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

Balslev, E. (1968). Discreteness of the Spectrum of a Second Order Elliptic Differential Operator in $L^2 (\mathsf{R}^n)$. MATHEMATICA SCANDINAVICA, 22, 42–50. https://doi.org/10.7146/math.scand.a-10869