  The influence of chemical potential oscillations on the magnetization
oscillations in two-dimensional multiband metals is investigated. In a first
part, the analytical derivation of Alexandrov and Bratkovsky [Phys. Rev. B
\textbf{63}, 033105 (2001)] of magnetic quantum oscillations in two-dimensional
multiband metals with a fixed number of electrons is shown to be mathematically
incorrect ; the chemical potential oscillations appearing in the arguments of
the Fourier components were not taken into account. In a second part, we derive
an approximative Fourier series of the magnetization oscillations in the regime
of small chemical potential oscillations.
  The main result is that combination frequencies with significant amplitudes
are found if the individual band frequencies differ significantly.
