Danilin, A. R.; Tanana, V. P. Necessary and sufficient conditions for convergence of approximations of linear ill-posed problems in Hilbert space. (Russian) Zbl 0562.65034 Zh. Vychisl. Mat. Mat. Fiz. 24, No. 5, 633—639 (1984). Reviewer: G.Vainikko MSC: 65J10 47A50