Mihailova, Ana, and Neko Georgiev. “An Estimate of the Period of Periodical Solution of an Autonomous System of Differential Equations”. Ann. Sofia Univ. Fac. Math. and Inf. 91 (December 12, 1999): 107–114. Accessed November 22, 2024. https://annual.uni-sofia.bg/index.php/fmi/article/view/273.