Danilin, A. R.; Tanana, V. P. Necessary and sufficient conditions for the approximations of linear ill- posed problems in a Hilbert space to converge. (English) Zbl 0601.65037 U.S.S.R. Comput. Math. Math. Phys. 24, No. 3, 5-9 (1984). MSC: 65J10 47A50