Monotonicity Properties of the Hurwitz Zeta Function

Abstract

AbstractLetbe the Hurwitz zeta function and letwhereα, β> 1 anda,b> 0 are real numbers. We prove: (i) The functionQis decreasing on (0, ∞) iffαa−βb≥ max(a−b, 0). (ii)Qis increasing on (0, ∞) iffαa−βb≤ min(a−b, 0). An application of part (i) reveals that for allx> 0 the functions⟼ [(s− 1)ζ(s,x)]1/(s−1)is decreasing on (1, ∞). This settles a conjecture of Bastien and Rogalski.