Weak focus, limit cycles, and bifurcations for bounded quadratic systems
This paper is an interesting contribution to the qualitative study of two-dimensional autonomous ordinary differential equations. For quadratic nonlinearities, the global phase portrait (i.e. taking into account the Poincaré's equator), and its qualitative changes when the parameters equation vary, are studied. It is a pity that the authors do not refer in this field to the fundamental and ancient contribution of the ``Andronov School''. Indeed, the books and papers on the qualitative theory of dynamical systems of Andronov, Gordon, Mayer, Leontovitch, Bautin, have considered equivalent (but not the same) problems from 1939 to 1976. It is particularly the case of the excellent book ``Methods and applications of the qualitative study of the dynamical systems in the plane'', Nauka, Moscow, 1976, written by \textit{N. N. Bautin} and \textit{E. A. Leontovitch}.
two-dimensional autonomous ordinary differential equations
scientific article; zbMATH DE number 714693