Perturbation theoryi si ntroducedb ym eans of models borrowedf romC elestial Mechanics, namely the two-bodyand three-body problems.Suchmodels allowo ne to introduce in as imple way the concepts of integrable and nearlyintegrable systems, whichcan be conveniently investigated usingHamiltonian formalism.Afterdiscussingthe problemofthe convergenceofperturbativeseries expansions, we introduce the basic notions of KAM theory,w hicha llows (under quite general assumptions) to state the persistenceo fi nvariantt ori. Thev aluea tw hichs uchs urfacesb reak-down canbed etermined by means of numerical algorithms. Among the others, we reviewt hree methods to which we refera sG reene, Padéa nd Lyapunov. We presents ome concretea pplications to discrete models of the three differentt echniques, in order to provide complementary informationa bout the break-down of invariant tori.
1I ntroductionThe dynamics of theplanets and satellites is ruled by Newton's law, according to whicht he gravitational force is proportional to the product of them asses of theinteracting bodiesand it is inversely proportional to thesquare of their distance.T he description of thet rajectoriess panned by the celestial bodies starts with the simplest model in whichone considers only theattraction exerted by the Sun, neglecting all contributions duet oo ther planets or satellites. Such model is known as the two-bodyp roblem andi ti sf ullyd escribed by Kepler's laws, according to whicht he motion is represented by ac onic.C onsider,f or example,t he trajectory of an asteroid moving on an elliptico rbit around the Sun. In the two-bodya pproximation thes emimajor axis and the eccentricityo ft he ellipse arefi xed in time. However, suche xample represents