FERROMAGNETIC GROUND-STATES OF THE HUBBARD-MODEL ON A COMPLETE GRAPH

We use group theory to derive the exact analytical expression of the f erromagnetic ground states of the Hubbard model on a complete graph fo r arbitrary lattice sites f and for arbitrary fillings N. We find that for t >0 and for N=f+1 the ground state is maximally ferromagnetic wi th total spin S=(f-1)/2. For N >f+1 the ground state is still ferromag netic but becomes degenerate with respect to S.