Variational principles for the scattering problem in the presence of a strong electromagnetic wave

A stationary approach to the problem of scattering of a nonrelativistic particle on a potential in the presence of a strong monochromatic electromagnetic wave is formulated in the language of quasienergy states. Functionals that are stationary on exact solutions of the problem are constructed. The variational principle for the scattering amplitude and Schwinger's variational principle are generalized.