1. C. Balázs, Approximation inL 2-space by interpolatory type operators,Acta Math. Acad. Sci. Hungar.,35 (1980), 403–408.
2. H. Berens and R. A. DeVore, Quantitative Korovkin theorems forL p -spaces, inApproximation Theory II (ed. by G. G. Lorentz, C. K. Chui and L. L. Schumaker), pp. 289–298. Academic Press (New York, 1976)
3. E. Egerváry and P. Turán, Notes on interpolation.V: On the stability of interpolation,Acta Math. Acad. Sci. Hungar.,9 (1958), 259–267.
4. G. Grünwald, On the theory of interpolation,Acta Math.,75 (1942), 219–245.
5. H. B. Knoop,Hermite-Fejér-interpolation mit Randbedingungen, Habilitationsschrift, Universität Duisburg (1981).