Equivalence after extension for compact operators on Banach spaces

TL;DR

This paper explores the concept of equivalence after extension for compact operators on Banach spaces, revealing differences from Hilbert spaces and establishing conditions related to operator ideals, s-numbers, and Banach space properties.

Contribution

It extends the understanding of operator equivalence after extension from Hilbert spaces to Banach spaces, highlighting necessary conditions and differences in various Banach space contexts.

Findings

Equivalence after extension implies same operator ideal but is not sufficient.

Operators on different -spaces cannot be equivalent after one-sided extension.

Compact operators cannot be equivalent after extension if acting on essentially incomparable Banach spaces.

Abstract

In recent years the coincidence of the operator relations equivalence after extension and Schur coupling was settled for the Hilbert space case, by showing that equivalence after extension implies equivalence after one-sided extension. In this paper we investigate consequences of equivalence after extension for compact Banach space operators. We show that generating the same operator ideal is necessary but not sufficient for two compact operators to be equivalent after extension. In analogy with the necessary and sufficient conditions on the singular values for compact Hilbert space operators that are equivalent after extension, we prove the necessity of similar relationships between the $s$-numbers of two compact Banach space operators that are equivalent after extension, for arbitrary $s$-functions. We investigate equivalence after extension for operators on $\ell^{p}$-spaces. We show that two operators that act on different $\ell^{p}$-spaces cannot be equivalent after one-sided extension. Such operators can still be equivalent after extension, for instance all invertible operators are equivalent after extension, however, if one of the two operators is compact, then they cannot be equivalent after extension. This contrasts the Hilbert space case where equivalence after one-sided extension and equivalence after extension are, in fact, identical relations. Finally, for general Banach spaces $X$ and $Y$, we investigate consequences of an operator on $X$ being equivalent after extension to a compact operator on $Y$. We show that, in this case, a closed finite codimensional subspace of $Y$ must embed into $X$, and that certain general Banach space properties must transfer from $X$ to $Y$. We also show that no operator on $X$ can be equivalent after extension to an operator on $Y$, if $X$ and $Y$ are essentially incomparable Banach spaces.