  Working in the effective-mass approximation, we apply a powerful convergent
perturbative technique of Turbiner's to the calculation of the ground state
energy and the wave function of an exciton confined to a three-dimensional
parabolic quantum dot. Unlike the usual Rayleigh-Schrodinger perturbation
theory, Turbiner's approach works well even in the regime of strong coupling
and does not require the knowledge of the full solution to the undisturbed
problem. The second-order convergent calculation presented below is in
excellent agreement with the results of exact numerical simulations for a wide
range of system's confinement parameters.
