Recent Developments in the Theory of Lorentz Spaces and Weighted Inequalities

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 motivation: 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