1.
Balslev E. Discreteness of the Spectrum of a Second Order Elliptic Differential Operator in $L^2 (\mathsf{R}^n)$. Math. Scand. [Internet]. 1968 Jun. 1 [cited 2024 Apr. 28];22:42-50. Available from: https://www.mscand.dk/article/view/10869