Analytic properties of the parametric harmonic zeta function, with applications to harmonic Stieltjes constants

Abstract

This paper investigates the analytic structure of the parametric harmonic zeta function $ζ_{H}\left(s, a, b\right)$, a Dirichlet series associated with generalized harmonic numbers. We establish its meromorphic continuation to the complex plane and determine the residues at all of its poles. We then derive a Taylor expansion for $ζ_{H}\left(s, a, b+t\right)$, leading to harmonic analogues and extensions of classical identities due to Landau, Singh-Verma, and Srivastava. We further develop a systematic theory of the associated harmonic Stieltjes constants by deriving explicit formulas, including previously unknown cases, together with a limit representation for the first-order constants. Finally, we construct the harmonic analogue of the classical digamma function, establish its principal analytic properties and its connection with the harmonic Stieltjes constants, and obtain Raabe-type formulas for the parametric harmonic zeta function, the harmonic Stieltjes constants, and the harmonic digamma function.