We find a quantum group structure in two-dimensional motions of a
nonrelativistic electron in a uniform magnetic field and in a periodic
potential. The representation basis of the quantum algebra is composed of
wavefunctions of the system. The quantum group symmetry commutes with the
Hamiltonian and is relevant to the Landau level degeneracy. The deformation
parameter q of the quantum algebra turns out to be given by the fractional
filling factor ν=1/m (m odd integer).Comment: (revised version), 10 pages, OS-GE-36-9