  A connection of a variety of tight-binding models of noninteracting electrons
on a rectangular lattice in a magnetic field with theta functions is
established. A new spectrum generating symmetry is discovered which essentialy
reduces the problem of diagonalization of these models. Provided that one knows
one eigenvector at one point in the parameter space of the corresponding Harper
equation one knows an eigenfunction of the corresponding model in the whole
range of momentum singlet out by the Landau gauge.
