Linear homogeneous boundary value problems. Eigen values and eigen functions. Strum-Liouville boundary value problems. Orthogonality of eigen functions. Reality of eigen values.
Links:
[1] https://maths.iisuniv.ac.in/courses/subjects/advanced-differential-equations-4
[2] https://maths.iisuniv.ac.in/academic-year/2014-2015