1. P. L. Butzer, R. J. Nessel and W. Trebels, ‘On Summation Processes of Fourier Expansions in Banach Spaces, I: Comparison Theorems, II: Saturation Theorems’, Tôhoku Math. J. 24 (1972) 127–140, 551-559.
2. J. Favard, ‘On the Comparison of the Processes of Summation’, SIAM J. Numer. Anal Ser. B 1 (1964) 38–52.
3. G. Freud, ‘On an Inequality of Markov Type’, Soviet Math. Dokl. 12 (1971) 570–573.
4. G. Freud and S. Knapowski, ‘On Linear Processes of Approximation III’, Studia Math. 25 (1965) 374–383.
5. E. Görlich, R. J. Nessel and W. Trebels, ‘Bernstein-Type Inequalities for Families of Multiplier Operators in Banach Spaces with Cesàro Decompositions, I: General Theory, II: Applications’, Acta Sci. Math. (Szeged) 34 (1973) 121–130, 36 (1974) 39-48.