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Journal of Convex Analysis 20 (2013), No. 1, 233--242
Copyright Heldermann Verlag 2013

Non-Archimedean Quantitative Grothendieck and Krein's Theorems

Jerzy Kakol
Faculty of Mathematics and Informatics, A. Mickiewicz University, 61-614 Poznan, Poland

Albert Kubzdela
Institute of Civil Engineering, University of Technology, 61-138 Poznan, Poland


We show that the non-archimedean version of Grothendieck's theorem about weakly compact sets for $C(X,\mathbb{K})$, the space of continuous maps on $% X $ with values in a locally compact non-trivially valued non-archimedean field $\mathbb{K}$, fails in general. Indeed, we prove that if $X$ is an infinite zero-dimensional compact space, then there exists a relatively compact set $H:=\{g_{n}:n\in \mathbb{N}\}\subset C(X,\mathbb{K})$ in the pointwise topology $\tau _{p}$ of $C(X,\mathbb{K})$ which is not $w-$% relatively compact, i.e. compact in the weak topology of $C(X,\mathbb{K})$, such that all $\Vert g_{n}\Vert =1$ and $\gamma (H):=\sup \{|\lim_{m}\lim_{n}f_{m}(x_{n})-\lim_{n}\lim_{m}f_{m}(x_{n})|:(f_{m})_{m}% \subset B,(x_{n})_{n}\subset H\}>0$, where $B$ is the closed unit ball in the dual $C(X,\mathbb{K})^{\ast }$ and the involved limits exist. The latter condition $\gamma (H)>0$ shows in fact that a quantitative version of Grothendieck's theorem for real spaces (due to Angosto and Cascales) fails in the non-archimedean setting. The classical Krein and Grothendieck's theorems ensure that for any compact space $X$ every uniformly bounded set $% H $ in a real (or complex) space $C(X)$ is $\tau _{p}$-relatively compact if and only if the absolutely convex hull $acoH$ of $H$ is $\tau _{p}$% -relatively compact. In contrast, we show that for an infinite zero-dimensional compact space $X$ the absolutely convex hull $acoH$ of a $% \tau _{p}-$relatively compact and uniformly bounded set $H$ in $C(X,\mathbb{K% })$ needs not be $\tau _{p}-$relatively compact for a locally compact non-archimedean $\mathbb{K}$. Nevertheless, our main result states that if $% H\subset C(X,\mathbb{K})$ is uniformly bounded, then $acoH$ is $\tau _{p}-$% relatively compact if and only if $H$ is $w$-relatively compact.

Keywords: Grothendieck's theorem, Krein's theorem, locally compact non-archimedean field, compactness, space of continuous functions.

MSC: 46S10, 46A50, 54C35

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