The main purpose of this paper is to supply a proof of formulas for constru
cting a perturbative solution to the perturbed Kepler problem by utilizing
quaternion algebra of the Kustaanheimo-Stiefel formulation. The main advant
age of this approach is a removal, from the corresponding solution, of fast
oscillations (in the case of conservative forces) and small divisors (in t
he case of time-dependent forces).