ElnazarovA. A., and SamoilenkoA. M. “On Invariant Tori of Countable Systems of Differential Equations With Delay”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 51, no. 9, Sept. 1999, pp. 1292–1295, https://umj.imath.kiev.ua/index.php/umj/article/view/4726.