Akymenko, A. M., and V. P. Yakovets. 2002. “On Periodic Solutions of Degenerate Singularly Perturbed Linear Systems With Multiple Elementary Divisor”. Ukrains’kyi Matematychnyi Zhurnal 54 (10): 1403-15. https://umj.imath.kiev.ua/index.php/umj/article/view/4178.