  We analyzed the magnetoresistivity of a two-dimensional electron system
excited by microwave radiation in a regime of high intensities and low
frequencies. In such a regime, recent experiments show that different features
appear in the magnetoresistivity response which suggest an anharmonic behavior.
These features consist mainly in distorted oscillations and new resonance peaks
at the subharmonics of the cyclotron frequency. We follow the model of
microwave-driven electron orbits motion which become anharmonic when the ratio
of microwave intensity to microwave frequency is large enough.
