Nagaoka State and Degeneracy in the U=infinity Hubbard Model
  The U=infinity Hubbard model on a hypercubic lattice with exactly one hole is studied. It is seen that a degeneracy can occur in the maximal-spin sector. It is proved variationally that the Nagaoka (maximal spin) state is not a ground state, if degeneracy exists. We give an explicit construction of a variational state, an incommensurate spin-wave state, with energy lower than that of the Nagaoka state. For the case of more than one hole, a few examples are discussed where a degeneracy exists. 
V. Subrahmanyam and Fajian Shi (Max-Planck, Dresden)