Abstract
Let T be a self-adjoint operator on a Hilbert space H with domain \(\mathscr{D}(T)\). Assume that the spectrum of T is contained in the union of disjoint intervals Δk = [α2k−1,α2k], k ∈ ℤ, the lengths of the gaps between which satisfy the inequalities
Suppose that a linear operator B is p-subordinate to T, i.e.,
with some b′ > 0 and M ⩾ 0. Then, in the case of b > b′, for large |k| ⩾ N, the vertical lines γk = {∈ ℂ | Re λ = (α2k + α2k+1)/2} lie in the resolvent set of the perturbed operator A = T + B. Let Qk be the Riesz projections associated with the parts of the spectrum of A lying between the lines γk and γk+1 for |k| ⩾ N, and let Q be the Riesz projection onto the bounded remainder of the spectrum of A. The main result is as follows: The system {Qk(H)}|k|⩾Nof invariant subspaces together with the invariant subspace Q(H) forms an unconditional basis of subspaces in the space H. We prove also a generalization of this theorem to the case where any gap (α2k,α2k+1), k ∈ ℤ, may contain a finite number of eigenvalues of T.
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This work was carried out with the support of the Russian Science Foundation (project no. 19-01-00140).
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Russian Text © The Author (s), 2019. Published in Funktsional’ nyi Analiz i Ego Prilozheniya, 2019, Vol. 53, No. 3, pp. 45–60.
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Motovilov, A.K., Shkalikov, A.A. Preservation of the Unconditional Basis Property under Non-Self-Adjoint Perturbations of Self-Adjoint Operators. Funct Anal Its Appl 53, 192–204 (2019). https://doi.org/10.1134/S0016266319030043
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DOI: https://doi.org/10.1134/S0016266319030043