Abstract
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The interaction of an electromagnetic wave with a two-dimensional electron gas subjected to a magnetic field and a weak periodic potential is investigated. The electron-impurity interaction is included to the lowest order. The periodic modulation of the Landau states has a profound effect on the optical conductivity. Simultaneous excitation of an electron-hole pair contributes significantly to the absorption around and below the cyclotron frequency. This is in contrast to a uniform electron gas under a perpendicular magnetic field where electron-hole pair excitations are absent.