  We apply a diagrammatic expansion method around the atomic limit (U >> t) for
the U-t-t' Hubbard model at half filling and finite temperature by means of a
continued fraction representation of the one-particle Green's function. From
the analysis of the spectral function A(\vec{k},\omega) we find an energy
dispersion relation with a (cos k_x-cos k_y)^2 modulation of the energy gap in
the insulating phase. This anisotropy is compared with experimental ARPES
results on insulating cuprates.
