Illinois Journal of Mathematics Volume 48, Number 2, Summer 2004, Pages 491–503 S 0019-2082 EXPLICIT FORMULAS FOR DIRICHLET AND HECKE L-FUNCTIONS XIAN-JIN LI Abstract. In 1997, thePauthor proved that the Riemann hypothesis holds if and only if λn = [1−(1−1/ρ)n ] > 0 for all positive integers n, where the sum is over all complex zeros of the Riemann zeta function. In 1999, E. Bombieri and J. Lagarias generalized this result and obtained a remarkable general theorem about the location of zeros. They also gave an arithmetic interpretation for the numbers λn . In this note, the author extends Bombieri and Lagarias’ arithmetic formula to Dirichlet L-functions and to L-series of elliptic curves over rational numbers. 1. Introduction Let K be a finite field with q elements, and let E be an elliptic curve over K. In the 1930s, H. Hasse proved the inequality √ |#E(K) − q − 1| ≤ 2 q, where #E(K) is the number of K-rational points on E; see [12]. Let a = 1 + q − #E(K) and LE (s) = 1 − az + qz 2 , where z = q −s . By Hasse’s inequality we have LE (s) = (1 − αz)(1 − βz) with |α| = |β| = √ q. Hence − ∞ X d log LE (s) = λE (n + 1)z n , dz n=0 where λE (n) = αn + β n . It is clear that √ |λE (n)| ≤ 2 q n Received April 24, 2003; received in final form October 17, 2003. 2000 Mathematics Subject Classification. Primary 11M26, 11M36. Research supported by National Security Agency. c 2004 University of Illinois 491 492 XIAN-JIN LI for n = 1, 2, . . .. This estimate implies that all zeros of LE (s) lie on the line <s = 1/2. Let ξ(s) = s(s − 1)π −s/2 Γ(s/2)ζ(s), where ζ(s) is the Riemann zeta function, and let λζ (n), n = 1, 2, . . . , be a sequence of numbers defined by X ∞ d 1 ln ξ = λζ (n + 1)z n . dz 1−z n=0 In 1997, the author obtained the following criterion for the Riemann hypothesis. Theorem 1 ([9]). All complex zeros of ζ(s) lie on the line <s = 1/2 if and only if λζ (n) > 0 for n = 1, 2, . . . . In 1952, A. Weil [13] proved a famous criterion for the validity of the Riemann hypotheses for number fields. The following is Bombieri’s refinement of Weil’s criterion. Bombieri’s refinement ([2]). All complex zeros of ζ(s) lie on the line <s = 1/2 if and only if X fb(ρ)fb̄(1 − ρ) ≥ 0 ρ for every complex-valued f ∈ C0∞ (0, ∞) which is not identically 0, where the Mellin transform of f is given by Z ∞ b f (s) = f (x)xs−1 dx. 0 Let f, g ∈ C0∞ (0, ∞). The multiplicative convolution of f and g is given by Z ∞ dy (f ∗ g)(x) = f (x/y)g(y) . y 0 If fe(x) = x−1 f (x−1 ), the Mellin transform of f ∗ fē is fb(s)fb̄(1 − s). Let gn (x) be the inverse Mellin transform of 1 − (1 − 1/s)n for n = 1, 2, . . . . E. Bombieri and J. Lagarias observed in [3] that n n 1 1 1− 1− + 1− 1− s 1−s n n 1 1 = 1− 1− 1− 1− , s 1−s and that gn (x) + gen (x) = (gn ∗ gen )(x). EXPLICIT FORMULAS FOR DIRICHLET AND HECKE L-FUNCTIONS 493 Hence, the positivity in the author’s criterion has the same meaning as that in Weil’s criterion. In 1999, Bombieri and Lagarias obtained the following remarkable theorem. Theorem 2 (Bombieri-Lagarias [3]). Let R be a set of complex numbers ρ whose elements have positive integral multiplicities assigned to them, such that 1 6∈ R and X 1 + |<ρ| < ∞. (1 + |ρ|)2 ρ Then the following conditions are equivalent: (1) <ρ ≤ 1/2 every ρ in R; for −n P 1 (2) ≥ 0 for n = 1, 2, . . . . ρ< 1− 1− ρ An arithmetic interpretation for the numbers λζ (n) was given in [3]. Theorem 3 (Bombieri-Lagarias [3]). We have ( N ) n X X Λ(k) n (−1)j j−1 j λζ (n) = lim j (ln k) − (ln N ) j j! N →∞ k j=1 k=1 n X n n + 1 − (ln 4π + γ) + (−1)j (1 − 2−j )ζ(j) 2 j j=2 for n = 1, 2, . . . , where γ = 0.5772 . . . is Euler’s constant and where Λ(k) = ln p when k is a power of a prime p and Λ(k) = 0 otherwise. Let χ be a primitive Dirichlet character of modulus r > 1, and L(s, χ) the Dirichlet L-function of character χ. If s+a − 12 (s+a) ξ(s, χ) = (π/r) Γ L(s, χ), 2 where ( 0, if χ(−1) = 1, a= 1, if χ(−1) = −1, then ξ(s, χ) is an entire function of order one and satisfies the functional equation ξ(s, χ) = χ ξ(1 − s, χ̄), where χ is a constant of absolute value one. By Theorem 2 of [1] we have Y ξ(s, χ) = ξ(0, χ) (1 − s/ρ), ρ where the product is over all the zeros of ξ(s, χ) in the order given by |=ρ| < T for T → ∞. 494 XIAN-JIN LI For n = 1, 2, . . . let λχ (n) = X [1 − (1 − 1/ρ)n ] , ρ where the sum on ρ runs over all zeros of ξ(s, χ) in the order given by |=ρ| < T for T → ∞. First, we give an arithmetic interpretation for the numbers λχ (n). Theorem 4. Let χ be a primitive Dirichlet character of modulus r > 1. Then we have n ∞ X n (−1)j−1 X Λ(k) n r χ̄(k)(ln k)j−1 + ln − γ + τχ (n), λχ (n) = − j (j − 1)! k 2 π j=1 k=1 where τχ (n) = (Pn n 1 j j (−1) 1 − 2j Pj=2 n n j −j j=2 j (−1) 2 ζ(j), ζ(j) − n 2 P∞ 1 l=1 l(2l−1) , if χ(−1) = 1, if χ(−1) = −1. Let E be an elliptic curve over Q with conductor N . For each prime p, we denote by Ẽp the reduction of E at p. Let p + 1 − #Ẽp (Fp ), if E has good reduction at p, 1, if E has split multiplicative reduction at p, ap = −1, if E has non-split multiplicative reduction at p, 0, if E has additive reduction at p. We define the L-series associated to E by the Euler product Y Y LE (s) = (1 − ap p−s + p1−2s )−1 (1 − ap p−s )−1 p|N p-N for <s > 3/2; see [12]. Let k and N be positive integers, and let χ be a multiplicative character of modulus N with χ(1) = 1 and χ(−1) = (−1)k . Let Γ be the Hecke congruence subgroup Γ0 (N ) of level N . We denote by S0 (Γ, k, χ) the space of all cusp forms of weight k and character χ for Γ. That is, f belongs to S0 (Γ, k, χ) if and only if f is holomorphic in the upper half-plane H, satisfies az + b f = χ(d)(cz + d)k f (z) cz + d for all ac db ∈ Γ, satisfies the usual regularity conditions at the cusps of Γ, and vanishes at each cusp of Γ. The Hecke operators Tn are defined by X az + b 1 X (Tn f )(z) = f χ(a)ak n d ad=n 0≤b<d EXPLICIT FORMULAS FOR DIRICHLET AND HECKE L-FUNCTIONS 495 for any function f on H. A function f in S0 (Γ, k, χ) is called a Hecke eigenform if Tn f = λ(n)f for all positive integers n with (n, N ) = 1. The Fricke involution W is defined by (W f )(z) = N −k/2 z −k F (−1/N z), and the complex conjugation operator K is defined by (Kf )(z) = f¯(−z̄). Set W̄ = KW . Then f is a newform if it is an eigenfunction of W̄ and of all the Hecke operators Tn . Let f be a newform in S0 (Γ, k, χ) normalized so that its Fourier coefficient is 1. Then it has the Fourier expansion ∞ X f (z) = λ(n)e2πinz n=1 with the Fourier coefficients equal to the eigenvalues of Hecke operators. Since f is an eigenfunction of the involution W̄ , we can assume that W̄ f = ηf for a constant η. Let ∞ X λ(n) Lf (s) = ns n=1 for <s > (k + 1)/2. This L-series has Euler product Y Lf (s) = (1 − λ(p)p−s + χ(p)pk−1−2s )−1 p and satisfies the functional identity √ !s N Γ(s)Lf (s) = ik η̄ 2π √ N 2π !k−s Γ(k − s)L̄f (k − s̄). When χ is primitive, we have η = τ (χ̄)λ(N )N −k/2 with τ (χ) being the Gauss sum for χ. For the theory of Hecke L-functions see [8]. We denote by S2 (N ) the space of cusp forms of weight 2 with the principal character for Γ0 (N ). Shimura-Taniyama Conjecture ([4], [14]). S2 (N ) such that Lf (s) = LE (s). There is a newform f ∈ The Shimura-Taniyama conjecture has now been proved ([4], [14]). If 1 1 ξE (s) = cE N s/2 (2π)−s Γ + s LE +s , 2 2 496 XIAN-JIN LI where cE is a constant chosen so that ξE (1) = 1, then ξE (s) is an entire function and satisfies ξE (s) = wξE (1 − s), r where w = (−1) with r being the vanishing order of ξE (s) at s = 1/2. Let n X 1 λE (n) = 1− 1− ρ ρ for n = 1, 2, . . . , where the sum is over all the zeros of ξE (s) in the order given by |=ρ| < T for T → ∞. Since ξE (s) is an entire function of order one, the conditions of Bombieri-Lagarias’ theorem are satisfied, and hence all the zeros of ξE (s) lie on the line <s = 1/2 if and only if λE (n) > 0 for n = 1, 2, . . . . For each prime number p, we let αp and βp be the roots of T 2 − ap T + p. Let b(pk ) = akp if p|N and b(pk ) = αpk + βpk if (p, N ) = 1. Next, we give an arithmetic interpretation for the numbers λE (n). We have ! √ n ∞ X N n (−1)j−1 X Λ(k) λE (n) = n ln −γ − b(k)(ln k)j−1 3/2 2π j (j − 1)! k j=1 k=1 ! ∞ n ∞ X n X 2 X 3 1 +n − + + (−1)j . 3 l(2l + 3) j (l + 1/2)j j=2 Theorem 5. l=1 l=1 The author wishes to thank Brian Conrey for his help during the preparation of the manuscript, and William Duke for his valuable suggestions. He also wants to thank the referee for carefully reading the manuscript and for his/her valuable suggestions. 2. Proof of Theorem 4 Weil’s explicit formula for L(s, χ) ([1], [13]). defined on R such that Let F (x) be a function 2F (x) = F (x + 0) + F (x − 0) for all x ∈ R, such that F (x) exp((b + 1/2)|x|) is of bounded variation on R for a constant b > 0, and such that F (x) + F (−x) = 2F (0) + O(|x|` ) EXPLICIT FORMULAS FOR DIRICHLET AND HECKE L-FUNCTIONS 497 as x → 0 for a constant ` > 0. Then ∞ r X X Λ(n) √ (χ(n)F (ln n) + χ̄(n)F (− ln n)) Φ(ρ) = F (0) ln − γ − π n ρ n=1 Z ∞ dx + F (x)e(3/2−a)|x| − F (0) , 1 − e2|x| −∞ where the sum on ρ runs over all zeros of ξ(s, χ) in the order given by |=ρ| < T for T → ∞, and Z ∞ (2.1) Φ(s) = F (x)e(s−1/2)x dx. −∞ A multiset is a set whose elements have positive integral multiplicities assigned to them [3]. Lemma 2.1 ([3, (2,4)]). Formally, if Y z f (z) = 1− ρ ρ and λn = X [1 − (1 − 1/ρ)n ] , ρ then we have d ln f dz 1 1−z = ∞ X λ−n−1 z n . n=0 Lemma 2.2 ([3, Corollary 1]). Let R be a multiset of complex numbers such that (1) 0, 1 6∈ R; (2) if ρ ∈ R, then 1 − ρ and ρ̄ are in R and have the same multiplicity as ρ; P 2 (3) ρ (1 + |<ρ|)/(1 + |ρ|) < ∞. Then <ρ = 1/2 for all ρ ∈ R if, and only if, X λn = [1 − (1 − 1/ρ)n ] ≥ 0 ρ∈R for n = 1, 2, 3, . . .. Lemma 2.3 ([3, Lemma 2]). For n = 1, 2, . . ., let Pn n xj−1 x/2 e j=1 j (j−1)! , if −∞ < x < 0, Fn (x) = n/2, if x = 0, 0, if 0 < x. 498 XIAN-JIN LI Then n 1 , Φn (s) = 1 − 1 − s where Φn is related to Fn as in (2.1). Proof of Theorem 4. For a sufficiently an integer let Fn (x), Fn,X (x) = 12 Fn (− ln X), 0, large positive number X that is not if − ln X < x < ∞, if x = − ln X, if −∞ < x < − ln X. Then Fn,X (x) satisfies all conditions of Weil’s explicit formula for L(s, χ). Let Φn,X (s) = Z ∞ Fn,X (x)e(s−1/2)x dx. −∞ By using the Weil explicit formula, we obtain that r X Φn,X (ρ) = Fn,X (0) ln − γ π ρ ∞ X Λ(k) √ (χ(k)Fn,X (ln k) + χ̄(k)Fn,X (− ln k)) k k=1 Z ∞ dx + Fn,X (x)e(3/2−a)|x| − Fn,X (0) , 1 − e2|x| −∞ − where the sum on ρ runs over all zeros of ξ(s, χ) in the order given by |=ρ| < T for T → ∞. It follows that (2.2) X Φn,X (ρ) lim X→∞ ρ n ∞ X n r n (−1)j−1 X Λ(k) = ln − γ − χ̄(k)(ln k)j−1 2 π j (j − 1)! k j=1 k=1 (Pn P∞ n 1 n 1 j j=2 j (−1) 1 − 2j ζ(j) − 2 l=1 l(2l−1) , if χ(−1) = 1, + Pn n j −j if χ(−1) = −1. j=2 j (−1) 2 ζ(j), Note that the infinite series in the second term on the right side of (2.2) converges by the prime number theorem for arithmetic progressions; see §1920 of [5]. EXPLICIT FORMULAS FOR DIRICHLET AND HECKE L-FUNCTIONS 499 We have (2.3) n X n j−1 X (ln X)j−k−1 s−k−1 j (j − k − 1)! j=1 k=0 n −s X n (− ln X)j−1 (ln X)n−2 −<s X +O X . = s j=1 j (j − 1)! |s|2 Φn (s) − Φn,X (s) = X −s (−1)j−1 Let ρ be any zero of ξ(s, χ) which is not the Siegel zero when χ is a real nonprincipal character. By §14 of [5] we have c c ≤ <ρ ≤ 1 − ln r(|ρ| + 2) ln r(|ρ| + 2) for a positive constant c. An argument similar to that made in the proof of (3.9) of [3] shows that X X −<ρ (2.4) |ρ|2 ρ 0 e−c √ ln X + X −β β2 for a positive constant c0 , where the second term on the right side of the inequality exists only when β is the Siegel zero of L(s, χ). Let X ψ0 (x, χ) = χ(n)Λ(n) n≤x when x is not a prime power. By §19–20 of [5] we have − X X ρ̄ ρ̄ ρ = ψ0 (X, χ̄) + L0 (0, χ̄) 1 X + 1 − ln L(0, χ̄) 2 X −1 when χ(−1) = −1, and − X X ρ̄ ρ ρ̄ = ψ0 (X, χ̄) + b(χ̄) + ln p X2 − 1 when χ(−1) = 1, where b(χ) is the constant term in the expansion of L0 /L near s = 0, L0 (s, χ) 1 = + b(χ) + · · · , L(s, χ) s and ψ0 (X, χ) = − √ 0 Xβ + O Xe−c ln X β 500 XIAN-JIN LI for a positive constant c0 . Since X X −(1−ρ̄) X X −ρ = ρ 1 − ρ̄ ρ ρ ! X X −(1−<ρ) 1 X X ρ̄ =− +O X ρ ρ̄ |ρ|2 ρ √ 0 X β−1 1 X X ρ̄ + O e−c ln X + , =− X ρ ρ̄ β2 we have (2.5) X X −ρ ρ ρ 0 X β−1 + e−c √ ln X for a positive constant c0 , where the term X β−1 exists only when β is the Siegel zero of L(s, χ). It follows from (2.3), (2.4) and (2.5) that X X lim Φn,X (ρ) = Φn (ρ). X→∞ ρ ρ This completes the proof of the theorem. 3. Proof of Theorem 5 Explicit formula for LE (s) ([10]). Let F (x) be a function defined on R such that 2F (x) = F (x + 0) + F (x − 0) for all x ∈ R, such that F (x) exp(( + 1/2)|x|) is integrable and of bounded variation on R for a constant > 0, and such that (F (x) − F (0))/x is of bounded variation on R. Then √ ∞ X N X Λ(n) − b(n) [F (ln n) + F (− ln n)] Φ(ρ) = 2F (0) ln 2π n ρ n=1 Z ∞ e−x F (x) + F (−x) − − 2F (0) dx, ex − 1 x 0 where the sum on ρ runs over all zeros of ξE (s) in the order given by |=ρ| < T for T → ∞, and Z ∞ Φ(s) = F (x)e(s−1/2)x dx. −∞ Lemma 3.1 ([6], [7], [11]). Let f be a newform of weight 2 for Γ0 (N ). Then there an absolute effective constant c > 0 such that Lf (s) has no zeros in the region c s = σ + it : σ ≥ 1 − . ln(N + 1 + |t|) EXPLICIT FORMULAS FOR DIRICHLET AND HECKE L-FUNCTIONS 501 Proof of Theorem 5. Since ξE (s) is an entire function of order one satisfying ξE (1) = 1 and ξE (s) = wξE (1 − s), we have ξE (s) = w Y (1 − s/ρ), ρ where the product is over all the zeros of ξE (s) in the order given by |=ρ| < T for T → ∞. If ϕE (z) = ξE (1/(1 − z)), then ∞ ϕ0E (z) X = λE (n + 1)z n . ϕE (z) n=0 For a sufficiently large positive number X that is not an integer let if − ln X < x < ∞, Fn (x), Fn,X (x) = 12 Fn (− ln X), if x = − ln X, 0, if −∞ < x < − ln X, where Fn (x) is given as in Lemma 2.3. Then Fn,X (x) satisfies all conditions of the explicit formula for LE (s). Let Φn,X (s) = Z ∞ Fn,X (x)e(s−1/2)x dx. −∞ By using the explicit formula, we obtain that √ X ∞ N X Λ(k) − b(k) [Fn,X (ln k) + Fn,X (− ln k)] 2π k k=1 Z ∞ Fn,X (x) + Fn,X (−x) e−x − − 2F (0) dx, n,X ex − 1 x 0 Φn,X (ρ) = 2Fn,X (0) ln ρ where the sum on ρ runs over all zeros of ξE (s) in the order given by |=ρ| < T for T → ∞. It follows that X lim Φn,X (ρ) X→∞ ρ √ n ∞ X n (−1)j−1 X Λ(k) b(k)(ln k)j−1 3/2 j (j − 1)! k j=1 k=1 ! ∞ n ∞ X n X 2 X 3 1 +n − + + (−1)j . 3 l(2l + 3) j (l + 1/2)j j=2 N = n ln −γ 2π ! − l=1 l=1 502 XIAN-JIN LI We have (3.1) Φn (s) − Φn,X (s) = X −s n X n j−1 X (ln X)j−k−1 s−k−1 j (j − k − 1)! j=1 k=0 n −s X n (− ln X)j−1 (ln X)n−2 −<s X +O X . = s j=1 j (j − 1)! |s|2 (−1)j−1 Let ρ be any zero of ξE (s). By Lemma 3.1 and the Shimura-Taniyama conjecture we have c c ≤ <ρ ≤ 1 − ln(N + 1 + |ρ|) ln(N + 1 + |ρ|) for a positive constant c. An argument similar to that made in the proof of (3.9) of [3] shows that (3.2) X X −<ρ |ρ|2 ρ 0 e−c √ ln X for a positive constant c0 . Since X X −ρ X X −(1−ρ) = ρ 1−ρ ρ ρ X X −(1−<ρ) 1 X Xρ =− +O X ρ ρ |ρ|2 ρ 0√ 1 X Xρ =− + O e−c ln X , X ρ ρ ! and since (ln X)j−1 X X ρ =0 X→∞ X ρ ρ lim for j = 1, 2, . . . , n by Theorem 4.2 and Theorem 5.2 of [11], we have (3.3) lim (ln X)j−1 X→∞ X X −ρ ρ ρ =0 for j = 1, 2, . . . , n. 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