  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, nearest-neighbor hopping and half-filling. The obtained four-band
structure of the spectrum and the shapes of the spectral function are close to
those observed in Monte Carlo calculations. It is shown that the maxima forming
the bands are of a dissimilar origin in different regions of the Brillouin
zone.
