Comment Those lecture notes assume familiarity with basic properties of Hilbert spaces and Lebesgue theory, as presented in my online document "Lecture Notes and Background Materials on Lebesgue Theory from a Hilbert. and Banach Space Perspective, Including an Application to Fractal Image Compression.1' A major reference for the material on Sturm-Liouville operators is the book "Differential Operators of Mathematical Physics. An Introduction" by Gunter Hellwig, Addison-Wesley, Reading, Massachusetts, 1967.