PetryshynR. I., & PokhylaO. M. (1997). Estimate of an error of the averaging method on a semiaxis for a multifrequency resonance system. Ukrains’kyi Matematychnyi Zhurnal, 49(5), 685–690. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/5049