For a positive irrational number α, we study the ordinary Dirichlet series ζ α (s) = n≥1 ⌊αn⌋ −s and S α (s) = n≥1 (⌈αn⌉ − ⌈α(n − 1)⌉)n −s . We prove relations between them and J α (s) = n≥1 {αn} − 1 2 n −s . Motivated by the previous work of Hardy and Littlewood, Hecke and others regarding J α , we show that ζ α and S α can be continued analytically beyond the imaginary axis except for a simple pole at s = 1. Based on the latter results, we also prove that the series ζ α (s; β) = n≥0 (⌊αn⌋ + β) −s can be continued analytically beyond the imaginary axis except for a simple pole at s = 1.