joint spreading models and uniform approximation of bounded operators

S. A. Argyros, A. Georgiou, A. -R. Lagos, P. Motakis

Published 2017-12-20Version 1

We investigate the following property for Banach spaces. A Banach space $X$ satisfies the Uniform Approximation on Large Subspaces (UALS) if there exists $C>0$ such that for $A\in\mathcal{L}(X)$, $W\subset\mathcal{L}(X)$ with $W$ convex and compact such that for some $\varepsilon>0$ and for every $x\in X$, $x\neq0$, there exists $B\in W$ with $\|A(x)-B(x)\|<\varepsilon\|x\|$ then there exists a subspace $Y$ of $X$ of finite codimension and a $B\in W$ with $\|(A-B)|_Y\|_{\mathcal{L}(Y,X)}<C\varepsilon$. We prove that a class of separable Banach spaces including $\ell_p$, for $1\le p< \infty$, and $c_0$ satisfy the UALS. On the other hand every $L_p[0,1]$, for $1\le p\le \infty$ and $p\neq2$, fails the property and the same holds for $C(K)$, where $K$ is an uncountable metrizable compact space. Our sufficient conditions for UALS are based on joint spreading models, a multidimensional extension of the classical concept of spreading model, introduced and studied in the present paper.