Weak greedy algorithms and the equivalence between semi-greedy and almost greedy Markushevich bases

Abstract

We introduce and study the notion of weak semi-greedy systems -which is inspired in the concepts of semi-greedy and Branch semi-greedy systems and weak thresholding sets-, and prove that in the context Markushevich bases in infinite dimensional Banach spaces, the notions of \textit{ semi-greedy, branch semi-greedy, weak semi-greedy, and almost greedy} Markushevich bases are all equivalent. This completes and extends some results from \cite{Berna2019}, \cite{Dilworth2003b}, and \cite{Dilworth2012}. We also exhibit an example of a semi-greedy system that is neither almost greedy nor a Markushevich basis, showing that the Markushevich condition cannot be dropped from the equivalence result. In some cases, we obtain improved upper bounds for the corresponding constants of the systems.

- The title is updated. - Definition 1.1 is fixed. - Several typos are fixed. - "the weak thresholding set" -> "the only weak thresholding set" (proof of Theorem 4.2). - An inaccuracy after equation (17) is corrected. - Some references are corrected, and some are added