1. Discrete and Continuous Boundary Value Problems;Atkinson,1964
2. Expansions in Eigenfunctions of Selfadjoint Operators;Berezanskii;AMS Translations of Mathematical Monographs, Providence, R.I.,1968
3. Direct Methods of Qualitative Spectral Analysis of Singular Differential Operators;Glazman,1965
4. Nonoscillation theorems;Hille;Trans. Amer. Math. Soc.,1948
5. Finite Differences and Difference Equations in the Real Domain;Fort,1948