EXACT RESULTS ON FERROMAGNETISM IN CORRELATED ELECTRON-SYSTEMS

Exact results, both old and new, on the stability of saturated ferromagnetism in the Hubbard model and its extensions are discussed. For the Hubbard model with all nearest-neighbor interactions a new set of rigorous criteria, valid ed half filling, is derived and a generalization of Nagaoka's theorem to finite interactions is presented. Of all coupling parameters only the on-side and the exchange interaction are found do be essential. Al strong coupling comparison with the effective t-J model is made. Further extensions, e.g. to the band-degenerate case, are discussed.