English

Building highly conditional almost greedy and quasi-greedy bases in Banach spaces

Functional Analysis 2018-03-23 v1

Abstract

It is known that for a conditional quasi-greedy basis B\mathcal{B} in a Banach space X\mathbb{X}, the associated sequence (km[B])m=1(k_{m}[\mathcal{B}])_{m=1}^{\infty} of its conditionality constants verifies the estimate km[B]=O(logm)k_{m}[\mathcal{B}]=\mathcal{O}(\log m) and that if the reverse inequality logm=O(km[B])\log m =\mathcal{O}(k_m[\mathcal{B}]) holds then X\mathbb{X} is non-superreflexive. Indeed, it is known that a quasi-greedy basis in a superreflexive quasi-Banach space fulfils the estimate km[B]=O(logm)1ϵk_{m}[\mathcal{B}]=\mathcal{O}(\log m)^{1-\epsilon} for some ϵ>0\epsilon>0. However, in the existing literature one finds very few instances of spaces possessing quasi-greedy basis with conditionality constants "as large as possible." Our goal in this article is to fill this gap. To that end we enhance and exploit a technique developed in [S. J. Dilworth, N. J. Kalton, and D. Kutzarova, On the existence of almost greedy bases in Banach spaces, Studia Math. 159 (2003), no. 1, 67-101] and craft a wealth of new examples of both non-superreflexive classical Banach spaces having quasi-greedy bases B\mathcal{B} with km[B]=O(logm)k_{m}[\mathcal{B}]=\mathcal{O}(\log m) and superreflexive classical Banach spaces having for every ϵ>0\epsilon>0 quasi-greedy bases B\mathcal{B} with km[B]=O(logm)1ϵk_{m}[\mathcal{B}]=\mathcal{O}(\log m)^{1-\epsilon}. Moreover, in most cases those bases will be almost greedy.

Keywords

Cite

@article{arxiv.1803.08351,
  title  = {Building highly conditional almost greedy and quasi-greedy bases in Banach spaces},
  author = {Fernando Albiac and Jose L. Ansorena and Stephen Dilworth and Denka Kutzarova},
  journal= {arXiv preprint arXiv:1803.08351},
  year   = {2018}
}

Comments

arXiv admin note: text overlap with arXiv:1712.04004