Schauder bases having many good block basic sequences
Tom 254 / 2020
In the study of asymptotic geometry in Banach spaces, a basic sequence which gives rise to a spreading model has been called a good sequence. It is well known that every normalized basic sequence in a Banach space has a subsequence which is good. We investigate the assumption that every normalized block tree relative to a basis has a branch which is good. This combinatorial property turns out to be very strong and is equivalent to the space being $1$-asymptotic $\ell _p$ for some $1\leq p\leq \infty $. We also investigate the even stronger assumption that every block basic sequence of a basis is good. Finally, using the Hindman–Milliken–Taylor theorem, we prove a stabilization theorem which produces a basic sequence all of whose normalized constant coefficient block basic sequences are good, and we present an application of this stabilization.