(r; n, p, q). Then X(E(r; n, p, q)) N(E(r; n, p, q)) 1/2+ɛ. Proof. It is easy to see that the Szpiro conjecture holds true for our family (combine (E(r; n, p, q)) = 2 8 p 2 q 2 r 2 with Proposition 1(iii)). Now use the main result from [GS]. Conjecture B. For a fixed prime r of the type 4k + 3 and ɛ > 0 there exist an infinite sequence {(n, p, q)} satisfying Conjecture A and L(E(r, n; p, q), 1) N(E(r, n; p, q)) ɛ. Proof of the conjecture 2. Assume the conjecture of Birch and Swinnerton- Dyer and hypothesis B hold true. Then we have, using proposition 1, X(E(r, n; p, q)) c 1 fin rn+(1/2) L(E(r, n; p, q), 1) N(E(r, n; p, q)) (1/2) ɛ. 3. Examples of elliptic curves with large X(E) In this section we compute the analytic order of X(E), i.e., the quantity (21) X(E) = L(E, 1) E(Q) tors 2, c (E)c fin (E) for certain special curves of rank zero. We use the following approximation of L(E, 1) (within an error of size 10 k ) [Co]: (22) S m = 2 m n=1 a n n e 2πn N, with m N 2π (2 log 2 + k log 10 log(1 exp( 2π/ N)). Consider the family E(n, p) : y 2 = x(x + p)(x + p 4 3 2n+1 ),

13 with (n, p) N (Z \ {0}). Any member of the family admits three isogenous (over Q) curves E i (n, p) (i = 2, 3, 4): and E 2 (n, p) : y 2 = x 3 + 4(2 3 2n+1 p)x n+2 x, E 3 (n, p) : y 2 = x 3 + 2(4 3 2n+1 + p)x 2 + (4 3 2n+1 p) 2 x, E 4 (n, p) : y 2 = x 3 + 2(p 8 3 2n+1 )x 2 + p 2 x. Isogenous curves do have the same L-series and ranks but may have different torsion subgroups, periods, Tamagawa numbers and orders of X(E). In our situation (of 2-isogenies) the order of X(E) can only change by a power of 2.

14 3.1. Tables. (n, p) N(n, p) X X 2 X 3 X 4 (11, 336) (11, 301) (11, 865) (11, 489) (11, 163) (12, 63) (12.24) (12, 22) (12, 262) (12, 605) (12, 56) (12, 934) (12, 694) (12, 382) (12, 257) (12, 466) (13, 136) (13, 69) (13, 60) (13, 96) (13, 876) (13, 672) (13, 544) (13, 928) (13, 73) (13, 42) (13, 160) (13, 3) (13, 66) (13, 125) (13, 17) (13, 5) (13, 708) Table 1. Elliptic curves E(n, p) (11 n 13; 1000 p 1000) with max 1 i 4 X i 10 6.

15 (n, p) N(n, p) X X 2 X 3 X 4 (14, 212) (14, 3) (14, 96) (14, 948) (14, 528) (14, 800) (14, 268) (14, 281) (14, 652) (14, 12) (14, 240) (14, 100) (14, 596) (14, 11) (14, 33) (14, 672) (14, 12) (15, 852) (15, 12) (15, 1) (15, 80) (15, 84) (15, 88) (15, 172) (15, 96) (15, 48) (15, 12) (15, 240) (15, 212) (15, 375) (15, 60) (15, 248) (15, 6) (15, 1) (15, 116) Table 2. Examples of elliptic curves E(n, p) (n = 14, 15, 1000 p 1000)

16 with max 1 i 4 X i (16, 8) (16, 92) (16, 588) (16, 624) (16, 408) (16, 300) (16, 12) (16, 96) (16, 592) (16, 48) (16, 32) (16, 268) (16, 472) (16, 33) (17, 404) (17, 68) (19, 32) Table 3. Examples of elliptic curves E(n, p) (16 n 19; 1000 p 1000) with max 1 i 4 X i Values of the Goldfeld-Szpiro ratio. Let GS(E) denote the ratio X(E) / N(E). In [dw] and [Ni] together there are 58 examples of elliptic curves of conductor less than with GS(E) greater than 1 (the largest value being ). The largest values of GS(E) we observed for our curves are as follows:

17 E X(E) GS(E) E 2 (9, 544) E 2,4 (16, 48) E 2 (10, 204) E 2,4 (19, 32) E 2 (11, 160) E 4 (17, 404) E 4 (16, 33) E 2,4 (16, 472) E 2,4 (17, 68) E 2,4 (16, 32) E 2,4 (16, 268) Table 4. Elliptic curves E i (n, p) (9 n 19, 1000 p 1000; 1 i 4) with the largest X(E). Notation E i,j (n, p) means that the given values of X(E) and GS(E) are shared by curves the isogeneous curves E i (n, p) and E j (n, p) Small non-zero and large values of L(E, 1). Let S k (N) denote the linear space of holomorphic cusp forms of weight k and level N. Let H + k (N) S k(n) be the subset of newforms with ɛ = 1. Iwaniec and Sarnak [IS] proved that the percentage of f s in H + k (N) for which L(f, k 2 ) (log N) 2 is at least 50 as N, with technical assumption φ(n) N (N squarefree). Note that L(E(7, p), 1) (log N(7, p)) 2 for any 1000 p 1000) in a rank zero case. It may not be true in general: L(E(8, 131), 1) = < = (log N(8, 131)) 2, L(E(9, 160), 1) = < = (log N(9, 160)) 2, L(E(10, 142), 1) = < = (log N(10, 142)) 2, L(E(11, 168), 1) = < = (log N(11, 168)) 2, L(E(12, 800), 1) = < = (log N(12, 800)) 2.

18 Here are some examples of large and small values of L(E, 1): E L(E, 1) E(11, 733) E(13, 160) E(12, 466) E(7, 433) E(10, 687) E(9, 767) E(9, 93) E(10, 837) Table 5. Elliptic curves E(n, p) (n 19, 1000 p 1000) with the largest values of L(E, 1) known to us. E L(E, 1) E(12, 800) E(10, 142) E(11, 168) E(14, 672) E(9, 160) E(10, 534) E(10, 408) Table 6. Elliptic curves E(n, p) (n 19, 1000 p 1000) with the smallest positive values of L(E, 1) known to us. Note that (23) L(E(10, 408)) L(E(10, 534)) = This is the smallest gap between the values of L(E, 1) of two elliptic curves which is known to us. The analytic orders of the Tate-Shafarevich group are 2, 4, 1, 4 for the isogeneous curves E i (10, 408), and 2, 8, 8, 8 for E i (10, 534) where i as usual takes values from 1 to 4.

19 References [BCFT] C. Breuil, B. Conrad, F. Diamond and R. Taylor, On the modularity of elliptic curves over Q: wild 3-adic exercises, J. Amer. Math. Soc. 14 (2001), [Ca] J.W.S. Cassels, Arithmetic on curves of genus 1 (VI). The Tate-Safarevic group can be arbitrarily large, J. reine angew. Math. 214/215 (1964), [Ch] J.R. Chen, On the representation of a large even number as the sum of a prime and the product of at most two primes, Sc. Sinica 16 (1973), [Co] H. Cohen, A course in Computational Algebraic Number Theory. Graduate Texts in Math. 138 Springer-Verlag (1993) [Cr] J.E. Cremona, Algorithms for modular elliptic curves. Cambridge University Press 1997; mwrank programs [dw] B.M.M. de Weger, A + B = C and big X s, Quart. J. Math. 49 (1998), [DS-P] W. Duke and R. Schulze-Pillot, Representation of integers by positive ternary quadratic forms and equidistribution of lattice points on ellipsoids, Invent. Math. 99 (1990), [Fe] K. Feng, On the rank and the BSD conjecture of elliptic curves E D : y 2 = x 3 D 3. In: Algebraic Geometry and Algebraic Number Theory (F. Ke-Quiu, Li Ke- Zheng, eds.), World Scientific, Singapore 1992, Naukai Series in Pure, Applied Math. and Theoretical Physics, vol. 3 [Fr1] G. Frey, Der Rang der Lösungen von y 2 = x 3 ± p 3 über Q, Manuscr. Math. 48 (1984), [Fr2] G. Frey, A relation between the value of the L-series of the curve y 2 = x 3 k 3 in s = 1 and its Selmer group, Arch. Math. 45 (1985), [GS] D. Goldfeld and L. Szpiro, Bounds for the order of the Tate-Shafarevich group, Compos. Math. 97 (1995), [GV] D. Goldfeld and C. Viola, Mean values of L-functions associated to elliptic, Fermat and other curves at the center of the critical strip, J. Number Theory 11 (1979), [Iw] H. Iwaniec, On the order of vanishing of modular L-functions at the critical point. Séminaire de Theorie des Nombres, Bordeaux 2 (1990), [IS] H. Iwaniec and P. Sarnak, The non-vanishing of central values of automorphic L-functions and Landau-Siegel zeros, Israel J. Math. 120 (2000), [Ko1] W. Kohnen, A remark on the Shimura correspondence, Glasgow Math. J. 30 (1988), [Ko2] W. Kohnen, Fourier coefficients of modular forms of half-integral weight, Math. Ann. 271 (1985), [KZ] W. Kohnen and D. Zagier, Values of L-series of modular forms at the center of the critical strip, Invent. Math. 64 (1981), [Ko] V.A. Kolyvagin, Finitness of E(Q) and X(E) for a subclass of Weil curves, Izv. Akad. Nauk SSSR 52 (1988),

20 [Kr] K. Kramer, A family of semistable elliptic curves with large Tate-Shafarevich groups, Proceed. AMS 89 (1983), [Le] J.L. Lehman, Rational points on elliptic curves with complex multiplication by the ring of integers in Q( 7), J. Number Theory 27 (1987), [Lo] G.A. Lomadse, On representations of numbers by positive ternary diagonal quadratic forms, I,II, Acta Arithm. 19 (1971), , [MM] L. Mai and M.R. Murty, A note on quadratic twists of an elliptic curve. In: Elliptic Curves and Related Topics. CRM Proceed. and Lecture Notes 4 (1994), [MuM] K. Murty and R. Murty, Mean values of derivatives of modular L-series, Ann. Math. 133 (1991), [Ne] J. Nekovár, Class numbers of quadratic fields and Shimura s correspondence, Math. Ann. 287 (1990), [Ni] A. Nitaj, Invariants des courbes de Frey-Hellegouarch et grands groupes de Tate- Shafarevich, Acta Arithm. 93 (2000), [On] K. Ono, Tate-Shafarevich groups of the congruent number elliptic curves, Acta Arithmetica 81 (1997), [Ra] C.S. Rajan, On the size of the Shafarevich-Tate group of elliptic curves over function fields, Compos. Math. 105 (1997), [Ro] H.E. Rose, On some elliptic curves with large Sha, Experimental Math. 9 (2000), [Ru] K. Rubin, Tate-Shafarevich groups of elliptic curves with complex multiplication. Adv. Studies in Pure Math. 17 (1989), [Sh] G. Shimura, On modular forms of half-integral weight, Ann. Math. 97 (1973), [Si] J.H. Silverman, The arithmetic of elliptic curves. Graduate Texts in Math. 106, Springer, New York 1986 [Tu] J. Tunnell, A classical diophantine problem and modular forms of weight 3/2, Invent. Math. 72 (1983), [Wa] J.L. Waldspurger, Sur les coefficients de Fourier des formes modulaires de poids demi-entiers, J. Math. Pure et Appl. 60 (1981), [ZK] D. Zagier and G. Kramarz, Numerical investigations related to the L-series of certain elliptic curves, Journal of the Indian Math. Soc. 52 (1987), A. Dabrowski: University of Szczecin, Institute of Mathematics, ul. Wielkopolska 15, Szczecin, Poland; dabrowsk@sus.univ.szczecin.pl and M. Wodzicki: Department of Mathematics, Evans Hall, University of California, Berkeley, CA USA; wodzicki@math.berkeley.edu