Posilicano, A. “Boundary Triples and Weyl Functions for Singular Perturbations of Self-Adjoint Operators”. Methods of Functional Analysis and Topology 10, no. 2 (June 25, 2004): 57–63. Accessed December 17, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/241.