“…In fact the Abel equations are of the form ṙ = a(θ) + b(θ)r + c(θ)r 2 + d(θ)r 3 , and they appeared by the first time in the works of Niels Henryk Abel, see [7]. Today there are more than 1400 papers in MathSciNet with the name "Abel equation" in their tittle, see for instance the papers [1,2,3,5,6,8] for results on centers in the Abel equations and the references quoted therein.…”