Аннотация:In order to calculate a priori or a posteriori error estimates for solutions of an ill-posed operator equationwith an injective operator we need to describe a set of approximate solutions that contains an exact solution.After that we have to calculate a diameter of this set or maximal distance from a fixed approximate solution toany element of this set. I will describe three approaches for constructing error estimates and also their practicalapplications.This work was supported by the RFBR-NSCF grant 19-51-53005.