The planetary problem is the n -body problem in the case that one of the masses is much larger than all the others. A prototypical example of a planetary problem is the Sun– Jupiter – Saturn system, where the mass of the Sun is about 1000 times larger than the masses of Jupiter or Saturn. An approximate solution to the problem is to decompose it into n − 1 pairs of star–planet Kepler problems, treating interactions among the planets as perturbations. Perturbative approximation works well as long as there are no orbital resonances in the system, that is none of the ratios of unperturbed Kepler frequencies is a rational number. Resonances appear as small denominators in the expansion.