1. On the impossibility of constructing a linear polynomial operator furnishing an approximation of the order of the best approximation;Berman, D. L.;Dokl. Akad. Nauk SSSR,1958
2. 2. M. Golomb, Lectures on theory of approximation, Argonne National Laboratory, Argonne, Illinois, 1962.
3. Prentice-Hall Series in Modern Analysis;Hoffman, Kenneth,1962