We show that the index defined via a trace for Fredholm elements in a Banach algebra has the property that an index zero Fredholm element can be decomposed as the sum of an invertible element and an element in the socle. We identify the set of index zero Fredholm elements as an upper semiregularity with the Jacobson property. The Weyl spectrum is then characterized in terms of the index.

Let T₁,...,Tₙ be bounded linear operators on a complex Hilbert space H. Then there are compact operators K₁,...,Kₙ ∈ B(H) such that the closure of the joint numerical range of the n-tuple (T₁-K₁,...,Tₙ-Kₙ) equals the joint essential numerical range of (T₁,...,Tₙ). This generalizes the corresponding result for n = 1. We also show that if S ∈ B(H) and n ∈ ℕ then there exists a compact operator K ∈ B(H) such that $\left|\right|(S-K)ⁿ\left|\right|={\left|\right|Sⁿ\left|\right|}_e$. This generalizes results of C. L. Olsen.

Let W(A) and $W_e\left(A\right)$ be the joint numerical range and the joint essential numerical range of an m-tuple of self-adjoint operators A = (A₁, ..., Aₘ) acting on an infinite-dimensional Hilbert space. It is shown that $W_e\left(A\right)$ is always convex and admits many equivalent formulations. In particular, for any fixed i ∈ 1, ..., m, $W_e\left(A\right)$ can be obtained as the intersection of all sets of the form $cl\left(W(A₁,...,A_{i+1},A_i+F,A_{i+1},...,Aₘ)\right)$, where F = F* has finite rank. Moreover, the closure cl(W(A)) of W(A) is always star-shaped with the elements in $W_e\left(A\right)$ as star centers....

Let $T\in ℒ\left(X\right)$ be a bounded operator on a complex Banach space $X$. If $V$ is an open subset of the complex plane such that $\lambda -T$ is of Kato-type for each $\lambda \in V$, then the induced mapping $f\left(z\right)\mapsto (z-T)f\left(z\right)$ has closed range in the Fréchet space of analytic $X$-valued functions on $V$. Since semi-Fredholm operators are of Kato-type, this generalizes a result of Eschmeier on Fredholm operators and leads to a sharper estimate of Nagy’s spectral residuum of $T$. Our proof is elementary; in particular, we avoid the sheaf model of Eschmeier and...

Let $𝒜$ be a commutative complex semisimple Banach algebra. Denote by $\mathrm{kh}\left(\mathrm{soc}\right(𝒜\left)\right)$ the kernel of the hull of the socle of $𝒜$. In this work we give some new characterizations of this ideal in terms of minimal idempotents in $𝒜$. This allows us to show that a “result” from Riesz theory in commutative Banach algebras is not true.

We study the infinitesimal generator of the Lax-Phillips semigroup of the automorphic scattering system defined on the Poincaré upper half-plane for SL₂(ℤ). We show that its spectrum consists only of the poles of the resolvent of the generator, and coincides with the poles of the scattering matrix, counted with multiplicities. Using this we construct an operator whose eigenvalues, counted with algebraic multiplicities (i.e. dimensions of generalized eigenspaces), are precisely the non-trivial zeros...

We introduce the minimal operator on weighted grand Lebesgue spaces, discuss some weighted norm inequalities and characterize the conditions under which the inequalities hold. We also prove that the John-Nirenberg inequalities in the framework of weighted grand Lebesgue spaces are valid provided that the weight function belongs to the Muckenhoupt $A_p$ class.

A study is made of a symmetric functional calculus for a system of bounded linear operators acting on a Banach space. Finite real linear combinations of the operators have real spectra, but the operators do not necessarily commute with each other. Analytic functions of the operators are formed by using functions taking their values in a Clifford algebra.

The spectral problem (s²I - ϕ(V)*ϕ(V))f = 0 for an arbitrary complex polynomial ϕ of the classical Volterra operator V in L₂(0,1) is considered. An equivalent boundary value problem for a differential equation of order 2n, n = deg(ϕ), is constructed. In the case ϕ(z) = 1 + az the singular numbers are explicitly described in terms of roots of a transcendental equation, their localization and asymptotic behavior is investigated, and an explicit formula for the ||I + aV||₂ is given. For all a ≠ 0 this...

We study the numerical radius of Lipschitz operators on Banach spaces. We give its basic properties. Our main result is a characterization of finite-dimensional real Banach spaces with Lipschitz numerical index 1. We also explicitly compute the Lipschitz numerical index of some classical Banach spaces.

Let T be a power-bounded operator on a (real or complex) Banach space. We study the convergence of the one-sided ergodic Hilbert transform $li{m}_n{\sum }_{k=1}^n\left(T^{k}x\right)/k$. We prove that weak and strong convergence are equivalent, and in a reflexive space also $su{p}_n\left|\right|{\sum }_{k=1}^n\left(T^{k}x\right)/k\left|\right|<\infty$ is equivalent to the convergence. We also show that $-{\sum }_{k=1}^{\infty }\left(T^k\right)/k$ (which converges on (I-T)X) is precisely the infinitesimal generator of the semigroup ${(I-T)}_{|\overline{(I-T)X}}^r$.