Elnazarov, A. A., and A. M. Samoilenko. 1999. “On Invariant Tori of Countable Systems of Differential Equations With Delay”. Ukrains’kyi Matematychnyi Zhurnal 51 (9), 1292–1295. https://umj.imath.kiev.ua/index.php/umj/article/view/4726.