“…One of the classical tools in the classification of all trajectories of a dynamical system is to find first integrals. For more details about first integral see for instance [3,5,8,11,12,14] , see the references quoted in those articles. We recall that in the phase plane, a limit cycle of system (1) is an isolated periodic orbit in the set of all periodic orbits of system (1) System (1) is integrable on an open set Ω of R 2 if there exists a non constant C 1 function H : Ω → R, called a first integral of the system on Ω , which is constant on the trajectories of the system (1) contained in Ω, i.e.…”