1. S. Bochner (1932), Vorlesungen über Fouriersche Integrale, Akademische Verlagsgesellschaft, Leipzig.
2. J. Boman (1971), Saturation problems and distribution theory, published in: Topics in Approximation Theory, Lecture Notes in Mathematics Vol. 187 (Author: H.S. Shapiro), Springer-Verlag, Berlin—Heidelberg—New York, 1971, Appendix I, 249–266.
3. P.L. Butzer and R.J. Nessel (1966), Contributions to the theory of saturation for singular integrals in several variables I, General Theory, Indag. Math. 28, Ser. A, 515–531.
4. P.L. Butzer and R.J. Nessel (1971), Fourier Analysis and Approximation, Vol. I, One-Dimensional Theory, Birkhäuser Verlag, Basel—Stuttgart.
5. R.A. DeVore (1972), The Approximation of Continuous Functions by Positive Linear Operators, Lecture Notes in Mathematics Vol. 293, Springer-Verlag, Berlin—Heidelberg—New York.