A. Giorgilli et U. Locatelli, KOLMOGOROV THEOREM AND CLASSICAL PERTURBATION-THEORY, Zeitschrift fur angewandte Mathematik und Physik, 48(2), 1997, pp. 220-261
We reconsider the original proof of Kolmogorov's theorem in the light
of classical perturbation methods based on expansions in some paramete
r. With a careful analysis of the accumulation of small divisors we pr
ove that their effect is bounded by a geometrically increasing numeric
al sequence. This allows us to achieve the proof without using the so
called quadratic method.