Basic functional properties of certain scale of rearrangement-invariant spaces

Abstract: Let $X$ be a rearrangement-invariant space over a non-atomic $\sigma$-finite measure space $(\mathscr{R},\mu)$ and let $\alpha\in(0,\infty)$. We define the functional \begin{equation*} |f|{X^{\langle \alpha \rangle}} = |((|f|^\alpha)^{**})^{\frac{1}{\alpha}}|{\overline{X}(0,\mu(\mathscr{R}))}, \end{equation*} in which $f$ is a $\mu$-measurable scalar function defined on $(\mathscr{R},\mu)$ and $\overline{X}(0,\mu(\mathscr{R}))$ is the representation space of $X$. We denote by $X^{\langle \alpha \rangle}$ the collection of all almost everywhere finite functions $f$ such that $|f|_{X^{\langle \alpha \rangle}}$ is finite. These spaces recently surfaced in connection of optimality of target function spaces in general Sobolev embeddings involving upper Ahlfors regular measures. We present a variety of results on these spaces including their basic functional properties, their relations to customary function spaces and mutual embeddings and, in a particular situation, a characterization of their associate structures. We discover a new one-parameter path of function spaces leading from a Lebesgue space to a Zygmund class and we compare it to the classical one.