Mean-Periodicity and Zeta Functions
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MEAN-PERIODICITY AND ZETA FUNCTIONS IVAN FESENKO, GUILLAUME RICOTTA, AND MASATOSHI SUZUKI ABSTRACT. This paper establishes new bridges between the class of complex functions, which contains zeta functions of arithmetic schemes and closed with respect to product and quotient, and the class of mean-periodic functions in several spaces of functions on the real line. In particular, the meromorphic continuation and functional equation of the Hasse zeta function of arithmetic scheme with its expected analytic shape is shown to imply the mean-periodicity of a certain explicitly defined function associ- ated to the zeta function. Conversely, the mean-periodicity of this function implies the meromorphic continuation and functional equation of the zeta function. This opens a new road to the study of zeta functions via the theory of mean-periodic functions which is a part of modern harmonic analysis. The case of elliptic curves over number fields and their regular models is treated in more details, and many other examples are included as well. CONTENTS 1. Introduction and flavour of the results2 1.1. Boundary terms in the classical one-dimensional case2 1.2. Mean-periodicity and analytic properties of Laplace transforms3 1.3. Hasse zeta functions and higher dimensional adelic analysis4 1.4. Boundary terms of Hasse zeta functions and mean-periodicity4 1.5. The case of Hasse zeta functions of models of elliptic curves5 1.6. Other results 6 1.7. New perspectives 6 1.8. Organisation of the paper6 2. Mean-periodic functions7 2.1. Generalities 7 2.2. Quick review on continuous and smooth mean-periodic functions8 2.3. Some relevant spaces with respect to mean-periodicity9 2.4. Mean-periodic functions in these relevant spaces 10 2.5. Mean-periodicity and analytic properties of Laplace transforms I 11 2.6. Mean-periodicity and analytic properties of Laplace transforms II 13 3. Mean-periodicity and analytic properties of boundary terms 15 3.1. The general issue 15 3.2. Mean-periodicity and meromorphic continuation of boundary terms 16 3.3. Mean-periodicity and functional equation of boundary terms 16 3.4. Statement of the result 17 4. On a class of mean-periodic functions arising from number theory 18 4.1. Functions which will supply mean-periodic functions 18 4.2. Mean-periodicity of these functions 19 4.3. On single sign property for these mean-periodic functions 23 5. Zeta functions of arithmetic schemes and mean-periodicity 23 5.1. Background on Hasse zeta functions of schemes 23 Date: Version of December 11, 2009. 1 2 I. FESENKO, G. RICOTTA, AND M. SUZUKI 5.2. Conjectural analytic properties of L-functions of elliptic curves 24 5.3. Hasse–Weil zeta functions of elliptic curves 25 5.4. Hasse zeta functions of models of elliptic curves 29 5.5. Hasse zeta functions of schemes and mean-periodicity 32 6. Other examples of mean-periodic functions 35 6.1. Dedekind zeta functions and mean-periodicity 35 6.2. Cuspidal automorphic forms and mean-periodicity 37 6.3. Eisenstein series and mean-periodicity 37 Appendix A. A useful analytic estimate for L-functions 39 References 42 Acknowledgements– The authors would like to thank A. Borichev for his enlightening lectures on mean- periodic functions given at the University of Nottingham in December 2007. This work was partially supported by ESPRC grant EP/E049109. The first author would like to thank the JSPS for its support. The second author is partially financed by the ANR project "Aspects Arithmétiques des Matrices Aléa- toires et du Chaos Quantique". 1. INTRODUCTION AND FLAVOUR OF THE RESULTS 1.1. Boundary terms in the classical one-dimensional case. The completed zeta function of a num- ber field K is defined on e (s) 1 by < È ³bK(s) : ³ ,K(s)³K (s) Æ 1 where ³K(s) is the Dedekind zeta function of K and ³K, (s) is a finite product of ¡-factors defined in 1 (5.3). It satisfies the integral representation (see [51]) Z s ³bK(s) f (x) x d¹A (x) Æ j j K£ AK£ where f is an appropriately normalized function in the Schwartz–Bruhat space on AK and stands j j for the module on the ideles A£ of K. An application of analytic duality on K AK leads to the K ½ decomposition ¡ ¢ ¡ ¢ ³bK(s) » f ,s » fb,1 s ! (s) Æ Å ¡ Å f where fb is the Fourier transform of f , »(f ,s) is an entire function and Z 1 s dx !f (s) h f (x)x Æ 0 x with Z Z ¡ 1 ¡ 1 ¢¢ h f (x) : f (xγβ) x¡ fb x¡ γβ d¹(¯)d¹(γ). Æ¡ γ A1 /K ¯ @K ¡ 2 K £ 2 £ Every continuous function on AK which vanishes on K£ vanishes on K, and the boundary @K£ of K£ is K \ K£ {0}. The meromorphic continuation and the functional equation for ³bK(s) are equivalent Æ to the meromorphic continuation and the functional equation for !f (s). Let us remark that !f (s) is ¡ t ¢ t the Laplace transform of H (t) : h e¡ thanks to the change of variable x e¡ . The properties of f Æ f Æ the functions h f (x) and H f (t), which are called the boundary terms for obvious reason, are crucial in order to have a better understanding of !f (s). We have ¡ 1 ± ¢¡ 1 ¢ h (x) ¹ A K£ f (0) x¡ fb(0) , f Æ¡ K ¡ ¡ 1 ± ¢¡ t ¢ H (t) ¹ A K£ f (0) e fb(0) f Æ¡ K ¡ MEAN-PERIODICITY AND ZETA FUNCTIONS 3 since @K£ is just the single point 0, with the appropriately normalized measure on the idele class group. As a consequence, ! (s) is a rational function of s invariant with respect to f fb and s f 7! 7! (1 s). Thus, ³bK(s) admits a meromorphic continuation to C and satisfies a functional equation with ¡ respect to s (1 s). 7! ¡ 1.2. Mean-periodicity and analytic properties of Laplace transforms. The previous discussion in the one-dimensional classical case naturally leads to the analytic study of Laplace transforms of specific functions. In particular, the meromorphic continuation and functional equation for Mellin transforms of real-valued functions f on R£ of rapid decay at and polynomial order at 0Å is Å Å1 equivalent to the meromorphic continuation and functional equation of Z 1 Z s dx Å1 st !f (s) : h f (x)x H f (t)e¡ dt Æ 0 x Æ 0 with 1 ¡ 1¢ h (x) f (x) "x¡ f x¡ f Æ ¡ ¡ t ¢ and H (t) h e¡ , where " 1 is the sign of the expected functional equation (see Section3). f Æ f Ƨ The functions h f (x) and H f (t) are called the boundary terms by analogy. The main question is the following one. What property of h f (x) or H f (t) implies the meromorphic continuation and functional 1 equation for !f (s)? One sufficient answer is mean-periodicity . Mean-periodicity is an easy general- ization of periodicity; a function g of a functional space X is X -mean-periodic if the space spanned by its translates is not dense in X . When the Hahn-Banach theorem is available in X , g is X -mean- periodic if and only if g satisfies a convolution equation g ' 0 for some non-trivial element ' of ¤ Æ the dual space X ¤ and a suitable convolution . Such a convolution equation may be thought as a ¤ generalized differential equation. Often2 mean-periodic functions are certain limits of exponential polynomials, and the latter were used already by Euler in his method of solving ordinary differential equations with constant coefficients. The functional spaces X , which are relevant for number theory, are mainly 8 >C (R£) the continuous functions on R£ (Section 2.2), <> Å Å X C 1(R£) the smooth functions on R£ (Section 2.2), £ Æ > Å Å :> R R Cpoly1 ( £) the smooth functions on £ of at most polynomial growth (see (2.4)) Å Å in the multiplicative setting, which is related to h f (x) and 8 >C (R) the continuous functions on R (Section 2.2), <> X C 1(R) the smooth functions on R (Section 2.2), Å Æ > :> Cexp1 (R) the smooth functions on R of at most exponential growth (see (2.3)) in the additive setting, which is related to H f (t). A nice feature is that the spectral synthesis holds in both X and X . The general theory of X -mean-periodic functions shows that if h f (x) is X - Å £ £ £ mean-periodic then !f (s) has a meromorphic continuation given by the Mellin–Carleman transform of h f (x) and satisfies a functional equation. Similarly, if H f (t) is X -mean-periodic then !f (s) has a Å meromorphic continuation given by the Laplace–Carleman transform of H f (t) and satisfies a func- tional equation. See Theorem 3.2 for an accurate statement. Note that in general !f (s) can have a meromorphic continuation to C and can satisfy a functional equation without the functions h f (x) and H f (t) being mean-periodic (see Remark 3.1 for explicit examples). 1The general theory of mean-periodicity is recalled in Section2. 2In this case one says that the spectral synthesis holds in X (see Section 2.1). 4 I. FESENKO, G. RICOTTA, AND M. SUZUKI 1.3. Hasse zeta functions and higher dimensional adelic analysis. For a scheme S of dimension n its Hasse zeta function Y s 1 ³S(s) : (1 k(x) ¡ )¡ Æ x S ¡ j j 2 0 whose Euler factors correspond to all closed points x of S, say x S , with finite residue field of car- 2 0 dinality k(x) , is the most fundamental object in number theory.