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, Vol. 49, no. 5, May 1997, pp. 685–690, https://umj.imath.kiev.ua/index.php/umj/article/view/5049.