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- [13] DISCONJUGACY CRITERIA FOR NONSELFADJOINT DIFFERENTIAL EQUATIONS OF EVEN ORDER NOTICES OF THE AMERICAN MATHEMATICAL SOCIETY, 1970, 17 (05): : 776 - &
- [20] CRITERIA FOR DISCRETE SPECTRUM OF SINGULAR SELFADJOINT DIFFERENTIAL OPERATORS NOTICES OF THE AMERICAN MATHEMATICAL SOCIETY, 1971, 18 (07): : 1057 - &