1. R. A. Adams, "Sobolev Spaces," Academic Press, New York, 1975.
2. V. V. Arestov, "Exact inequalities between norms of functions and their derivatives," Acta Scientiarum Mathematicarum, 33(1972), 243–267.
3. _____, "The best approximation to differentiation operators," Matematicheskie Zametkie, 1(1967), part 2; 149–154.
4. _____, "On the best approximation of the operators of differentiation and related questions," Approximation Theory, Proceeding of Conference in Poznan, Poland 1972. Reidel Publishing Co., Boston.
5. V. I. Berdyshev, "The best approximation in L(0, ∞) to the differentiation operator," Matematicheskie Zametki, 5(1971), 477–481.