Threshold singularities in the correlators of the one-dimensional models

We calculate the threshold singularities in one-dimensional models using the universal low-energy formfactors obtained in the framework of the non-linear Luttinger liquid model. We find the reason why the simplified picture of the impurity moving in the Luttinger liquid leads to the correct results. We obtain the prefactors of the singularities including their $k$- dependence at small $k<<p_F$.