https://scholarworks.utrgv.edu/mss_fac/814/
"A Necessary and Sufficient Condition for General Rayleigh–Liénard Syst" by Xiuli Cen, Hebai Chen...
The elementary center–focus problem is studied for a general separable planar analytic Rayleigh–Liénard system dxdt=y,dydt=−g(x)+f(x)ψ(y)y, where g(x), f(x),...
necessary and sufficientconditiongeneralsystxiuli