New series for some special values of $L$-functions

Abstract

Dirichlet's $L$-functions are natural extensions of the Riemann zeta function. In this paper we first give a brief survey of Apéry-like series for some special values of the zeta function and certain $L$-functions. Then, we establish two theorems on transformations of certain kinds of congruences. Motivated by the results and based on our computation, we pose 48 new conjectural series (most of which involve harmonic numbers) for such special values and related constants. For example, we conjecture that \begin{align*}\sum_{k=1}^\infty\frac1{k^4\binom{2k}k}\bigg(\frac1k+\sum_{j=k}^{2k}\frac1j\bigg)=&\frac{11}9ζ(5), \\\sum_{k=1}^\infty\frac{(-1)^{k-1}}{k^3\binom{2k}k}\bigg(\frac1{5k^3}+\sum_{j=1}^{k}\frac1{j^3}\bigg)=&\frac{2}5ζ(3)^2, \end{align*} and $$\sum_{k=1}^\infty\frac{48^k}{k(2k-1)\binom{4k}{2k}\binom{2k}k}=\frac{15}2\sum_{k=1}^\infty\frac{(\frac k3)}{k^2},$$ where $(\frac k3)$ denotes the Legendre symbol.