Petryshyn, R. I., and O. M. Pokhyla. “Estimate of an Error of the Averaging Method on a Semiaxis for a Multifrequency Resonance System”. Ukrains’kyi Matematychnyi Zhurnal 49, no. 5 (May 25, 1997): 685–690. Accessed April 11, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5049.