  We have applied the Fast Fourier transform (FFT), which allows to compute
efficiently convolution sums, to solve the set of self-consistent T-matrix
equations to get the Green function of the two dimensional attractive-U Hubbard
modelbelow $T_c$, extending previous calculations of the same authors. Using a
constant order parameter $\Delta(T)$, we calculated $T_c$ as a function of
electron density and interaction strength $U$. These global results deviate
from the BCS behavior remarkably.
