  In two-dimensional (2D) electron systems in a magnetic field, the Coulomb
interaction among charge carriers, under Landau quantization, essentially
governs a variety of many-body phenomena while there are also phenomena, such
as the (integer) quantum Hall effect, that appear unaffected by the
interaction. It is pointed out that the response of 2D electrons to
spatially-uniform potentials and fields enjoys a long-wavelength gauge
symmetry, associated with cyclotron motion of electrons, that leaves the
Coulomb interaction invariant and that thus naturally explains why cyclotron
resonance (as implied by Kohn's theorem) and the quantized Hall conductance
appear insensitive to the interaction. It is discussed, in the light of this
new long-wavelength gauge symmetry, how Dirac electrons in graphene and
conventional 2D electrons differ in cyclotron-resonance characteristics and the
quantum Hall effect.
