1. Representation and approximation of functions by general singular integrals, IA, IB;Butzer,1960
2. Approximation with Kernels of Finite Oscillations;Hoff,1968
3. Linear operators and approximation theory;Korovkin,1959
4. Approximation of Functions;Lorentz,1966
5. On linear positive operators in approximation theory;Schurer,1965