We discuss the universal nature of relaxation in isolated many-body quantumsystems subjected to global and strong periodic driving. Our rigorous Floquetanalysis shows that the energy of the system remains almost constant up to anexponentially long time in frequency for arbitrary initial states and that aneffective Hamiltonian obtained by a truncation of the Floquet-Magnus expansionis a quasi-conserved quantity for long time scale. These two general propertieslead to intriguing classification on the initial stage of relaxation, one ofwhich is similar to the prethermalization phenomenon in quasi-integrablesystems.
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