“…e definitions and basic properties of (ω, c)-periodic and (ω, c)-pseudo-periodic functions were introduced and analyzed by Alvarez, Gómez, and Pinto in [108,109], motivated by some known results regarding the qualitative properties of the solution to Mathieu's linear differential equation y″(t) + [a − 2q cos 2 t]y(t) � 0, arising in modeling of railroad rails and seasonally forced population dynamics (ω > 0 and c ∈ C\ 0 { }). e linear delayed equations can have (ω, c)-periodic solutions as well (see, e.g., [109], Example 2.5).…”