5 citations · 5 across the 4 of their papers we have counts for
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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.…