The oscillation theory of neutral differential equations has been investigated extensively in the last decades and increasingly attract much interest due to the valuable applications of these type of equations in many fields; see [5–8]. This theory, generally, investigates the problem of existence of an infinite number of zeros of all solutions; see [1–3,6] for the recent
advances in oscillation theory.
A crucial question in the theory is to determine the zeros locations for each solution of a given equation. This problem, for functional differential equations, did not receive the deserved interest, although it gives more insight into the properties of the solutions which means better understanding for some phenomenon that can be modeled by these equations