Il’kivV. S., and PtashnikB. 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. https://umj.imath.kiev.ua/index.php/umj/article/view/3560.