Embedding $\ell_2$ and $J$ into subspaces of $JT$ and $JT^*$

TL;DR

This paper investigates the structure of subspaces within the Banach spaces $JT$ and $JT^*$, showing the presence of $ ext{ell}_2$ and $J$ subspaces, and characterizing sequences in $JT$.

Contribution

It demonstrates that all subspaces of $JT$ and $JT^*$ contain complemented $ ext{ell}_2$ subspaces and characterizes the structure of sequences in $JT$.

Findings

Every subspace of $JT$ or $JT^*$ contains complemented $ ext{ell}_2$.

Non-reflexive subspaces of $JT$ contain an isomorphic copy of $J$.

Sequences in $JT$ have subsequences equivalent to $ ext{ell}_2$ or $J$ basis.

Abstract

In the first part of the paper we show that every closed subspace of $JT$ or $JT^*$ contains $\ell_2$ complemented in $JT$ or $JT^*$ respectively, and $JT$ contains uncomplemented copies of $\ell_2$. As a result, the predual $\B$ of $JT$, as well as the spaces $JT$ and $JT^*$, are subprojective and superprojective. In the second part, we prove that every weakly Cauchy sequence that is not weakly convergent in $JT$ has a subsequence equivalent to the basis of $J$. Hence, every non-reflexive subspace of $JT$ contains an isomorphic copy of $J$, and every Schauder basic sequence in $JT$ has a subsequence which is equivalent either to the basis of $\ell_2$ or to the basis of $J$. Moreover these subspaces may be selected to be complemented in $JT$.