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, Note that, although we explicitly deal with the case of a 3-level atom in laser fields, the model is fairly general and can be applied to a wide range of physical systems

, Note that (?, ? k=a,b , ?) are not uniquely defined by these relations: the point is only to identify a common infinitesimal parameter ? for systematic expansion

, ? rel can easily be shown to be equal, up to second order terms in ?, to the average of ? over the period of its highest harmonic component ? 3 ? ?