BALSLEV, E. Discreteness of the Spectrum of a Second Order Elliptic Differential Operator in $L^2 (\mathsf{R}^n)$. MATHEMATICA SCANDINAVICA, [S. l.], v. 22, p. 42–50, 1968. DOI: 10.7146/math.scand.a-10869. Disponível em: https://www.mscand.dk/article/view/10869. Acesso em: 28 apr. 2024.