“…Other references relevant to the topic are, for instance, the article by Henríquez and Hernández,
18 which was devoted to the analysis of the approximate controllability of control systems given by second‐order semilinear functional differential equations with infinite delay; the work by Sakthivel et al,
19 which is focused on the study of the exact controllability of certain second‐order nonlinear impulsive control differential systems; the study of Shoukaku,
20 about the oscillatory behavior of certain hyperbolic equations with continuous distributed deviating arguments; or Liu and Huang,
21 where the coincidence degree theory was applied to obtain results on the existence and uniqueness of
‐periodic solutions for a class of second‐order neutral functional differential equations. The same approach, coincidence degree theory, was applied in
22 to analyze the existence of periodic solutions for higher‐order differential equations with deviating arguments. In this last case, the dependence on the different derivatives
is linear.…”