I was reading the Wiki page on the Sturm-Liouville theory. Why are those tenets true? Are there any (not too advanced) reference material?
I have also read that
"There are countably infinite number of eigenvalues which satisfy the self-adjoint problem due to the property of completeness." -- someone's old notes
Firstly, why is the property of completeness true?
Secondly, why is this quoted statement true?
Thanks