Criteria for discrete spectrum of singular selfadjoint differential operators

L. W. Rollins
1972 Proceedings of the American Mathematical Society  
Under certain conditions on the coefficients of symmetric singular differential operators of order 2n, selfadjoint extensions are shown to have a discrete spectrum. The results are proven specifically for the Friedrichs extension.
doi:10.1090/s0002-9939-1972-0291883-x fatcat:kbevqhbe4zdqzo4icnwolwtxda