In the attractive Hubbard Model (and some extended versions of it), the ground state is proved to have spin angular momentum S=0 for every (even) electron filling. In the repulsive case, and with a bipartite lattice and a half-filled band, the ground state has S=(1/2∥B‖-‖A‖‖, where ‖B‖ (‖A‖) is the number of sites in the B (A) sublattice. In both cases the ground state is unique. The second theorem confirms an old, unproved conjecture in the ‖B‖=‖A‖ case and yields, with ‖B‖≠‖A‖, the first provable example of itinerant-electron ferromagnetism. The theorems hold in all dimensions without even the necessity of a periodic lattice structure.

• Received 12 December 1988

DOI:https://doi.org/10.1103/PhysRevLett.62.1201