Resolvent of the generator of the C0-group with non-basis family of eigenvectors and sharpness of the XYZ theorem

The paper presents an explicit form of the resolvent and characterisation of the spectrum for the class of generators of C_0C0​-groups with purely imaginary eigenvalues, clustering at i\inftyi∞, and complete minimal non-basis family of eigenvectors, constructed recently by the authors in [28]. The discrete Hardy inequality serves as the cornerstone for the proofs of the corresponding results. Furthermore, it is shown that the main result on the Riesz basis property for invariant subspaces of the generator of the C_0C0​-group (the XYZ theorem), obtained a decade ago by G. Q. Xu, S. P. Yung and H. Zwart in [31] and [32], is sharp.