  We report on the observation of microwave-induced resistance oscillations
associated with the fractional ratio n/m of the microwave irradiation frequency
to the cyclotron frequency for m up to 8 in a two-dimensional electron system
with high electron density. The features are quenched at high microwave
frequencies independent of the fractional order m. We analyze temperature,
power, and frequency dependencies of the magnetoresistance oscillations and
discuss them in connection with existing theories.
