1. Andronov A.A., Leontovich E.A., Gordon I.I., Maier A.G.: Theory of Bifurcations of Dynamic Systems on a Plane. Wiley, New York (1967)
2. Bautin N.N.: On the number of limit cycles which appear with the variation of coefficients from an equilibrium position of focus or center type. Mat. Sb. 30(72), 181–196 (1952)
3. Bautin N.N.: On the number of limit cycles which appear with the variation of coefficients from an equilibrium position of focus or center type. Am. Math. Soc. Transl. 100, 397–413 (1954)
4. Caubergh M.: Configurations of zeroes of analytic functions. C. R. Acad. Sci. Paris Sér. I Math. 333, 307–312 (2001)
5. Caubergh, M.: Limit cycles near vector fields of center type, PhD thesis. Limburgs Universitair Centrum, Belgium (2004)