Some Global Properties of the Attractive Hubbard Model in the   Superconducting Phase: T-Matrix Approximation in 2D
  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. 
S. Schafroth (Uni-Zurich), J.J. Rodriguez (UFF), H. Beck (Neuchatel)