BIRKELAND, Bent. A singular Sturm-Liouville problem treated by non-standard analysis. Mathematica Scandinavica, Berkeley, CA, United States, v. 47, p. 275–294, 1980. DOI: 10.7146/math.scand.a-11890. Disponível em: https://journals.msp.org/mscand/article/view/1757. Acesso em: 25 jul. 2026.