A general class of centers for the Poincaré problem
For the system \[ {dx\over dt}= -y- p(x,y),\quad {dy\over dt}= x+ q(x,y), \] where \(p\), \(q\) are homogeneous polynomials of degree \(n\geq 2\), it is obtained sufficient center conditions in case of \(n= 4,5,6,7,8\) are obtained.
scientific article; zbMATH DE number 5577095