Averaging theory at any order for computing periodic orbits

We provide an explicit expression for the solutions of the perturbed first order differential equations. Using it we give an averaging theory at any order in ε for the following two kinds of analytic differential equations dx/dθ =Xk≥1εk Fk(θ, x), dx/dθ =Xk≥0εk Fk(θ, x). We apply these results for studying the limit cycles of planar polynomial differential systems after passing them to polar coordinates