Akymenko, A. M., & Yakovets, V. P. (2002). On Periodic Solutions of Degenerate Singularly Perturbed Linear Systems with Multiple Elementary Divisor. Ukrains’kyi Matematychnyi Zhurnal, 54(10), 1403-1415. https://umj.imath.kiev.ua/index.php/umj/article/view/4178