Zadiraka К. V., and Lоs G. A. 1966. “On the Stability and Instability of Solutions of Linear Differential Equations of the $n$th Order With Periodic Coefficients”. Ukrains’kyi Matematychnyi Zhurnal 18 (6): 117-22. https://umj.imath.kiev.ua/index.php/umj/article/view/8467.