“…The determination of the Kreȋn-von Neumann extension and other nonnegative extensions can be found in [212,350]. For singular perturbations associated with Sturm-Liouville operators, see for instance [331] and the later papers [9,28,29,124,171,182,282,315,316,479,480,481,482,507,531,532,549,550]; for δ-point interactions we refer to [8,293]. Special properties of the Titchmarsh-Weyl coefficient have been studied in many papers; we just mention [130,292,366,367].…”