“…Here we present a rather general and effective method for approximating the solutions (1), as well as a number of useful bounds or qualitative properties of the latter, that we have not been able to find in the very broad literature (see e.g. the already cited references, the more mathematical ones [7,28,16,23,21,34,32,27,30,35,10,29,17,2], and the references therein) on the subject. Our key observation is that, passing (section 4.1) from the canonical coordinates q, p to the angle-action variables ψ, I = H/ω, via…”