1. Altomare, F., Limit semigroups of Bernstein-Schnabl operators associated with positive projections, Ann. Sc. Norm. Sup. Pisa. Cl. Sci., (4) 19 (1989), no. 2, 259–279.
2. Altomare, F. and Campiti, M., Korovkin-type Approximation Theory and its Applications, De Gruyter Studies in Mathematics, 17, W. De Gruyter, 1994.
3. Campiti, M., A generalization of Stancu-Mühlbach operators, Constr. Approx., 7 (1991), 1–18.
4. Campiti, M., Limit semigroups of Stancu-Mühlbach operators associated with posive projections, Ann. Sc. Norm. Sup. Pisa, Cl. Sci., (4) 19 (1992), No. 1, 51–67.
5. Campiti, M. and Metafune, G., Approximation properties of recursively defined Bernstein-type operators. J. Approx. Theory, 87 (1996), No. 3, 243–269.