“…In (1.1), A is a possibly unbounded operator defined on a Banach space X and f : Z × X → X is given. This class of (N, λ)-periodic functions was introduced in the reference [6] as the discrete counterpart of the notion of (ω, c)-periodic functions [10], a notion that has been studied by various authors, see, e.g., [8,9,[15][16][17][18][19]26] and [30]. It is worth noting that class of (N, λ)-periodic functions contains the classes of discrete periodic (λ = 1), discrete anti-periodic (λ = −1), discrete Bloch-periodic (λ = e ikN , k ∈ Z fixed), and unbounded functions.…”