Shchegolev, S. A. 1998. “On One Class of Solutions of a Countable Quasilinear System of Differential Equations With Slowly Varying Parameters”. Ukrains’kyi Matematychnyi Zhurnal 50 (8), 1121–1128. https://umj.imath.kiev.ua/index.php/umj/article/view/4861.