MitropolskyY. A., and FоdchukV. I. 1966. “Asymptotic Methods of Nonlinear Mechanics As Applied to Nonlinear Differential Equations With Delayed Argument”. Ukrains’kyi Matematychnyi Zhurnal 18 (3), 65-84. https://umj.imath.kiev.ua/index.php/umj/article/view/8738.