Il’kiv VS, Ptashnik BI. Problems for partial differential equations with nonlocal conditions. Metric approach to the problem of small denominators. Ukr. Mat. Zhurn. [Internet]. 2006Dec.25 [cited 2024Apr.27];58(12):1624–1650. Available from: https://umj.imath.kiev.ua/index.php/umj/article/view/3560