1. Andronov, A.A., Leontovich, E.A., Gordon, I.I., Maĭer, A.G.: Theory of Bifurcations of Dynamic Systems on a Plane. Halsted Press [A division of John Wiley & Sons], New York. (Israel Program for Scientific Translations, Jerusalem–London, 1973. Translated from the Russian)
2. Bai, J., Liu, Y.: A class of planar degree n (even number) polynomial systems with a fine focus of order $$n^2-n$$ n 2 - n . Chin. Sci. Bull. 12, 1063–1065 (1992)
3. Bautin, N.: On the number of limit cycles appearing with variation of the coefficients from an equilibrium state of the type of a focus or a center. Mat. Sb. 30, 181–196 (1952)
4. Bautin, N.: On the number of limit cycles appearing with variation of the coefficients from an equilibrium state of the type of a focus or a center. Am. Math. Soc. Transl. 100, 397–413 (1954)
5. Bondar, Y., Sadovskii, A.: On a Zoladek theorem. Differ. Equ. 44, 263–265 (2008)