Periodic orbits from second order perturbation via rational trigonometric integrals

• Along the number, we study the shape and the period of the perturbed periodic orbits.
• First a second order studies are given for concrete planar vector fields.
• Same technique apply simultaneously to Abel equations and planar vector fields.
• The computations share the same integrals of rational trigonometric functions.
• Isochronous quadratic systems have at most 2 limit cycles up to second order.

The second order Poincaré-Pontryagin-Melnikov perturbation theory is used in this paper to study the number of bifurcated periodic orbits from certain centers. This approach also allows us to give the shape and the period up to the first order. We address these problems for some classes of Abel differential equations and quadratic isochronous vector fields in the plane. We prove that two is the maximum number of hyperbolic periodic orbits bifurcating from the isochronous quadratic centers with a birational linearization under quadratic perturbations of second order. In particular the configurations (2,0)(2,0) and (1,1)(1,1) are realizable when two centers are perturbed simultaneously. The required computations show that all the considered families share the same iterated rational trigonometric integrals.