ShevelоV. N., and ОdarіchO. N. “Some Questions of the Theory of Oscillation (non-Oscillation) of the Solutions of Second Order Delay Differential Equations ”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 23, no. 4, June 1971, pp. 507-15, https://umj.imath.kiev.ua/index.php/umj/article/view/8584.