Lagoda, V. A., I. O. Parasyuk, and A. M. Samoilenko. 2012. “Lipschitzian Invariant Tori of Indefinite Monotone System”.
Ukrains’kyi Matematychnyi Zhurnal
64 (3): 363-83.
https://umj.imath.kiev.ua/index.php/umj/article/view/2582
.