Mihailova, A. and Georgiev, N. (1999) “An estimate of the period of periodical solution of an autonomous system of differential equations”, Ann. Sofia Univ. Fac. Math. and Inf, 91, pp. 107–114. Available at: https://ftl5.uni-sofia.bg/index.php/fmi/article/view/273 (Accessed: 21 November 2024).