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 2026 Aug. 14];22:42-50. Available from: https://tidsskrift.dk/math/article/view/10869