8 - Extension of C(K)-Valued Operators

Summary

The chapter is devoted to the single topic of extending $\mathscr C$-valued operators. Its first section presents the global approach to the extension of operators: Zippin’s characterisation of $\mathscr C$-trivial embeddings by means of weak*-continuous selectors and a few noteworthy applications. The second section presents the Lindenstrauss-Pe\l czy\’nski theorem with two different proofs: the first one combines homological techniques with the global approach, while the second is Lindenstrauss-Pe\l czy\’nski’s original proof. The analysis of their proof is indispensable for understanding Kalton’s imaginative inventions that lead to the so-called $L^*$ and $m_1$-type properties and to a decent list of $\mathscr C$-extensible spaces. The next two sections contain, respectively, those points of the Lipschitz theory that are necessary to develop the linear theory and different aspects of Zippin’s problem: which separable Banach spaces $X$ satisfy $\operatorname{Ext}(X, C(K))=0\,$? The problem admits an interesting gradation in terms of the topological complexity of $K$. The final section reports the complete solution of the problem of whether $\operatorname{Ext}(C(K), c_0)\neq 0$ for all non-metrisable compacta $K$.