| Abstract: | In this paper we study the action of a bounded linear operator over different kinds of sequences of a Banach space. Our work is mainly devoted to minimal and M - basic sequences. Plans and García Castellón have characterized the boundedness of a linear operator T by requiring the minimality of any sequence whose image is a minimal sequence (e.g. P, 1969], GC, 1990]). We extend these results to other types of sequences like M-basic, basic, strong M-basic, etc., We are also interested on conditions that ensure the minimality of the image of a given minimal sequence. Thus in Corollary 3.7 we characterize semi - Fredholm operators as those which transform every p-minimal sequence into q-minimal. In the last section we deal with M - basis whose image is M - basis or norming M - basis or basis or in general the “best” possible sequence. |
