1. Andreev, A.: Investigation on the behaviour of the integral curves of a system of two differential equations in the neighborhood of a singular point. Translations Amer. Math. Soc. 8, 187–207 (1958)
2. Artés, J.C., Llibre, J., Schlomiuk, D., Vulpe, N.: Geometric configurations of singularities of planar polynomial differential systems - a global classification in the quadratic case. Springer, Cham (2020)
3. Berthier, M., Moussu, R.: Réversibilité et classification des centres nilpotents. Ann. Inst. Fourier (Grenoble) 44, 465–494 (1994)
4. Conti, R.: Uniformly isochronous centers of polynomial systems in $${\mathbb{R} }^2$$. Lecture Notes in Pure and Appl Math. 152, 21–31 (1994)
5. Conti, R.: Centers of planar polynomial systems a review. Le Matematiche 53(2), 207–240 (1998)