Some theorems of Korovkin type

• Autorzy: Tomoko Hachiro , Takateru Okayasu
• Języki publikacji: EN

Abstrakty

EN

We take another approach to the well known theorem of Korovkin, in the following situation: X, Y are compact Hausdorff spaces, M is a unital subspace of the Banach space C(X) (respectively, $C_{ℝ}(X)$) of all complex-valued (resp., real-valued) continuous functions on X, S ⊂ M a complex (resp., real) function space on X, ϕₙ a sequence of unital linear contractions from M into C(Y) (resp., $C_{ℝ}(Y)$), and $ϕ_{∞}$ a linear isometry from M into C(Y) (resp., $C_{ℝ}(Y)$). We show, under the assumption that $Π_{N} ⊂ Π_{T}$, where $Π_{N}$ is the Choquet boundary for $N = Span(⋃_{1≤n≤∞}Nₙ)$, Nₙ = ϕₙ(M) (n = 1,2,..., ∞), and $Π_{T}$ the Choquet boundary for $T = ϕ_{∞}(S)$, that {ϕₙ(f)} converges pointwise to $ϕ_{∞}(f)$ for any f ∈ M provided {ϕₙ(f)} converges pointwise to ${ϕ_{∞}(f)}$ for any f ∈ S; that {ϕₙ(f)} converges uniformly on any compact subset of $Π_{N}$ to $ϕ_{∞}(f)$ for any f ∈ M provided {ϕₙ(f)} converges uniformly to $ϕ_{∞}(f)$ for any f ∈ S; and that, in the case where S is a function algebra, {ϕₙ} norm converges to $ϕ_{∞}$ on M provided {ϕₙ(f)} norm converges to $ϕ_{∞}$ on S. The proofs are in the spirit of the original one for the theorem of Korovkin.

Kategorie tematyczne

• 46E15: Banach spaces of continuous, differentiable or analytic functions
• 41A36: Approximation by positive operators

• Wydawca: Institute of Mathematics Polish Academy of Sciences
• Czasopismo: Studia Mathematica
• Rocznik: 2003
• Tom: 155
• Numer: 2
• Strony: 131-143

Daty

wydano

2003

Twórcy

autor

Tomoko Hachiro

• Mathematics Division, Sendai Shirayuri Gakuen High School, Murasaki-Yama 1-2-1, Izumi-ku, Sendai 981-3205, Japan

autor

Takateru Okayasu

• Department of Mathematical Sciences, Faculty of Science, Yamagata University, Yamagata 990-8560, Japan

Identyfikatory

DOI

10.4064/sm155-2-3