On the range space of Yano's extrapolation theorem and new extrapolation estimates at infinity.

Abstract

Given a sublinear operator T satisfying that !Tf!Lp(ν) ≤ C p−1 !f!Lp(µ), for every 1 < p ≤ p0, with C independent of f and p, it was proved in [C] that sup r>0 ! ∞ 1/r λν T f (y) dy 1 + log+ r ! ' M |f(x)|(1 + log+ |f(x)|) dµ(x). This estimate implies that T : L log L → B, where B is a rearrangement invariant space. The purpose of this note is to give several characterizations of the space B and study its associate space. This last information allows us to formulate an extrapolation result of Zygmund type for linear operators satisfying !Tf!Lp(ν) ≤ Cp!f!Lp(µ), for every p ≥ p0.