“…Necessary and sufficient conditions (in terms of the characteristic equation) for the oscillation of all solutions of first order neutral differential equations have been obtained by Sficas and Stavroulakis [22], Grove, Ladas and Meimaridou [13], Kulenovic, Ladas and Meimaridou [16], Grammatikopoulos, Sficas and Stavroulakis [10], Farrell [7], and Grammatikopoulos and Stavroulakis [11,12]. Necessary and sufficient conditions for the oscillation of higher order equations have been obtained by Ladas, Sficas and Stavroulakis [18], Ladas, Partheniadis and Sficas [17], and Wang [25].…”