Non-asymptotic $\ell_1$ spaces with unique $\ell_1$ asymptotic model

Spiros A. Argyros, Alexandros Georgiou, Pavlos Motakis

Published 2019-10-05Version 1

A recent result of D. Freeman, E. Odell, B. Sari, and B. Zheng states that whenever a separable Banach space not containing $\ell_1$ has the property that all asymptotic models generated by weakly null sequences are equivalent to the unit vector basis of $c_0$ then the space is Asymptotic $c_0$. We show that if we replace $c_0$ with $\ell_1$ then this result is no longer true. Moreover, a stronger result of B. Maurey - H. P. Rosenthal type is presented, namely, there exists a reflexive Banach space with an unconditional basis admitting $\ell_1$ as a unique asymptotic model whereas any subsequence of the basis generates a non-Asymptotic $\ell_1$ subspace.