Abstract
In this paper, we establish some properties for a uniformly continuous cosine family \((C(t))_{t\in \mathbb {R}}\) which has the property that C(t) is demicompact for some (resp. every) \(t>0\). More precisely, we prove that this property is equivalent to the demicompactness of \(I-A\) where A is the infinitesimal generator of the uniformly continuous cosine family \((C(t))_{t\in \mathbb {R}}\). The obtained result is used to study the spectral inclusion for a uniformly continuous cosine family for an upper semi-Fredholm spectrum. In addition, we give some perturbation results on demicompactness.
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Benkhaled, H., Elleuch, A. & Jeribi, A. Demicompactness Properties for Uniformly Continuous Cosine Families. Mediterr. J. Math. 19, 157 (2022). https://doi.org/10.1007/s00009-022-02083-6
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DOI: https://doi.org/10.1007/s00009-022-02083-6
Keywords
- Cosine family
- demicompact linear operator
- perturbation theory
- upper semi-Fredholm spectrum
- spectral inclusion