Il’kiv, V. S., & Ptashnik, B. I. (2006). Problems for partial differential equations with nonlocal conditions. Metric approach to the problem of small denominators. Ukrains’kyi Matematychnyi Zhurnal, 58(12), 1624–1650. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/3560