References
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- Titchmarsh E 1962 Eigenfunction Expansions Associated with Second-Order Differential Equations 2nd Edn Oxford Claredon Press
- Levitan B Sargsyan I 1975 Introduction to Spectral Theory Translations of Mathematical Monographs 39 Providence R.I. The American Mathematical Society
- Kostyuchenko A Sargsyan I 1979 Distribution of Eigenvalues: Self-Adjoint Ordinary Differential Operators Moscow Nauka (Russian)
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