Zadiraka К. V., and Lоs G. A. “On the Stability and Instability of Solutions of Linear Differential Equations of the $n$th Order With Periodic Coefficients”. Ukrains’kyi Matematychnyi Zhurnal, vol. 18, no. 6, Oct. 1966, pp. 117-22, https://umj.imath.kiev.ua/index.php/umj/article/view/8467.