## Diagonal convergence of the remainder Padé approximants for the Hurwitz zeta function.(English)Zbl 1475.11152

This interesting paper constructs sequences of complex numbers that rapidly converge to the Hurwitz zeta function $\zeta(s,a)=\sum_{k=0}^{\infty}\,\frac{1}{(k+a)^s},\ \operatorname{Re}(a)>0\text{ and }\operatorname{Re}(s)>1.$ The method uses diagonal Padé approximants to the remainder series $\sum_{k=n}^{\infty}\,\frac{1}{(k+a)^s}.$ An important tool is is the series $\Phi_s(z)=\sum_{n=0}^{\infty}\,\frac{(s)_{2n+1}}{(2n+2)!}B_{2n+2}(-z)^n\, z\not=0\text{ and }s>0,$ with $$(B_{2n+2})_{n\geq 0}$$ the Bernoulli numbers, which is the asymptotic expansion of $\hat{\Phi}_s(z)=\int_0^{\infty}\,\frac{\mu_s(x)}{1-zx}\,dx,$ with $$\mu_s(x)=\omega_s(\sqrt{x})/2\sqrt{x}\in L^1(\mathbb{R}^{+}$$ for $$\operatorname{Re}(s)>0$$ and $\omega_s(x)=\frac{2(-1)^mx^s}{\Gamma(s)\Gamma(m+1-s)}\,\int_x^{\infty}\,(t-x)^{m-s}\frac{d^m}{dt^m}\left(\frac{1}{e^{2\pi t}-1}\right)\,dt.$
The main results are:
Theorem 1. (§1) Let $$s>0,\,s\not= 1$$ and $$a\in\mathbb{C}$$ such that $$\operatorname{Re}(a)>0$$. Set $$a_n=n+a$$. Then, for every large enough integer $$n$$ and any integer $$k\geq 1$$, we have $\zeta(s,a)=\sum_{j=0}^{n-1}\frac{1}{(j+a)^s}+\frac{1}{(s-1)a_n^{s-1}}+\frac{1}{2a_n^s}+\frac{1}{a_n^{s+1}}[k/k]_{\Phi_s}\left(-\frac{1}{a_n^2}\right)+ \varepsilon_{k,s} \left(\frac{1}{a_n^2}\right),$ where $| \varepsilon_{k,s} (1/a_n^2)|\leq D_s\frac{(2k+2\rho)\Gamma(2k+\rho+1)^2}{|a_n|^{4k+2}(4k+2\rho+1)(2k+1)\left(\begin{matrix}4k+2\rho\\ 2k+1\end{matrix}\right)^2},$ where $$\rho=(m+7)/2$$ and $$D_s=(2\pi)^sm!/\gamma(s)$$ and $$m=[s]$$.
Corollary 1. (§1) Let $$r\in\mathbb{Q}$$ such that $$0<r<2e$$. Let $$s>0,s\not= 1$$. Then, for every integer $$n\geq 1$$ such that $$rn$$ is an integer, we have $\zeta(s)=\sum_{k=1}^n\frac{1}{k^s}+\frac{1}{(s-1)n^{s-1}}-\frac{1}{2n^s}+\frac{1}{n^{s+1}}[rn/rn]-{\Phi_s}\left(-\frac{1}{n^2}\right)+\delta_{r,s,n},$ where $\limsup_{n\rightarrow\infty}\,|\delta_{r,s,n}|^{1/n}\leq\left(\frac{r}{2e}\right)^{4r}.$
Proposition 1. (§3) For any $$s>0$$ and any $$x\geq 0$$, we have $0<\Gamma(s)x\omega_s(x)\leq 2(2\pi)^{s-1}m!G\left(\frac{m+5}{2},1,x\right),$ where $$m=[s]$$ and $G(\alpha,\beta,x)=|\Gamma(\alpha+ix)\Gamma(\beta+ix)|^2.$
The layout of the paper is as follows:
§1. Introduction ($$\frac{1}{2}$$ pages)
§2. Consequences of an integral representation of $$\zeta(s,a)$$ ($$1\frac{1}{2}$$ pages)
§3. Bounds for the weight $$\omega_s(x)$$ ($$3$$ pages)
§4. Wilson’s polynomials ($$1\frac{1}{2}$$ pages)
§5. A bound for the Padé approximant of $$\Phi_s(z)$$ ($$2$$ pages)
§6. Proofs of Theorem 1 and Corollary 1 ($$\frac{1}{2}$$ page)
§7. The case $$s$$ real (negative)
In this section the main results given above are generalized to the case $$s<0$$
§8. The case $$a=1$$ and $$s\in\mathbb{N}$$ ($$1$$ page)
References ($$10$$ items)

### MSC:

 11M35 Hurwitz and Lerch zeta functions 11Y60 Evaluation of number-theoretic constants 41A21 Padé approximation 11J72 Irrationality; linear independence over a field 41A60 Asymptotic approximations, asymptotic expansions (steepest descent, etc.)
Full Text:

### References:

  Apéry, R., Irrationality of $$\zeta(2)$$ and $$\zeta(3)$$, Astérisque, 61, 11-13 (1979) · Zbl 0401.10049  Askey, R.; Wilson, J., A set of hypergeometric orthogonal polynomials, SIAM J. Math. Anal., 13, 4, 651-655 (1982) · Zbl 0496.33007  Brezinski, C., Padé-Type Approximation and General Orthogonal Polynomials, International Series of Numerical Mathematics, vol. 50 (1980), Birkhauser · Zbl 0418.41012  Cuyt, A., Handbook of Continued Fractions for Special Functions (2008), Springer Science  Matala-aho, T., Type II Hermite- Padé approximations of generalized hypergeometric series, Constr. Approx., 33, 289-312 (2011) · Zbl 1236.41017  Prévost, M., A new proof of the irrationality of $$\zeta(2)$$ and $$\zeta(3)$$ using Padé approximants, J. Comput. Appl. Math., 67, 2, 219-235 (1996) · Zbl 0855.11037  Prévost, M., Remainder Padé approximants for the Hurwitz zeta function, Results Math., 74, 1, Article 51 pp. (2019), 22 · Zbl 1408.41008  Prévost, M.; Rivoal, T., Application of Padé approximation to Euler’s constant and Stirling’s formula, Ramanujan J. (2020), in press  Rivoal, T., Nombres d’Euler, approximants de Padé et constante de Catalan, Ramanujan J., 11, 2, 199-214 (2006) · Zbl 1152.11337  Wilson, J., Some hypergeometric orthogonal polynomials, SIAM J. Math. Anal., 11, 690-701 (1980) · Zbl 0454.33007
This reference list is based on information provided by the publisher or from digital mathematics libraries. Its items are heuristically matched to zbMATH identifiers and may contain data conversion errors. It attempts to reflect the references listed in the original paper as accurately as possible without claiming the completeness or perfect precision of the matching.