The generalized resolvents for a certain class of perturbed symmetric operators with equal and finite deficiency indices are investigated. Using the Weinstein-Aronszajn formula, we give a classification of the spectrum.
A. Hebbeche. "Generalized resolvents and spectrum for a certain class of perturbed symmetric operators." J. Appl. Math. 2005 (1) 81 - 92, 16 February 2005. https://doi.org/10.1155/JAM.2005.81