5 citations · 5 across the 4 of their papers we have counts for
8 papers
Spectral sets and operator radii
Catalin Badea, Michel Crouzeix, Hubert Klaja
We study different operator radii of homomorphisms from an operator algebra into and show that these can be computed explicitly in terms of the usual norm. As an application…
Spectral sets for numerical range
Hubert Klaja, Javad Mashreghi, Thomas Ransford
We define and study a numerical-range analogue of the notion of spectral set. Among the results obtained are a positivity criterion and a dilation theorem, analogous to those alrea…
Non-commutativity of exponential spectrum
Hubert Klaja, Thomas Ransford
In a Banach algebra, the spectrum satisfies for each pair of elements . We show that this is no longer true for the exponential spe…
On mapping theorems for numerical range
Hubert Klaja, Javad Mashreghi, Thomas Ransford
Let be an operator on a Hilbert space with numerical radius . According to a theorem of Berger and Stampfli, if is a function in the disk algebra such that $f…
Dirichlet spaces with superharmonic weights and de Branges-Rovnyak spaces
Omar El-Fallah, Karim Kellay, Hubert Klaja +2
We consider Dirichlet spaces with superharmonic weights. This class contains both the harmonic weights and the power weights. Our main result is a characterization of the Dirichlet…
Rank one perturbations of diagonal operators without eigenvalues
Hubert Klaja
In this paper, we prove that every diagonal operator on a Hilbert space of which is of multiplicity one and has perfect spectrum admits a rank one perturbation without eigenvalues.…
Hyperinvariant subspaces for some compact perturbations of multiplication operators
Hubert Klaja
In this paper, a sufficient condition for the existence of hyperinvariant subspace of compact perturbations of multiplication operators on some Banach spaces is presented. An inter…
The numerical range and the spectrum of a product of two orthogonal projections
Hubert Klaja
The aim of this paper is to describe the closure of the numerical range of the product of two orthogonal projections in Hilbert space as a closed convex hull of some explicit ellip…