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), pp. 1624–1650. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/3560 (Accessed: 27July2024).