Approximation schemes satisfying Shapiro's Theorem

Abstract: An approximation scheme is a family of homogeneous subsets $(A_n)$ of a quasi-Banach space $X$, such that $A_1 \subsetneq A_2 \subsetneq ... \subsetneq X$, $A_n + A_n \subset A_{K(n)}$, and $\bar{\cup_n A_n} = X$. Continuing the line of research originating at a classical paper by S.N. Bernstein (in 1938), we give several characterizations of the approximation schemes with the property that, for every sequence ${\epsilon_n}\searrow 0$, there exists $x\in X$ such that $dist(x,A_n)\neq \mathbf{O}(\epsilon_n)$ (in this case we say that $(X,{A_n})$ satisfies Shapiro's Theorem). If $X$ is a Banach space, $x \in X$ as above exists if and only if, for every sequence ${\delta_n} \searrow 0$, there exists $y \in X$ such that $dist(y,A_n) \geq \delta_n$. We give numerous examples of approximation schemes satisfying Shapiro's Theorem.