Piotr Niemiec

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20112023

most citedUniversal valued Abelian groups

9 citations · 38 across the 24 of their papers we have counts for

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9 papers · 1 filter

math.FA2020★ 1 cited

Positive Hankel operators, positive definite kernels and related topics

Piotr Niemiec

It is shown that a positive (bounded linear) operator on a Hilbert space with trivial kernel is unitarily equivalent to a Hankel operator that satisfies double positivity condition…

math.FA2020

A note on functions preserving positive definiteness

Piotr Niemiec

The aim of the paper is to give a full characterization of functions f from I into the real line R (where I is an interval in R that satisfies certain natural conditions) such that…

math.FA2017★ 1 cited

Applications of amenable semigroups in operator theory

Piotr Niemiec, Paweł Wójcik

The paper deals with continuous homomorphisms $S ∋ s \mapsto T_{s} \in L (E)$ of amenable semigroups $S$ into the algebra $L (E)$ of all bounded linear operators on a Banach space $E…

math.FA2013

Bounded convergence theorems

Piotr Niemiec

There are presented certain results on extending continuous linear operators defined on spaces of E-valued continuous functions (defined on a compact Hausdorff space X) to linear o…

math.FA2013★ 1 cited

Direct integrals of matrices

Piotr Niemiec

It is shown that each linear operator on a separable Hilbert space which generates a finite type I von Neumann algebra has, up to unitary equivalence, a unique representation as a…

math.FA2012★ 4 cited

Functional calculus for diagonalizable matrices

Piotr Niemiec

For an arbitrary function f:Ω\rightarrow C (where Ωis a subset of the field C) and a positive integer k let f act on all diagonalizable complex matrices whose all eigenvalues lie i…

math.FA2011★ 1 cited

Borel structure of the spectrum of a closed operator

Piotr Niemiec

For a linear operator $T$ in a Banach space let $σ_{p} (T)$ denote the point spectrum of $T$ , $σ_{p [n]} (T)$ for finite $n > 0$ be the set of all $λ \in σ_{p} (T)$ such that $\dim \ker (T…

math.FA2011★ 2 cited

Norm closures of orbits of bounded operators

Piotr Niemiec

To every bounded linear operator $A$ between Hilbert spaces $H$ and $K$ three cardinals $ι_{r} (A)$ , $ι_{i} (A)$ and $ι_{f} (A)$ and a binary number $ι_{b} (A)$ are assigne…

math.FA2011

Central points and measures and dense subsets of compact metric spaces

Piotr Niemiec

For every nonempty compact convex subset $K$ of a normed linear space a (unique) point $c_{K} \in K$ , called the generalized Chebyshev center, is distinguished. It is shown that $c_K…