Danilov, V. Y., Martynyuk, D. I., & Pan’kov, V. G. (1996). Second bogolyubov theorem for systems of difference equations. Ukrains’kyi Matematychnyi Zhurnal, 48(4), 464-475. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/5311