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Item type:Publication, Access status: without files , Limit operators techniques on general metric measure spaces of bounded geometry(2020-09-15); We study band-dominated operators on (subspaces of) Lp-spaces over metric measure spaces of bounded geometry satisfying an additional property. We single out core assumptions to obtain, in an abstract setting, definitions of limit operators, characterizations of compactness and Fredholmness using limit operators; and thus also spectral consequences. In this way, we recover and unify the classical and recent results on limit operator techniques, but also gain new insights and are able to treat further applications.Publicationtype: Journal Article135 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , On symmetries of the feinberg-zee random hopping matrixIn this paper we study the spectrum Σ of the infinite Feinberg-Zee random hopping matrix, a tridiagonal matrix with zeros on the main diagonal and random ±1‘s on the first sub- and super-diagonals; the study of this non-selfadjoint random matrix was initiated in Feinberg and Zee (Phys. Rev. E 59 (1999), 6433-6443). Recently Hagger (Random Matrices: Theory Appl., 4 1550016 (2015)) has shown that the so-called periodic part Σπ of Σ, conjectured to be the whole of Σ and known to include the unit disk, satisfies p-1(Σπ) ⊂ Σπ for an infinite class S of monic polynomials p. In this paper we make very explicit the membership of S, in particular showing that it includes Pm(λ) = λUm-1(λ/2), for m ≥ 2, where Un(x) is the Chebychev polynomial of the second kind of degree n. We also explore implications of these inverse polynomial mappings, for example showing that Σπ is the closure of its interior, and contains the filled Julia sets of infinitely many p ∈ S, including those of Pm, this partially answering a conjecture of the second author.Publicationtype: Book Part153 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Essential pseudospectra and essential norms of band-dominated operators(2016-05-01); ; An operator A on an lp-space is called band-dominated if it can be approximated, in the operator norm, by operators with a banded matrix representation. The coset of A in the Calkin algebra determines, for example, the Fredholmness of A, the Fredholm index, the essential spectrum, the essential norm and the so-called essential pseudospectrum of A. This coset can be identified with the collection of all so-called limit operators of A. It is known that this identification preserves invertibility (hence spectra). We now show that it also preserves norms and in particular resolvent norms (hence pseudospectra). In fact we work with a generalization of the ideal of compact operators, so-called P-compact operators, allowing for a more flexible framework that naturally extends to lp-spaces with p∈(1, ∞) and/or vector-valued lp-spaces.Publicationtype: Journal Article127 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Fredholm Theory with Applications to Random Operators(2016); This thesis is concerned with the Fredholm theory of bounded linear operators acting on Banach space valued sequence spaces. As an application, random operators are considered and studied in detail. One of the most important tools in the study of operators on sequence spaces is the concept of limit operators. The correspondence between an operator and its limit operators is studied regarding properties like spectrum, pseudospectrum and numerical range. It turns out that similar theorems can be formulated for all these properties, respectively. These results prove to be particularly useful in the case of random operators. Special attention is directed to the so-called Feinberg-Zee random hopping matrix, which, despite its simple appearance, seems to have a very complicated spectrum. With the help of new methods, improved upper and lower bounds to the spectrum are obtained. One of these lower bounds is an infinite sequence of Julia sets, which emphasizes the complexity of the spectrum of this particular operator.Publicationtype: Doctoral Thesis848 2530