[1]
Balslev, E. 1968. Discreteness of the Spectrum of a Second Order Elliptic Differential Operator in $L^2 (\mathsf{R}^n)$. MATHEMATICA SCANDINAVICA. 22, (Jun. 1968), 42–50. DOI:https://doi.org/10.7146/math.scand.a-10869.