Hereditary Approximation under Hilbertian Extension
In this paper, we prove that if a Banach space $X$ has the hereditary approximation property, then $X\oplus_2 H$ has the hereditary approximation property for every Hilbert space $H$. This answers a question of Johnson and Szankowski. In addition, if $X$ is also reflexive, then every closed subspace of $X\oplus_2H$ has the metric approximation property. The proof uses Grothendieck's characterization of the approximation property, a factorization of nuclear operators through quotients of $X$, and a nuclear perturbation of the coordinate projection on a closed graph.
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