# Which primes are sums of two cubes? Fernando Rodriguez Villegas Don Zagier

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1 Which primes are sums of two cubes? Fernando Rodriguez Villegas Don Zagier Department of Mathematics Princeton University Princeton, NJ USA Max-Planck-Institut für Mathematik Gottfried-Claren-Str Bonn GERMANY MP1/95-61

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3 CanaJia.n Ma.thematical Sod.,ty Conf.,r.,nCfl Proc.,.,Jin,,' Volnme 00, 0000 Which priules are sums of two cubes? FERNANDO RODRIGUEZ VILLEGAS AND DON ZAGIER ABSTRACT. Let. Sp Le t.be "unkllown" part. or t.he L-Beriea of t.hc ellipt.ic curve x 3 + Sl 3 = P at. 6 = 1, 60 timt. conjecturally Sp =0 if p is a surn or t.wo dist.inct. cules and equals t.he order of a Tat.e-Shararevich group otherwillc. Thc quelltion or tbc title ia thcn to detel'llline whether Sp =O. For p ~ 1 (mod D) the ßllswer depellds only on p (mod 9) aud is weil known. We give three different criterin for the relnnining CMf:. Dur first formula represents Sp as the trllee of a certaiu algelraic number (the value of a specific modular functioll nt a CM point), the sccond representl!l Sp as the square or the trace of a t1i1uilar llumler, ami the third shows tllat Sp valli!lh~ if an ollly if plh(p-l)/3(o). where {!n(t)}n2: 0 ia a licquence or polynomials sntisfyins a simple recursion relation. 1. Illtroduetioll nnd results A classical problem of Diophantine analysis is to recoguize which numbers N are the sum of two rational cubes. For instance, 1 is not so represented (Fermat, Euler), whereas every prime af the form Uk -1 conjecturally is (Sylvester). If we assurne the Birch-Swinnerton-Dyer conjecture, then the question is equivalent to the vallishing at s = 1 of the L-series of the elliptic curve EN : X 3 + y3 = N. \Ve consider only the case w lien N = P is prime. 1f]J::;:: 2, 3 or 5 (mod 9), then L(E p, l) i:- 0, so p should not be a stirn of two cubes (except for = 2). This ia in fact t,rue and follows eit.ljer from a 3-desceut argument (given already in the 19th century by Sylvester, L\lcas and Pepill) or from the Coates-\Viles theorem. If p ::::: 4, 7 or 8 (mod 9) then t.he funetioll nl equation farces L (E p, 1) to vanish, so ]J should be a sum of two cubes, and for the first two of these three cases a proof of this has been annoullced by Noam Elkies. From now on we restriet to the remaining case p == 1 (mod 9). Here the L-series may or may not vanish. The question is numerically decidable for ally given prime, since 1991 Mathematic. Subject CfasJlijicativn. llfg7, 11085, llg40. ~oooo American M;o.thematlcal soc!.,ty /00 S1.oo + S.25 pclr pa"e

4 2 F. RODRIGUEZ VILLEGAS D. ZAGIER where Sp is known to be an int.eger (conjecturally equal to 0 if Ep(Q) i= {O} and to the order of the Tnte-Shafarevich grollp of E p otherwise), but a table of these numbers, such as the aue for }J < 2000 given at the end of this section, suggests no simple pattern. In t.his paper we will give three formulas for Sp and hence three conjectural answers to the question of the title. Most of the proofs rely on ideas similar to those in [5] alld [7] and have been omitted or only sketched, but, to quote from Sylvester's paper on the same subject [9], ui trust my readers will do me jnstice to believe that I am in possession of a strict demonstration of all that has been advanced withont proof." \Ve do inc1ude a few short proofs which use ideas different from those in the two papers cited. First auswer: \Ve nssoeiate to the prime p = 9k + 1 an algebraic number Op of degree 18k, defined as follows {IP 8(pE) 0' P (0), where 8(z) = t Lm,nEZ e2;t1"j(rn:l+mn+n:l)~and J = -!(1 +1/3.;=3). (The value of 8(J), by the way, is -3r(~)3/(21l')2.) Then Sp = Tr(op), A more detailed statement is given in Theo where Tr denotes alsolllte trace. rem 1 below. Secoud fillswcr: This has the same form, hut gives the square root of Sp as a trace (tlms proving, in particular, that Sp is a square, as expected). Of course Sp has two square roots whell nollzero. It turns out that they are canonically indexed by the two primes above p in J( =~. Let P be one of these peimes. Choosing P is equivalent, via P = (p, -r 41 - ), to choosing an integer r (mod 2p) with r 2 == -3 (mod 4p). Let Zo =(r + ~)/2 alld set ß {!P ry(pzo) p =.J±12 71(Zo/P), where ry(z) = c;tl"i~/l'2 [J~=l (1- e2;t1"in~) is Dedekind's eta funetiall, the aign ia +1 if p == 3 (mod 4) and -1 if p == 1 (mod 4), and the correct choice of the 6 th root of p will be explained Iater. Then ßp is algebraic of degree 6k over Q and we have Sp = [Tr(ßp )]2, and Tr(ßp) = -Tr(ßp). A more detailed statement is given in Theorem 2 below. Third Rllswer: Define polynomials In (t) by /o(t) =I, /I(t) =t 2 and

5 3 and let A k =f3d0). (It is trivial timt In (0) =0 if 3 does not divide n.) Then.e..=..!.!.!. ]) Sp == (-:~) 3 (-3-)! A 2 (p-l)/9 (mod p). This determiuet:l Sp since ISp I < p/2 ns we shall prove in 5. have L(Ep, 1) = 0 <=> pja:l(p-l)/9. In particular, we Thiru auswer (vru'iallt): In fact, the Ak 's of the third answer (which are normalized central values of certain Hecke L-series) are always squares, and we ean get their square roots as foliows. Define polynomials 9n (t) E Q[t] by 90(t) =1, 91 (t) = ~{l and 9n+dt) =(1 - t 3 ) 9~(t) - (271 + ~) t 2 9n(t} - n(n -!} t 9n-di} (n 2: I), and let Bk =93k(0). (Just a.s with In, 9n(0) =0 if 3 does not divide n.) Then A:!k = BZ I for au k 2: 0 and in particular and L(Ep 11) = 0 <==> pib(p-l)/91 fc rq e=l.2. P - 1 V Sp == ±(v -3)~-- (~)! B(p-l)/'J (mod p). \Ve conjecture tl1at this formula is always trne with the + sign if we interpret VS; and J=3 (mod ]) as Tr(ß'P) and r (mod])), respectively (wi th r, P, ßp ab in the second answer), and have checked this for p < We give a short table of vailles of the f1ulllbers Bk. k Bk This also gives the first values of the llumbers A:Z k =Br The odd-index values A2Ic+l, which ure not lieeded for Ollr Uthird answer," are also the squares of the constant terms of certain polynomials C3k+l (t) satisfying a recursion (cf. Theorem 3 below, wllcre a short. t.able is given).

6 4. F. RODRIGUEZ VILLEGAS D. ZAGIER The nllmbers Ak ami Bk have a different description in terms of generating fllnctions: and 1 t 1. ) ~ Ak r ( F 3' 3; 'jjx = L.J (3k)! h::o.j.r t _ Bk-T (I-x) F(3,'j'3'x) - L (3k)! 2 ' h::o where F =2Ft is Gallss'g hypergeomet.ric function and T :t :t F('j, 3' 3' x) = X I l:t. Pb, 3; 3; x)3 00 ()k That the coefficiellts in the first of these hypergeometrie expansions are the squares of the eoeffieiellts in the seeolld is a surprising and beautiful identity, quite apart from the colluection with L-serics. There are similar resltlts for the llumber SpJ correspollding to the elliptie eurve EpJ : x 3 + rr = p2, nnmely for a eertain root of unity (, and (pt fl(pzo) IP = ~ 1](-zolp), However in the rest of this note we will stay with Sp. Using auy of the "answers" given in this section, we ean easily ealculate Sp numerically. \Ve give a f.able for ]J == 1 (mod 0), p < Iu this range, the value of Sp is always 0, 1 or 4, as foliows: Sp =0: 19,37,127,163,271,379,307,433,523,631,820,883,919, 937,1063,1171,1459,1531,1567,1621,1657,1801 Sp = 1: 73,109,181,199,307,487,541,577,613,757,811,1009, Sp =4: 1117,1153,1279,1297,1423,1549,1693, ,001,1747,1873,1000 A eomplete table of SN for N < 1000 is given in [10]. 2. The fol'ullllns for Sp Let 0 =Z[w] C J( = Q(w) = Q(J=3), where w 2 +w + 1 = 0, v'=3 =~ + I, and embed [( ill C viaw H e:t7fi/3. The elliptieeurve E : x 3 +!l = 1 has eomplex multiplication by O. Hs L-series is L(l/J, s), where l/j is the Hecke eharacter of!( satisfying 1,b((O')) =0', for all 0' E 0, 0' == 1 (mod 3). Let p =: 1 (mod 3) be n prime. \Ve considcr the grollps ~ = (OlpO)*I(ZlpZ)* aud ~u = ~/O*,

7 5 whieh are cyclic of orders }) - 1 and (p - 1)/3 respeetively. \Ve let H3p (Hp) be the ring cluss field modulo 3p (modulo p) of K and identify A (Ao) with Gal(H 3P / J() (Gal(H p / K)) via t,he Artin map. Let X : ß.-r (w) be the cubic charact.er defined by ( _) (p-l)/3 X(u) == ~ (mod })) (u E 0, (u,p) = 1). Then L (t/jx, s) and L (t/jx'l, s) are the L-series of the curves E p and Ep'J, respectively. The sigll in their fulletiollal equation is +1 if and ouly if p == 1 (mod 9) or, equivalent.ly, if anel only if X fa.ctors tljrough A o. The formulas for Sp that we will obtaill involve linear eombinations of values of certain modular forms Oll CM POillts in the tipper-half plane eorresponding to A and A o. \Ve need to illtroduce the following notation in order to da this explieitly. 1) Let «5 = (-1-1/3.;=3")/2 E Kn'J-l, where 1l denotes the eomplex upper-half plane. As usllalt] will denote Dedekilld '8 eta fullct.ioil. As a set of reprcsentatives for A we take the numbers 1 ajl(i «5 - k, wit.h k E Z/pZ such timt «5 - k is prime to p (henee excllldillg two vallies and bringing the total to p - 1). Let Pp : A.-r C be giveil by }16(p«5) 8(o-k) Jlp{l) =~' Jlp{eS - k) = e(~) (k E Z/pZ, (es - k,p) = 1), with e(z) as in 1. The fl1uetioll I'p is weil defined and its values are conjugate algebraic integers in H 3p / /{. Note that pi E H 3p and that ps E Hp if and ollly if p == 1 (mod 9). \Ve define ~p = p-i 1 l p (l) = pi 0(pd)/0(J). It belohgs to H 3p and its conjugatea over K are {p-ix(u)llp(u) : U E A}. THEOREM 1. Let p == 1 (mac! U) be prime. Trl/31'IK(n-p) =p-t L: X(U)/'p(U) = 27 Sp, ue~ I'Vit./, the uoove notatioh we Iwve where Sp E Z is tije Bin:h-SwiulIertvu-Dyer wlmber defi"ecl in the introduclion. 2) \Ve choose w a primit.ive eube of unit.y modulo p; this corresponds, via P = (w - W, p), to choosillg a pri(ite P of 0 above p. As a set of representatives for Ao we take the numbers 1 ami w-k, where k runs over Z/pZ\{w, -l-w, 0, -1})/,..", and where,..", dcfined by k,.." -I/(k+1) /k I corresponds to orbits under muitipiicatioll by w. (The vallies of w - k for k = 0 or -1 represent the same class as 1 in Ao). Let Ap : Au --+ C oe givell by ( W-k) k w-k Ap{W - k) = ---::p- (:lr 1}(-p-)/7}(W) (k E Z/pZ,

8 6 F. RODRIGUEZ VILLEGAS D. ZAGIER ( W-k) (W-k). where -p = -p- is t.he quadratic symbol at P, (24 =e 2,n/"l4, vp > 0, and!p = 1, i if p == 1,3 (mod 4). The function >"1' is weil defined aod ita values are conjugate units in an abelian extension of [(, which is quadratic over Hp. Let r be a solutioll of,.2 == -3 (mod 4})) such that r == 2w + 1 (mod p), and let Zo = (r + ~)/2. \Ve defiue pp = (~) (:;t(r+i)/"l'l(zo/p)/"i(w), It is not E 0 generales P. Finally, we let fp = (p- i Pp 1,\1'(1), hard to check that p~ where ( E 0* is such that ( p~ == 1 (mod 2); it belongs to Hp and its conjugates over I< are {(p-ippix(u)'\p(u) : u E ßu}. THEOREM 2. Let p == 1 (mod 9)!Je prime. Witll tlle above notation we have TrI/pI K (~p) = p-i ppl L X( u) '\1'(u) = v!=3 R1', with R 1' E Z. The number R 1',mtüjies ue6 u where Sp is t.he Bin:Ii-SwiulIert.oll.Dyer "umher defiued in tlie introduclion. Reularks. 1. Theorem 2 holcls in more generalit.y. For any character fjj of ß let R1'(fjJ) = L.p(u),\p(u) ue6 (a Lagrauge rp..sol vent). Thell for 4> of odd order, Rp (4)) 'J is essentially the algebraic part of L(1/J4>-2, 1). 2. It is pos,i;jible to define, in a similar way, illtegers RA associated to any cubefree ideal A of 0 slich that. R~ = SN is the Birch-Swillnerton-Dyer number of the elliptic curve x = N, where N = N(A). Olle might hope timt these numbers are t.he Fourier coelficiclit.s of a modular form of seme sort. Theorem 1 is proved by writ.illg the special value L(1/Jx, 1) as a linear combina.- tion of values of an Eisenstein series as in [4) und using the Shimura reciprocity law. One thell dedllces Theorelll 2 from a variallt of the fa.ctorization formula of [7) and a careful chasing of 24 th roets of unit.y. Theorems 1 and 2 are easily seen to be eqllivalcnt t.o t.he analogons stat.ements given in the introduction.

9 1 3. COllgrllenccs Our third answer to t.he question when Sp vanishes was based on a congruence between Sp (which ia, up to a factor, the value of a certaill L-function at s =1) and another number Ak which, as we will discuss in amoment, ia (again up to a factor) a special value of an L-fundion independent of p at 80me other value of s. Congruences of this sort go back to Cauchy, Kummer, aod Hurwitz. For example [1], the dass number h(-p) of the quadratic field Q(FPJ for a prime p > 3, p == 3 (mod 4) satisfies h(-p) == -2B(p+l)/'J (mod p), where (here only!) Rn denotes the u th Bernolllli number. Olle way to interpret this ia to say that the two Dirichlet series nnd which are congruent term by term Illodulo p, also have congruent values at 8 =O. Turning this fact into a heuristic argument, we would expect that (suitable algebraic versions of) the values at s = 0 of the Dirichlet series 1 L: X(cr),.,...,,--- aeo\{o} 1/J(o)N(a)1 ami 0' ~ 1 L: ( Ci" ) 1/J(a)N(a)1 ' aeo\{o} which are easily seen to equal L(VJ;'(, 1) ami L(t/J'.lk-l, k) respectively, with k-l = 2(p - 1)/3, ShOlild also be congruent mod1110 p (see 2 for notations). This is indeed the case, at least if]j =: 1 (mod 3), where it follows from the existence of a p-adic L-fuuction iuterpolatiljg special values of Hecke L-series due to Manin Vishik alld Katz. \Ve 1I0W lilake this precise. For k E N defille the algebraic part. of L(1/J'Jk - 1, k) to be L = ~\v ( 2rr )k-l (k - 1)1 L(.f,2k-l k) k 3J3~V n 'f' " where v =2 if k == 2 (mod 6) ami v =1 otherwise, alld n =f(l/3)3/(2tr/3) = is the fundamental real period of the ellipt.ic curve :r: = 1. Then using t!je formllias of [2] we find that.e=..!..!.!. p- 1 2 Sp == (-3) 3 (-3-)! L(2p+l)/3 (mod p). This correspollds to t.he "third allswer" of the intradllction because L 3n + 1 =An for ab u, as we will now discliss. 4. Rccllrsiolls By the methods of [7] olle call obtaiu forrnttlas for L(1/J2k-l, k), k E N, in terms of derivatives of modular forlns alld thell dedllce recursive forrnulas giving the algebraic parts Lk, aud similarly for t.heir square raots. A typical formula for the values is the idelltity L 3n + 1 = An just ment,ioned, but since the results

10 8 F. RODRIGUEZ VILLEGAS D. ZAGIER for the square roots are more iuterestillg and give more precise information, we sha11 state the fuil results ollly there (buf. see Hemark 2 after Theorem 4 below). The formulas far the square roals of Lk can be divided llaturally into three branches: (a) k == 1 (mod 6), (b) k == 2 (mad 6), and (c) k == 4 (mod 6). (For other values of k the fuilct.ional equatioll forces L(t/J2k-l, k) to be zero.) For each branch there is a formula for.;r;in terms ofa higher non-holomorphic derivative of a fixed half-integral weight modular form at a fixed CM point (e.g. for J L 6n + 1 it is the n t h non-holomorphic derivative of 7]( z) al the point z = w), and this in turn leads to the following descript.ion of the square roots as the constant terms of a recursively defined sequellce of polynomials. THEOREM 3. Lel un(t), un(l), cn(l)!je the poly"omiuls defined by lhe recutslons an+dt) bn+ 1(t) Cn +l(t) = -(1-8t 3 ) u~.(t.) - (16n + 3) (1 un(t) - 4n(2u - 1) t On-dt), (1-8t 3 ) u~(t) + (I6n + 9) (J Un(t) - 411(2n + 1) t bn- 1 (t), = (1-8l 3 ) c~(t) + (Hin + 9) t J cu(t) - 4n(2n + 1) t Cn-l(t) fot n 2:: I, will, initial coihjitiuhs 213 Go(t) =1, udt) =-3l I uo(t) =I, udt) =9t I co(t) =t, cdt) =1 + t. Then for ull n E Z~u wllile Um (0) =Um (0) = Cm+l (0) =0 for fil ~ 0 (mod 3). \Ve give a short table. Note thai, ll311 (0) is the B n of the iutroduetion. 11 (13,,(0) U311(0) c3n+do) G :12U RCluurk. The constant terms ofthe polynamials *n(t) (* = a, bor c) satisfy the congruences *n+(p-1)tl(o) == (rr + 1f) *n (0) (mod 1)) for ab n > 1 and all p == 1 (mod 3) prime, where 1T denotes a generator of a prime in [( above p with 1T' == 1 (mod 3). The correspolldihg collgrueuce for the squares of the *n (0) (i.e., for the numbers Lk) was kllowil, and the possibility of'choosing the signs in such a way that this congrnence desceuds to the square roots for ab p simultaneously had been conjectured by I<oblitz [3]. 111 fact, I<oblitz conjectured the existence of a ~adic L-function interpolat.iug suitable modifications of the presumed square roots. This has been proved by Sofer [8] in otlter similar cascs (see also [6]).

11 9 There is anot.her way of obt.aillillg the Ilumbers an (OL b o (0) aud Co (0) directly in terms of gencratillg series. THEOREM 4. (1) Let u(,) = F(~,~; 3;,3) (md v(,) =, F(~,~; i;,3), where F = '1Fl is G(JUSS 's hypergeometric /ulletiou. Let and define ha( ~) -_ u(~)l/'j.(l _ ~3)1/24, I () I ()3 h () 1 h ( ),, tb' = t a " c' = 2" b ' Ha(x) = ha(,l Hb(x) = hb(,), Hc(x} = hc{,l where x = v/2u = ~(, + t, ,7 + 31J~o,lO +... ). Then X n X n X n Ha(z) =L:{-ltun(O)" Hb(x) = L: bn{o)" Hc(x) =E cn(o),. U. n. n. n'2:0 n'2:0 n'2: 0 (2) The series Ha (x) i.'i lh e ej.pansiou 0/ 1J(z) aijout w = (-1 + yc3)/2 in the /ollowiug sense:! (w - wx) (1 - x) - rj =Cl Ha (C2 XL 1 - x (lxi< ll where Cl =11(W) =ew (3 t 0/211") ~ uud C2 = /411' with 0 as in 2. Relnnrks. 1. As a corollary of the identity Hb(x) = H a (x)3 we obtain sornewhat surprising polyllomial relat.ions Let.weell the square roots of the L-values {L 60 +d ami {L 6n +:4}' Analogolls ident.i t.ies also hold for other CM cu rves, Iinking L-vailles ofolle c\lrve to L-valuf'..8 of it.s twist by Q( vc'3)/q; ultimately they boil down to classical Jacobi identit,y 0'(0) =211'1l. 2. \Ve briefly stat.e here the power series expansions involving the L k themselves'. With t.he notat.ioll of Theorem 4 let Go(x) = U(T) and Gdx) = TU(T}. Then 2-oG~0)(0)eqnals Cl o /2(O)'J. ifu == 0 (mod 6L C(n_l)/2(0)2 ifn == 3 (mod 6), and 0 otherwise; 2- n G\0)(ü) eqllals b(n_l)/2{o)2 if H == 1 (mod 6) and zero otherwise. \Ve may even separate the two brauches (a) und (b) in Go by considering the series U ( T) (1± (1- ;3)t ). It is presumably possible to prove cl irectly that the series Ha, lh, He, Go and GI have their Taylor coefficiellts related ab indicated, but we have not, done so. Proof (sketch). Part 1) of the theorem follo\\'s from part 2) and the interpretation of t he constallt terms Cl n (0L b Jl (0L Co (0) as I1on-holomorphic clerivatives of holomorphic modular fonns, toget.her with the general fact that the expansion of any modular form as apower series iu a modular function satisfies a linear differential equation. (Classical examples of this latter assertion are the expansion of 11'1 or ~ as apower serics in 1/j or of 0'1 as apower series in AI ab of which illvolve hypergeomet.ric fullctiolls.) It can also be proved directly from the recursive definitions of the polyllomials Cl n I b n and Cu without any apriori knowledge tllat modular forms are involved.

12 10 F. RODRIGUßZ VILLEGAS D. ZAGIER Part 2) is n consequellce of t.he following simple result abollt non-holomorphic derivatives. \Ve recall their defillition. For ally k E R we let tjk be the differential operator iz + irü acting on fuuctions / of z = x + iy E 1l. It has the property where lk has the usual meallillg. In particular, if f is a modular form of weight k on same grollp rc SI2(R), then {}k!' where {}k = t?k+2" 0 0 t?k+~ 0 t?kl is a (non-holomorphic) modular fonn of weight k + 2n on the same group. PROPOSITION 1. Let f : 1l ~ C be Oll OIwlyt.ic funtion and Zo = :ro + iyo a point in 1l. Then the Jollowing e:t1)(wsioh holds (Iwl < 1). Proof. Tt is easy to check by illduction that -!..?"/( ) = """ (i + I + k - 1) (_1_)' /(i)(zo),1k Zn L., I 2' '1' n.. lyo J. J+'=n """ {}"f( ) (2iY(lw)n """ fj)(zu) (2' )J """ (i + 1+ k - 1) I L., k Zu I = L., '1 Jyow L., I W, n. J. n~o J?U '?O and our claim follows from Taylor's ami the binomial theorems. RellU\l'k. Notice that. the subst.itution 4>(t) =(zu-zot)/(1-t) is au isomorphism from 11. to the ullit disk sellding Zu ta 0, with inverse,p-l(z) = (z - zo)/(z - zo). The proposit.ion then says t.h at. the noil- holomorphie derivatives t?kf (zo) are essentially the Taylor coefficients of flkcp at. t =o. 5. Estiluntes of Sp The last two criteria for the vanishing of Sp described in the intraduction are given in terms of the vanishing of Sp (Illod }J). Timt these two statements are in fact equivalent is a coliseqtlence of the following general estimate. PROPOSITION 2. Let E/Q be a modular elliptic curve 0/ conduetor N. Then IL(E,I)1 < {4N)I/4 (log: +,,) + Cu, where "'t = i.'t Etlle1"S COll.'itrmt am] Co = (( ~)2 =

13 11 P roof. BecaHse of the Hiliversal estimate tun I :\$ V7i 0"0 (u) for the coefficienta of L(E, s) we have where w = ± 1 ia tbe sigll in t.he fuuctional eq\latioll and F (x) = f: 0"0 ~) e-nz n;1 v n Using the fact that the Mellin trausform of F(x) is r(s)((s +!r J, we find the asymptotic expansion l 1 00 F(x) = ~ (log - +,) +L cnx n x 4x n=(i (x ~ 0) with Co = ((~)2 ami cl = -«(- ~) < O. Same numerical work shows that Ln> 1 CnX n < 0 for all X > O. (For l.he proposi t ion, we need this oniy for x :5-21T/v'IT.) Applying this to the curve E p, whose conductor is 27p'J, we find after a simple calculation the following estimate. COROLLARY. For l' == 1 (mod U) we Iwve 18 p t < O.61]JEi/6Iogp. In parficulur, Sp is del.enrü"ed by its vulue modulo ]J ulid Sp == 0 (mod p) Sp =O. RClunrk. 'Ve cah also estilllate Sp lisillg Theorems 1 or 2. For instance, from Theorem 2 aud the estünate 7J(w;k) = 0(1'1/4), we obtain Rp = 0(p2/3), so that Rp, and hence Sp, is def,cnnillcd by its value modulo p. The corresponding estimnte using Theorem 1 is more difficult, because e is not a cusp form, but seems to lead to the esl.imate Sp = O(JJ Ei / G +'), essentially the same as in the Corollary above. Note that Lo determine Sp from its value modulo p we need ooly the weaker estimaf,e SI' < 1'2/4, since we know that Sp is a square. REFERENCES 1. A. Hurwitz, J(faH~71za!ll binärer qlladmtijcher FO""H~n V(H1 negativer Dderminl1nte, Acta Math. 10 (1895), Mnthelllnti~he Werke, Bil. 11, Bil'khäuser-Verla&, BlUlel Stuttgnrt, 1003, pp N. M. Katz, p-i1jic intupolation oj n~al analy!ic EiJwJtein ~uiell, Ann. or Math. 104 (1916), N. KoLlitz, p-adic congrih~'lcell and modular Jornu oj 'lai/-integral weiglit, Math. Ann. ~14 (198G), F. Rodriguez VillegM, On thc "quan: root oj.p~cial value. oj certain L-lIenelJ, Invent. math. log (1991), , Square root Jonuula. J"r cwtral valuelj oj Hecke L-lJerie. 11, Duke Math. J. 1~ (1993),

14 12 F. RODHIGUEZ VILLEGAS D. ZAGIER 6., On the Tllylor cor:fficimt, oj theta JuuctiOf" oj GM elfiptic curve" Arithmetic Geometry (N. ChildreSll mlll J.W. Jones, ed:'l.), Contempornry Mllthemntics 174, AMS, F. Rodrigllez Villegas nnd D. Zngiel', Square roi)t. 1)1 centnji value.s oj Hecke L.urie" Advunces in NUlIlLel' Theory, (F. Q. GOllvea and N. Yui, cdlj.), Cla.rendoll Press, Oxford, H193, pp A. Sofer, p adic interpolation 01.square root. 0/ central value, 01 Hecke L lundion" ph,n. TheBiB, The Ohio Stute Vniversity, 19<J3. 9. S. Sylvetiter, On the equation in numbe,.., Ax 3 + B y 3 + GZ s =DrJlz, and it. auociate,y"tern 0/ equatio 08, Philmlilphical Magazine 31 (1847), MathelllaticaJ Papen, Vol. 1, Chelsea, New York, 1973, pp D. Z8gier alld G. Krn1ll8l"l., Numcricaf inve"tiuation, rdated to the L,erie. 0/ certain elliptic curve., J. Ind. Mnth. \$oe. tt2 (1987), DEPARTMENT 01" MATIIEMATICS, PRINCETON UNIVERSITY, PRINCETON, NJ 08ri44, USA E mail addreu: villegß..'iouli\th.prillceton.edu MAX-PLANCK-INSTITUT FÜR MATHEMATIK, G.-CLAREN-5TR. 20, BONN, GERMANY E maif addreu: zngierolilpilll-hollll.lilpg.de

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