EHRLING, G. On a type of eigenvalue problems for certain elliptic differential operators. MATHEMATICA SCANDINAVICA, [S. l.], v. 2, p. 266–284, 1954. DOI: 10.7146/math.scand.a-10414. Disponível em: https://www.mscand.dk/article/view/10414. Acesso em: 2 may. 2024.