1. Representation and approximation of functions by general singular integrals;Butzer,1960
2. Trigonometric convolution operators with kernels having alternating signs and their degree of convergence;Butzer;Jahresbericht der Deutschen Matrimatiker Verinigung,1967
3. Approximation with Kernels of Finite Oscillations;Hoff,1968
4. Approximation with kernels of finite oscillations I. Convergence;Hoff;J. Approximation Theory,1970
5. P. P. Korovkin, “Linear Operators and Approximation Theory,” Hindustan Publishing Corp., New Delhi, India.