Density of States of a d-wave Superconductor in the Presence of Strong   Impurity Scatterers: a Non Perturbative Result
  We present a method to compute the density of states induced by N non magnetic impurities in a d-wave superconductor, in the unitary limit of very strong scatering centers. For frequencies very small as compared to the superconducting gap ($\omega \ll \Delta_0$) the additional density of states has the leading divergence ${\displaystyle \delta \rho (\omega) \simeq n_i / [ | 2 \omega | (\ln^2 |\omega/\Delta_0| + (\pi/2)^2)]}$. This result is non perturbative. 
Catherine Pepin and Patrick A. Lee