Electron-electron interactions give rise to the correction, deltasigma^{int}(omega), to the ac magnetoconductivity, sigma(omega), of a clean 2D electron gas that is periodic in omega_c^{-1}, where omega_c is the cyclotron frequency. Unlike conventional harmonics of the cyclotron resonance, which are periodic with omega, this correction is periodic with omega^{3/2}. Oscillations in deltasigma^{int}(omega) develop at low magnetic fields, omega_cllomega, when the conventional harmonics are suppressed by the disorder. Their origin is a {em double} backscattering of an electron from the impurity-induced Friedel oscillations. During the time simomega^{-1} between the two backscattering events the electron travels only a {em small portion} of the Larmour circle.
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