arXiv2024
Finite rank perturbations of diagonalizable normal operators acting boundedly on infinite dimensional, separable, complex Hilbert spaces are considered from the standpoint of view of the existence of invariant subspaces. In particular, if $T=D_Λ+u\otimes v$ is a rank-one perturbation of a diagonalizable normal operator $D_Λ$ with respect to a basis $\mathcal{E}=\{e_n\}_{n\geq 1}$ and the vectors $u$ and $v$ have Fourier coefficients $\{α_n\}_{n\geq 1}$ and $\{β_n\}_{n\geq 1}$ with respect to $\mathcal{E}$ respectively, it is shown that $T$ has non trivial closed invariant subspaces provided that either $u$ or $v$ have a Fourier coefficient which is zero or $u$ and $v$ have non zero Fourier coefficients and $$ \sum_{n\geq 1} |α_n|^2 \log \frac{1}{|α_n|} + |β_n|^2 \log \frac{1}{|β_n|} < \infty.$$ As a consequence, if $(p,q)\in (0,2]\times (0,2]$ are such $\sum_{n\geq 1} (|α_n|^p + |β_n|^q )< \infty,$ it is shown the existence of non trivial closed invariant subspaces of $T$ whenever $$(p,q)\in (0,2]\times (0,2]\setminus \{(2, r), (r, 2):\; r\in(1,2]\}.$$ Moreover, such operators $T$ have non trivial closed hyperinvariant subspaces whenever they are not a scalar multiple of the identity. Likewise, analogous results hold for finite rank perturbations of $D_Λ$. This improves considerably previous theorems of Foiaş, Jung, Ko and Pearcy, Fang and Xia and the authors on an open question explicitly posed by Pearcy in the seventies.