Evtukhov, V. M., and N. S. Vasileva. “Conditions of Oscillatory or Nonoscillatory Nature of Solutions for a Class of Second-Order Semilinear Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, no. 4, Apr. 2007, pp. 458–466, https://umj.imath.kiev.ua/index.php/umj/article/view/3320.