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It is shown that for a definite cluster in the two-dimensional U= Hubbard model, there are states in any S^z sector, which are degenerate with the lowest state in the S₌₀ₗ^z sector. It is shown further that these states become the lowest ones in their sectors as the cluster becomes large enough, keeping the hole concentration constant. Then degenerate ground states of various S^z are obtained. The properties of the ground state of any S^zS₌₀ₗ^z can be found through those of the ground state of S₌₀ₗ^z.
Yang et al. (Sun,) studied this question.