CRENSHAW, J. A. Extreme Positive Linear Operators. MATHEMATICA SCANDINAVICA, [S. l.], v. 25, p. 195–217, 1969. DOI: 10.7146/math.scand.a-10957. Disponível em: https://www.mscand.dk/article/view/10957. Acesso em: 3 may. 2024.