  In the weak coupling BCS-approximation normal impurities do not influence
superconducting T_{c} in significant manner in case of isotropic s-wave
pairing. However, in case of d-wave pairing these are strongly pair-breaking.
This fact is in rather strong contradiction with many experiments on disordered
high-T_{c} superconductors assuming the d-wave nature of pairing in these
systems. With the growth of electron attraction within the Cooper pair the
system smoothly crosses over from BCS-pairs to compact Boson picture of
superconductivity. As pairing strength grows and pairs become compact
significant deviations from universal Abrikosov-Gorkov dependence of T_{c} on
disorder appear in case of d-wave pairing with superconducting state becoming
more stable than in the weak coupling case. As high-T_{c} superconductors are
actually in the intermediate region with Cooper pairs size of the order of few
interatomic lengths, these results can explain the relative stability of d-wave
pairing under rather strong disordering.
