Quadratic and cubic systems with degenerate infinity
The authors obtain an affine classification of quadratic systems with degenerate infinity in eight types and prove that they present 13 phase portraits on the Poincaré sphere. It seems that they did not know the classical paper of \textit{W. Buchel} [Mitt. Math. Ges. Hamburg 4, No. 4, 133-169 (1904)]. For cubic systems with degenerate infinity the authors show that there are examples of such systems with at least three concentric limit cycles; they also give a criterion to ensure that in some cases the number of such limit cycles is less than or equal to two.
affine classification of quadratic systems with degenerate infinity
scientific article; zbMATH DE number 902484