Aim of this work is the study of differential equations governing
non--dissipative non--linear oscillators; these arise in different physical
models such as the treatment of relativistic oscillators, up to generalizations
to Duffing's relativistic oscillators and in non--relativistic models which
deals with cables with an attached midpoint mass, or some harmonic Duffing
oscillators.Comment: 15 pages, 9 figures, 3 table