  A variational principle enabling one to compute individual Floquet states of
a periodically time-dependent quantum system is formulated, and successfully
tested against the benchmark system provided by the analytically solvable model
of a linearly driven harmonic oscillator. The principle is particularly well
suited for tracing individual Floquet states through parameter space, and may
allow one to obtain Floquet states even for very high-dimensional systems which
cannot be treated by the known standard numerical methods.
