Recent Developments in the Theory of Lorentz Spaces and Weighted Inequalities (Memoirs of the American Mathematical Society)

The main objective of this work is to bring together two well known and, a priori, unrelated theories dealing with weighted inequalities for the Hardy-Littlewood maximal operator $M$. For this, the authors consider the boundedness of $M$ in the weighted Lorentz space $Lambdap u(w) $. Two examples are historically relevant as a If $w=1$, this corresponds to the study of the boundedness of $M$ on $Lp(u) $, which was characterized by B. Muckenhoupt in 1972, and the solution is given by the so called $A p$ weights. The second case is when we take $u=1$. This is a more recent theory, and was completely solved by M.A. Arino and B. Muckenhoupt in 1991. It turns out that the boundedness of $M$ on $Lambdap(w) $ can be seen to be equivalent to the boundedness of the Hardy operator $A$ restricted to decreasing functions of $Lp(w) $, since the nonincreasing rearrangement of $Mf$ is pointwise equivalent to $Af*$. The class of weights satisfying this boundedness is known as $B p$. Even though the $A p$ and $B p$ classes enjoy some similar features, they come from very different theories, and so are the techniques used on each Calderon-Zygmund decompositions and covering lemmas for $A p$, rearrangement invariant properties and positive integral operators for $B p$. This work aims to give a unified version of these two theories. Contrary to what one could expect, the solution is not given in terms of the limiting cases above considered (i.e., $u=1$ and $w=1$) , but in a rather more complicated condition, which reflects the difficulty of estimating the distribution function of the Hardy-Littlewood maximal operator with respect to general measures.