BALSLEV, Erik. Discreteness of the Spectrum of a Second Order Elliptic Differential Operator in $L^2 (\mathsf{R}^n)$. Mathematica Scandinavica, Berkeley, CA, United States, v. 22, p. 42–50, 1968. DOI: 10.7146/math.scand.a-10869. Disponível em: https://journals.msp.org/mscand/article/view/2434. Acesso em: 25 jul. 2026.