Golets, V. L. “On the Problem of Disturbance of Stable Invariant Tore of the Dynamic System ”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 23, no. 1, Dec. 1970, pp. 130-7,
https://umj.imath.kiev.ua/index.php/umj/article/view/8495
.