1. Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables;Abramowitz,1992
2. Special Functions;Andrews,1999
3. Qualitative Theory of Planar Differential Systems;Artés,2006
4. On the number of limit cycles which appear with the variation of coefficients from an equilibrium position of focus or center type;Bautin;Transl. Am. Math. Soc.,1954
5. C. Chicone, Review in MathSciNet, Ref. 94h:58072.