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abstract: 'The paper presents an explicit form of the resolvent for the class of generators of $C_0$-groups with purely imaginary eigenvalues, clustering at $i\infty$, and complete minimal non-basis family of eigenvectors, constructed recently by the authors in [@Sklyar3]. The growth properties of the resolvent are described. The discrete Hardy inequality serves as the cornerstone for the proofs of the corresponding results. Moreover, it is shown that the main result on the Riesz basis property for invariant subspaces of the generator of the $C_0$-group, obtained a decade ago by G.Q. Xu, S.P. Yung and H. Zwart in [@Xu], [@Zwart], is sharp.'
address: 'G.M. Sklyar: Institute of Mathematics, University of Szczecin, Wielkopolska 15, 70451, Szczecin, Poland; V. Marchenko$^{*}$: B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, Mathematical Division, Prospekt Nauky 47, 61103, Kharkiv, Ukraine; $^{*}$ - Corresponding author'
author:
- 'Grigory M. Sklyar'
- 'Vitalii Marchenko$^{*}$'
date: '26.07.2018'
title: 'Resolvent of the generator of the $C_0$-group with non-basis family of eigenvectors and sharpness of the XYZ theorem'
---

Introduction
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Problems of spectral theory for nonselfadjoint (NSA) operators attract more and more growing interest of experts in different fields of mathematics and 