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Computing eigenvalues of regular Sturm-Liouville problems. Dwyer, H.I.; Zettl, A. — 1994 Electronic Journal of Differential Equations (EJDE) [electronic only]
Left-definite variations of the classical Fourier expansion theorem. Littlejohn, L.L.; Zettl, A. — 2007 ETNA. Electronic Transactions on Numerical Analysis [electronic only]
Inequalities and eigenvalues of Sturm-Liouville problems near a singular boundary. Everitt, W.N.; Marletta, M.; Zettl, A. — 2001 Journal of Inequalities and Applications [electronic only]
Inequalities among eigenvalues of Sturm-Liouville problems. Eastham, M.S.P.; Kong, Q.; Wu, H.; Zettl, A. — 1999 Journal of Inequalities and Applications [electronic only]
Singular Sturm-Liouville Problems with Nonnegative and Indefinite Weights. H.G. Kaper; M.K. Kwong; A. Zettl — 1984 Monatshefte für Mathematik