Generalized Lambert series, Raabe's integral and a two-parameter generalization of Ramanujan's formula for $\zeta(2m+1)$

Abstract

A comprehensive study of the generalized Lambert series $\displaystyle\sum_{n=1}^{\infty}\frac{n^{N-2h}\exp{(-an^{N}x)}}{1-\exp{(-n^{N}x)}}, 0<a\leq 1,\ x>0$, $N\in\mathbb{N}$ and $h\in\mathbb{Z}$, is undertaken. Two of the general transformations of this series that we obtain here lead to two-parameter generalizations of Ramanujan's famous formula for $\zeta(2m+1)$, $m>0$ and the transformation formula for $\log\eta(z)$. Numerous important special cases of our transformations are derived. An identity relating $\zeta(2N+1), \zeta(4N+1),\cdots, \zeta(2Nm+1)$ is obtained for $N$ odd and $m\in\mathbb{N}$. Certain transcendence results of Zudilin- and Rivoal-type are obtained for odd zeta values and generalized Lambert series. A criterion for transcendence of $\zeta(2m+1)$ and a Zudilin-type result on irrationality of Euler's constant $\gamma$ are also given. New results analogous to those of Ramanujan and Klusch for $N$ even, and a transcendence result involving $\zeta\left(2m+1-\frac{1}{N}\right)$, are obtained.