EHRLING, Gunnar. On a type of eigenvalue problems for certain elliptic differential operators. Mathematica Scandinavica, Berkeley, CA, United States, v. 2, p. 266–284, 1954. DOI: 10.7146/math.scand.a-10414. Disponível em: https://journals.msp.org/mscand/article/view/2908. Acesso em: 8 sep. 2026.