Er’omenko, V. O., and A. M. Aliluiko. 2010. “Quasiperiodic Solutions of Degenerate Linear Systems of Second-Order Ordinary Differential Equations”.
Ukrains’kyi Matematychnyi Zhurnal 62 (6): 773–783.
https://umj.imath.kiev.ua/index.php/umj/article/view/2908.