2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/146935We give a self-contained presentation of the theory of self-adjoint extensions using the technique of boundary triples. A description of the spectra of self-adjoint extensions in terms of the corresponding Krein maps (Weyl functions) is given. Applications include quantum graphs, point interactions, hybrid spaces, singular perturbations.81 pages, new references added, subsection 1.3 extended, typos correctedMathematical PhysicsSpectral Theorytext