Discreteness of the Spectrum of a Second Order Elliptic Differential Operator in $L^2 (\mathsf{R}^n)$.

E. Balslev, “Discreteness of the Spectrum of a Second Order Elliptic Differential Operator in $L^2 (\mathsf{R}^n)$”., Math. Scand., vol. 22, pp. 42–50, Jun. 1968, doi: 10.7146/math.scand.a-10869.