KRENGEL, Ulrich. Ein Approximationslemma in komplexen Banachräumen mit positivem Kegel. Mathematica Scandinavica, Berkeley, CA, United States, v. 15, p. 69–78, 1964. DOI: 10.7146/math.scand.a-10729. Disponível em: https://journals.msp.org/mscand/article/view/2618. Acesso em: 25 jul. 2026.