represents the center of gravity of the spectral function. In our Figures the center of gravity corresponds to the crossing of the real axis by the line $y=\omega -\Omega _{
{\bf k}}$. So, if the quasiparticle peak is far from this point we would have a great incoherent part.

Quite different features demonstrate $A_p({\bf k}_3,\omega )$ in Fig.\[f3\] (c). The broad lowest peak is determined by the appearance of nonzero ${\rm
Im}\Sigma ({\bf k}_3,\omega )$ in the region where ${\rm Re}G^{-1}({\bf k}
_3,\omega )$ has no zeroes. Two broad additional peaks at $\omega 
\approx -1.6$ and $\omega \approx -1.05$ are formed due to the zero values of ${\rm Re} G^{-1}({\bf k}_3,\omega )$ close to these $\omega $. But at the same time the ${\rm Im}\Sigma ({\bf 
k}_3,\omega )$ is strong in these regions. Moreover, the maximum of ${\rm Im}\Sigma ({\bf k}_3,\omega )$ (near the point $\omega 
\approx -1.37$ ) determines the local minimum of $A_p({\bf k} 
_3,\omega )$ despite of the fact, that this point is close to the frequency value where ${\rm Re}G^{-1}({\bf k}_3,\omega )=0$. It is clear that it is impossible to treat any of $A_p({\bf 
k}_3,\omega )$ peaks as a quasiparticle one. Let us mind that the 