  A three-body scattering theory previously proposed by one of the present
authors is developed to be applied to the saturated ferromagnetic state in the
two-dimensional Hubbard model. The single-particle Green's function is
calculated by taking account of the multiple scattering between two electrons
and one hole. Several limiting cases are discussed and the relation to the
variational principle is examined. The importance of the three-body correlation
is demonstrated in comparison with the results of the ladder approximation. A
possible phase boundary for the Nagaoka ground state is presented for the
square lattice, which improves the previous variational results.
