
doi: 10.4064/sm155-2-3
Summary: 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, \(C_{{\mathbb R}}(X)\)) of all complex-valued (resp., real-valued) continuous functions on~\(X\), \(S\subset M\) a complex (resp., real) function space on~\(X\), \(\{\phi_{n}\}\)~a~sequence of unital linear contractions from \(M\) into~\(C(Y)\) (resp., \(C_{{\mathbb R}}(Y)\)), and \(\phi_{\infty}\) a linear isometry from \(M\) into~\(C(Y)\) (resp., \(C_{{\mathbb R}}(Y)\)). We show, under the assumption that \({\varPi}_{N} \subset {\varPi}_{T}\), where \({\varPi}_{N}\) is the Choquet boundary for \(N = \text{Span} (\bigcup_{1\leq n\leq \infty}N_n)\), \(N_n=\phi_{n}(M)\) (\(n=1,2,\ldots,\infty\)), and \({\varPi}_{T}\) the Choquet boundary for \(T=\phi_{\infty}(S)\), that \(\{\phi_{n}(f)\}\) converges pointwise to \(\phi_{\infty}(f)\) for any \(f\in M\) provided \(\{\phi_{n}(f)\}\) converges pointwise to \(\phi_{\infty}(f)\) for any \(f\in S\); that \(\{\phi_{n}(f)\}\) converges uniformly on any compact subset of~\({\varPi}_N\) to \(\phi_{\infty}(f)\) for any \(f\in M\) provided \(\{\phi_{n}(f)\}\) converges uniformly to \(\phi_{\infty}(f)\) for any \(f\in S\); and that, in the case where \(S\) is a function algebra, the \(\{\phi_n\}\) norm converges to~\(\phi_{\infty}\) on~\(M\) provided the \(\{\phi_{n}(f)\}\) norm converges to~\(\phi_{\infty}\) on~\(S\). The proofs are in the spirit of the original one for the theorem of Korovkin.
function space, linear contraction, Banach spaces of continuous, differentiable or analytic functions, Approximation by positive operators, Korovkin type approximation
function space, linear contraction, Banach spaces of continuous, differentiable or analytic functions, Approximation by positive operators, Korovkin type approximation
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