Discrete spectra for some complex infinite band matrices

Under suitable assumptions the eigenvalues for an unbounded discrete operator A in l2 , given by an infinite complex band-type matrix, are approximated by the eigenvalues of its orthogonal truncations. Let [formula] where [formula] is the set of all limit points of the sequence (λn) and An is a finite dimensional orthogonal truncation of A. The aim of this article is to provide the conditions that are sufficient for the relations σ (A) ⊂ Λ (A) or Λ(A) ) ⊂ σ (A) to be satisfied for the band operator A.

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Opracowanie rekordu ze środków MNiSW, umowa Nr 461252 w ramach programu "Społeczna odpowiedzialność nauki" - moduł: Popularyzacja nauki i promocja sportu (2021).