POPA, E. C. Sheffer polynomials and approximation operators. Tamkang Journal of Mathematics, [S. l.], v. 34, n. 2, p. 117–128, 2003. DOI: 10.5556/j.tkjm.34.2003.258. Disponível em: https://journals.math.tku.edu.tw/index.php/TKJM/article/view/258. Acesso em: 8 may. 2024.