2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/57798We prove an analog of Krein's resolvent formula expressing the resolvents of self-adjoint extensions in terms of boundary conditions. Applications to quantum graphs and systems with point interactions are discussed.6 pagesMathematical PhysicsFunctional Analysis46N50, 47A06, 47A10text