Our study investigates the stability properties of specific essential spectra associated with 2 × 2 block matrices formed by linear relations with unbounded components. These matrices are characterized by a non-diagonal domain, meaning that the domain consists of vectors which satisfy certain relations between their components. The analysis focuses on linking these spectra to the union of the essential spectra of the restricted diagonal multivalued operator entries.
Wafa Selmi (Wed,) studied this question.