Crăciun, Maria. “Approximation Operators Constructed by Means of Sheffer Sequences”. Rev. Anal. Numér. Théor. Approx. 30, no. 2 (August 1, 2001): 135–150. Accessed April 15, 2025. https://ictp.acad.ro/jnaat/journal/article/view/2001-vol30-no2-art3.