  The diagram technique for the one-band Hubbard model is formulated for the
case of moderate to strong Hubbard repulsion. The expansion in powers of the
hopping constant is expressed in terms of site cumulants of electron creation
and annihilation operators. For Green's function an equation of the Larkin type
is derived and solved in a one-loop approximation for the case of two
dimensions and nearest-neighbor hopping. With decreasing the electron
concentration in addition to the four bands observed at half-filling a narrow
band arises near the Fermi level. The dispersion of the new band, its bandwidth
and the variation with filling are close to those of the spin-polaron band in
the t-J model.
