  Hofstadter's diagram, or the energy spectrum against the magnetic field in
tight-binding systems, is obtained for the models having flat (dispersionless)
one-electron band(s) that have originally been proposed for itinerant spin
ferromagnetism. Magnetic fields preserve those flat bands that arise from a
topological reason, while dispersions emerge in a singular manner for the flat
bands arising from interference, implying an anomalous orbital magnetism.
