Double integrals and infinite products for some classical constants via analytic continuations of Lerch's transcendent

Abstract

The two-fold aim of the paper is to unify and generalize on the one hand the double integrals of Beukers for $ζ(2)$ and $ζ(3),$ and those of the second author for Euler's constant $γ$ and its alternating analog $\ln(4/π),$ and on the other hand the infinite products of the first author for $e$, and of the second author for $π$ and $e^γ.$ We obtain new double integral and infinite product representations of many classical constants, as well as a generalization to Lerch's transcendent of Hadjicostas's double integral formula for the Riemann zeta function, and logarithmic series for the digamma and Euler beta functions. The main tools are analytic continuations of Lerch's function, including Hasse's series. We also use Ramanujan's polylogarithm formula for the sum of a particular series involving harmonic numbers, and his relations between certain dilogarithm values.