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Abstract
The behaviour of a multilevel system in a strong field is considered, when the interaction energy is much greater than the energy of one quantum and the latter is of the same order as the level spacing of the system. In the case of a monochromatic field the zero-order approximation for quasi-energy is found to be the diagonal matrix element of the unperturbed hamiltonian over the interaction operator eigenfunctions. The asymptotes to the quasi-energy spectrum for the hamiltonian with arbitrary periodic time dependence are also obtained.