A sharp form of the Marcinkiewicz Interpolation Theorem for Orlicz spaces

Abstract

An extension of Marcinkiewicz Interpolation Theorem, allowing intermediate spaces of Orlicz type, is proved. This generalization yields a necessary and sufficient condition so that every quasilinear operator, which maps the set, $S(X,\mu )$, of all $\mu$-measurable simple functions on $\sigma$- finite measure space $(X,\mu )$ into $M(Y,\nu )$, the class of $\nu$-measurable functions on $\sigma$- finite measure space $(Y,\nu )$, and satisfies endpoint estimates of type: $1<p<\infty$, $1\le r<\infty$, \begin{equation*} λ\, ν\left( \left\lbrace y \in Y : |(Tf)(y)| > λ\right\rbrace \right)^{\frac{1}{p}} \leq C_{p,r} \left( \int_{\mathbb{R_+}} μ\left( \left\lbrace x \in X : |(f)(x)| > t \right\rbrace \right)^{\frac{r}{p}} t^{r-1}dt \right)^{\frac{1}{r}}, \end{equation*} for all $f\in S(X,\mu )$ and $\lambda \in {\mathbb{R}}_{+}$; is bounded from an Orlicz space into another.