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Hahn-Banach extension operators and spaces of operators
Authors:Å  svald Lima  Eve Oja
Institution:Department of Mathematics, Agder College, Gimlemoen 257, Serviceboks 422, 4604 Kristiansand, Norway ; Faculty of Mathematics, Tartu University, Vanemuise 46, EE-51014 Tartu, Estonia
Abstract:Let $X\subseteq Y$ be Banach spaces and let $\mathcal A\subseteq \mathcal B$ be closed operator ideals. Let $Z$ be a Banach space having the Radon-Nikodým property. The main results are as follows. If $\Phi:\mathcal A(Z,X)^*\to \mathcal B(Z,Y)^*$ is a Hahn-Banach extension operator, then there exists a set of Hahn-Banach extension operators $\phi_i:X^*\to Y^*$, $i\in I$, such that $Z=\sum_{i\in I}\oplus_1 Z_{\Phi\phi_i}$, where $Z_{\Phi\phi_i}=\{z\in Z\colon \Phi(x^*\otimes z)=(\phi_i x^*)\otimes z, \forall x^*\in X^*\}$. If $\mathcal A(\hat{Z},X)$ is an ideal in $\mathcal B(\hat{Z},Y)$ for all equivalently renormed versions $\hat{Z}$ of $Z$, then there exist Hahn-Banach extension operators $\Phi:\mathcal A(Z,X)^*\to \mathcal B(Z,Y)^*$ and $\phi:X^*\to Y^*$ such that $Z=Z_{\Phi\phi}$.

Keywords:Hahn-Banach extension operators  spaces of operators  operator ideals  Radon-Nikod\'{y}m property
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