Ivanov , A. F., and P. Marushiyak. “On Oscilation and Asymptotic Behaviour of Solutions of One System of Differential-Functional Equations of Neutral Type”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 44, no. 8, Sept. 1992, pp. 1044-9,
https://umj.imath.kiev.ua/index.php/umj/article/view/8032.