Danilov, V.Y., Martynyuk, D.I. and Pan'kov, V.G. (1996) “Second bogolyubov theorem for systems of difference equations”, Ukrains’kyi Matematychnyi Zhurnal, 48(4), pp. 464–475. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/5311 (Accessed: 5 December 2025).