“…where 0 < ε ≪ 1 is the singular perturbation parameter, δ and η are called the delay and the advance parameters, sometimes referred to as negative shift and positive shift, respectively, as in [16,17]; precise assumptions will be given in the next section. This type of differential equation plays an important role in the mathematical modeling of various practical phenomena, for instance, variational problems in control theory [6], description of the so-called human pupil-light reflex [19], evolutionary biology [34], and a variety of models for physiological processes or diseases [21].…”