# Equivalence after extension and Schur coupling do not coincide, on essentially incomparable Banach spaces

## TL;DR

This paper demonstrates that equivalence after extension and Schur coupling do not always coincide for operators on essentially incomparable Banach spaces, providing new characterizations and concrete examples such as shift operators.

## Contribution

It shows that EAE and SC are not equivalent in general, especially on essentially incomparable Banach spaces, and offers new insights into their relationship and properties.

## Key findings

- U and V are EAE but not SC when acting on different ll^p spaces
- Schur coupling is not transitive in general
- Constructs an operator W that is SC to both U and V under certain conditions

## Abstract

In 1994 H. Bart and V.\'{E}. Tsekanovskii posed the question whether the Banach space operator relations matricial coupling (MC), equivalence after extension (EAE) and Schur coupling (SC) coincide, leaving only the implication EAE/MC $\Rightarrow$ SC open. Despite several affirmative results, in this paper we show that the answer in general is no. This follows from a complete description of EAE and SC for the case that the operators act on essentially incomparable Banach spaces, which also leads to a new characterization of the notion of essential incomparability. Concretely, the forward shift operators $U$ on $\ell^p$ and $V$ on $\ell^q$, for $1\leq p,q\leq \infty$, $p\neq q$, are EAE but not SC. As a corollary, SC is not transitive. Under mild assumptions, given $U$ and $V$ that are Atkinson or generalized invertible and EAE, we give a concrete operator $W$ that is SC to both $U$ and $V$, even if $U$ and $V$ are not SC themselves. Some further affirmative results for the case where the Banach spaces are isomorphic are also obtained.