  We study spectral properties of a spinless quantum particle confined to an
infinite planar layer with hard walls which interacts with a periodic lattice
of point perturbations and a homogeneous magnetic field perpendicular to the
layer. It is supposed that the lattice cell contains a finite number of
impurities and the flux through the cell is rational. Using the Landau-Zak
transformation, we convert the problem into investigation of the corresponding
fiber operators which is performed by means of Krein's formula. This yields an
explicit description of the spectral bands which may be absolutely continuous
or degenerate, depending on the parameters of the model.
