Mean Square of the Hurwitz Zeta-Function and Other Remarks R Balasubramanian, K Ramachandra
Total Page:16
File Type:pdf, Size:1020Kb
Mean square of the Hurwitz zeta-function and other remarks R Balasubramanian, K Ramachandra To cite this version: R Balasubramanian, K Ramachandra. Mean square of the Hurwitz zeta-function and other remarks. Hardy-Ramanujan Journal, Hardy-Ramanujan Society, 2004, 27, pp.8 - 27. hal-01109904 HAL Id: hal-01109904 https://hal.archives-ouvertes.fr/hal-01109904 Submitted on 27 Jan 2015 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 8 Hardy-Ramanujan Journal Vol.27 (2004) 8-27 MEAN SQUARE OF THE HURWITZ ZETA-FUNCTION AND OTHER REMARKS BY R.BALASUBRAMANIAN AND K.RAMACHANDRA (Accepted on 26-04-2005) ABSTRACT. The Hurwitz zeta-function associated with the parameter a(0 < a · 1) is a generalisation of the Riemann zeta-function namely the case a = 1: It is de¯ned by X1 ³(s; a) = (n + a)¡s; (s = σ + it; σ > 1) (A:1) n=0 and its analytic continuation. In fact à ! X1 Z n+1 1¡s ¡s du a ³(s; a) = (n + a) ¡ s + (A:2) n=0 n (u + a) s ¡ 1 gives the analytic continuation to (σ > 0). (This remark is due to E.LANDAU (see [EL]). A repetition of this process several times shows that a1¡s ³(s; a) ¡ (A:3) s ¡ 1 can be continued as an entire function to the whole plane. In Re(s) ¸ ¡1; t ¸ 2; ³(s; a) ¡ a¡s = O(t3) and by the functional equation (see x2) it is 0 1 à ! 1 ¡Re(s) jsj 2 O @ A 2¼ in Re(s) ·¡1; t ¸ 2: From these facts we can deduce an `Approximate functional equation' (see x3), which is a generalisation of the approximate functional equation for ³(s). (The two are not very much di®erent). Combining this with an important theorem due to van-der- CORPUT, (see [ECT], Theorem 5.9), we prove 1 Z T +T 3 ¡ 1 1 ¡ 1 ¡it 2 3 T 3 j³( + it; a) ¡ a 2 j dt ¿ (log T ) (A:4) T 2 Mean Square of the ... 9 uniformly in a(0 < a · 1): From this we deduce similar results for quasi- L functions (see [RB, KR]1) and more general functions: Let a1; a2;::: be any periodic sequence of complex numbers for which the sum over a period is zero. Let b1; b2;::: be any sequence of complex Pn " numbers for which j=2 jbj ¡ bj¡1j + jbnj · n for every " > 0 and every n ¸ n0("). Then we prove 1 Z T +T 3 X1 ¡ 1 anbn 2 " T 3 j j dt · T (A:5) 1 +it T n=1 (n + a) 2 for every " > 0 and every T ¸ T0(") Here as usual 0 < a · 1 and T0(") is independent of a. x1. INTRODUCTORY REMARKS. These concern problems on ³(s; a) which need serious attention. We begin with M.N.HUXLEY'S recent important achievement [MNH] 1 32 ³( + it) = O (t®+"); ® = ; t ¸ 2: (1.1) 2 " 205 89 The earlier important record was also due to him and it was ® = 570 : We wish him every 1 64 success in his venture regarding improvement of (A.5) with 3 replaced by 205 . The result Z T 1 1 2 T T T 1 j³( + it)j dt = log + (2γ ¡ 1) + O(T 3 ) (1.2) 2¼ 0 2 2¼ 2¼ 2¼ P1 1 R n+1 du is due to R.BALASUBRAMANIAN [RB]. (Here as usual γ = n=1( n ¡ n u ) is the EULER constant). It remains to consider Z T X1 a b j n n j2dt 1 +it 0 n=1 (n + a) 2 1 and obtain an analogue of (1.2). Some work in the direction of extending (1.2) to ³( 2 + ¡ 1 ¡it it; a) ¡ a 2 is in progress and it is due to a student of M.JUTILA. This extension gives (A.4) as a corollary and (A.4) implies 1 ¡ 1 ¡it 1 3 ³( + it; a) ¡ a 2 = O(t 6 (log t) ): (1.3) 2 The result 2 Z T +T 3 ¡ 2 1 ¡ 2 ¡it 4 " T 3 j³( + it; a) ¡ a 2 j dt ¿" T (1.4) T 2 uniformly in a seems to be hopeless at present although N.ZAVOROTNYI [NZ] has proved Z T 1 4 2 +" j³( + it)j dt = TP (log T ) + O(T 3 ): (1.5) 0 2 10 RB, KR where P (x) is a certain polynomial of degree 4 in x. (For further work by Y.MOTOHASHI 2 and A.IVIC see [AI]). Even (1.4) with 1 in place of 3 also seems to be hopeless. Mean value results such as (A.4) imply (by convexity principles) X ¡σ¡it 2 1 +" à max ! j³(σ + it; a) ¡ a j ¿ T 3 I t"I 1 σ ¸ 2 1 where I runs over all (abutting) unit intervals into which [T;T +T 3 ] can be divided. (Similar remarks apply to all the mean value upper bounds over short intervals considered in this paper). It gives, as a corollary, the theorem 1 ¡ 1 ¡it 1 +" j³( + it; a) ¡ a 2 j < t 6 2 for all t ¸ t0("): (This can certainly be obtained by the WEYL-HARDY-LITTLEWOOD method (see [ECT] Theorem 5.3)). The most important result in the theory of the Riemann zeta-function is, undoubtedly, the result (1¡σ)3=2 2 1 j³(σ + it)j · C t C (log t) 3 ; ( · σ · 1; t ¸ 3) (1.6) 1 2 2 where C1 > 0 and C2 > 0 are absolute constants. This result is due to the Soviet Mathe- matician I.M.VINOGRADOV. (See the chapter on VINOGRADOFF's (both the names are the same) method in [ECT]). He proved (1.6) with (log t)2=3 replaced by a higher power of log t: De¯nitely he knew (1.6) and published 2 ³(1 + it) = O((log t) 3 ) (1.7) also. Both (1.6) and (1.7) are nearly 50 years old. It is a great challenge to improve even (1.7). It is well-known that quasi Riemann hypothesis (³(s) 6= 0 in σ ¸ 1 ¡ ± for some constant ± > 0) implies easily ³(1 + it) = O(log log t); t ¸ 100: (1.8) It is not hard to prove (1.6) for ³(σ + it; a) ¡ a¡σ¡it, (VINOGRADOFF's work gives (1.6) for this function as well). For the latest economical constants C1 = 76:2 and C2 = 4:45 in (1.6) see KEVIN FORD [KF]. See also [KR, AS]1. x2 FUNCTIONAL EQUATION. (We borrow the functional equation for ³(s; a) from x2.17 of [ECT]). We have X1 ³(s; a) = (n + a)¡s; (σ > 1); n=0 Mean Square of the ... 11 X1 Z n+1 1¡s ¡s du a = ((n + a) ¡ s ) + ; (σ > 0); (2.1) n=0 n (u + a) s ¡ 1 a1¡s and repetition of this process several times shows that ³(s; a) ¡ s¡1 is entire. Also in σ ¸ ¡1; t ¸ 2 we have ³(s; a) ¡ a¡s = O(t3). Again (see[ECT]) we have the functional equation ( 2¡(1 ¡ s) s¼ X1 cos(2¼ma) ³(s; a) = 1¡s sin( ) 1¡s (2¼) 2 m=1 m ) s¼ X1 sin(2¼ma) + cos( ) 1¡s ; (σ < 0): (2.2) 2 m=1 m s¼ Note that jtan ( 2 )j lies between two positive constants for all σ provided t ¸ 2; and so the conversion factors 2¡(1 ¡ s) s¼ 2¡(1 ¡ s) s¼ à (s) = sin ( ) and à (s) = cos( ) (2.3) 1 (2¼)1¡s 2 2 (2¼)1¡s 2 are nearly the same (as that for the Riemann zeta-function) so far as the magnitudes are concerned. It is useful to record here that for t ¸ 2; we have à ! 1 ¡σ jsj + 1 2 jà (s)j + jà (s)j ¿ (2.4) 1 2 2¼ where the implied constant is absolute. x3. APPROXIMATE FUNCTIONAL EQUATION 1 4 10 Let 0 < " · 100 ;T ¸ T0(");L = log T;¿ = L ; h = " L; 1 · X · T; 1 T 1 · Y · T; 1 · H · T 2 and XY ¸ (1 + "): (3.1) 2¼ 1 P1 ¡s Let w = u + iv; s = 2 + it; T · t · T + H; ³(s; a) = n=0(n + a) ; (0 < a · 1); σ > 1: 1 (We put s = 2 + it for trivial simpli¯cation. The results are true with trivial modi¯cations with s = σ + it for any σ uniformly in any ¯xed interval a0 · σ · b0): Then we have à ! X1 µn + a¶h (n + a)¡sExp ¡ n=1 x 1 Z w dw = (³(s + w; a) ¡ a¡s¡w)¡( + 1)Xw + O(T ¡1) (3.2) 2¼i u=2;jvj·¿ h w 5L w ¡jIm(w)j Here we move the line of integration to u = ¡ " . (We use j¡( h + 1)j ¿ exp(¡ h ) in h ¡s jRe(w)j · 2 ): The pole at w = 0 contributes ³(s; a) ¡ a and the horizontal portions of 12 RB, KR the contour contribute O(T ¡1): We then use the functional equation and the fact that for jIm(z)j ¸ 2, we have à ! 1 ¡Re(z) jzj + 1 2 jà (z)j + jà (z)j ¿ (3.3) 1 2 2¼ a result mentioned already.