This paper is concerned with a neutral differential equation with four constant coefficients, one delay and one advancement. By means of the theory of envelopes, we consider all possible values of the parameters involved in the equation and obtain a complete set of necessary and sufficient conditions for all solutions to be oscillatory.

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This article is concerned with a linear functional difference equation with three constant coefficients and three delays. By means of the theory of envelopes, we consider all possible values of the parameters involved in the equation and we obtain a complete set of necessary and sufficient conditions for all solutions to be oscillatory. ᮊ 1998 Academic Press