1. de Gruyter Studies in Mathematics;F Altomare,1994
2. Butzer, P.L.: On the singular integral of de la Vallée-Poussin. Arch. Math. 7, 295–309 (1956)
3. Butzer, P.L.: Representation and approximation of functions by general singular integrals. IA, IB. Nederl. Akad. Wetensch. Proc. Ser. A63 (=Indag. Math. 22, 1–24 (1960))
4. Butzer, P.L., Nessel, R.J.: Fourier Analysis and Approximation, vol. 1, One-Dimensional Theory. Academic Press, New York, London (1971)
5. Butzer, P.L., Trebels, W.: Opérateurs de Gauss–Weiesrtrass et de Cauchy–Poisson et conditions lipschitzienne dans
$$L^{1}(E_n)$$
L
1
(
E
n
)
. C. R. Acad. Sci. Paris sér. A-B. 268, 700–703 (1969)