Closed ideals in the algebra of compact-by-approximable operators (2105.08403v1)

Abstract: We construct various examples of non-trivial closed ideals of the compact-by-approximable algebra $\mathfrak{A}X =:\mathcal K(X)/\mathcal A(X)$ on Banach spaces $X$ failing the approximation property. The examples include the following: (i) if $X$ has cotype $2$, $Y$ has type $2$, $\mathfrak{A}_X \neq {0}$ and $\mathfrak{A}_Y \neq {0}$, then $\mathfrak{A}{X \oplus Y}$ has at least $2$ closed ideals, (ii) there are closed subspaces $X \subset \ell^p$ for $4 < p < \infty$ and $X \subset c_0$ such that $\mathfrak{A}_X$ contains a non-trivial closed ideal, (iii) there is a Banach space $Z$ such that $\mathfrak{A}_Z$ contains an uncountable lattice of closed ideal having the reverse order structure of the power set of the natural numbers. Some of our examples involve non-classical approximation properties associated to various Banach operator ideals. We also discuss the existence of compact non-approximable operators $X \to Y$, where $X \subset \ell^p$ and $Y \subset \ell^q$ are closed subspaces for $p \neq q$.