Recke, L., and A. M. Samoilenko. “Conditions for Synchronization of One Oscillation System”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 57, no. 7, July 2005, pp. 922–945,
https://umj.imath.kiev.ua/index.php/umj/article/view/3654
.