Renorming spaces with greedy bases

Abstract

We study the problem of improving the greedy constant or the democracy constant of a basis of a Banach space by renorming. We prove that every Banach space with a greedy basis can be renormed, for a given ε>0, so that the basis becomes (1+ε)-democratic, and hence (2+ε)-greedy, with respect to the new norm. If in addition the basis is bidemocratic, then there is a renorming so that in the new norm the basis is (1+ε)-greedy. We also prove that in the latter result the additional assumption of the basis being bidemocratic can be removed for a large class of bases. Applications include the Haar systems in Lp[0,1], 1<p<∞, and in dyadic Hardy space H1, as well as the unit vector basis of Tsirelson space.