Embedding theorems for symmetric spaces of measurable functions
M. A. Muratova,
B. A. Rubshteinb a Crimea Federal University, Simferopol
b Ben-Gurion University of the Negev
Abstract:
Let
$m$ be the usual Lebesgue measure on
$\mathbb{R}_+ = [0,+\infty)$.
Dealing with symmetric (rearrangement invariant) spaces
$\mathbf{E}$ on the standard measure space
$(\mathbb{R}_+,m)$, we
treat the following embeddings:
$$
\mathbf{L}_1\cap\mathbf{L}_\infty \subseteq \mathbf{\Lambda}^0_{\widetilde{V}}\subseteq \mathbf{E}^0\subseteq \mathbf{E}\subseteq \mathbf{E}^{11}\subseteq \mathbf{M}_{V_*} \subseteq \mathbf{L}_1+\mathbf{L}_\infty \;,
$$
where $\mathbf{E}^0= cl_\mathbf{E}(\mathbf{L}_1\cap\mathbf{L}_\infty)$ is the closure of
$\mathbf{L}_1\cap\mathbf{L}_\infty$ in
$\mathbf{E}$,
$\mathbf{E}^{11}=(\mathbf{E}^1)^1$ is the second associate space of
$\mathbf{E}$,
$V(x)= \|1_{[0,x]}\|_\mathbf{E}$ is the fundamental function of the symmetric space
$\mathbf{E}$, $\displaystyle{V_*(x)= \frac{x}{V(x)}1_{(0,\infty)}(x)}$,
$\widetilde{V}$ is the least concave majorant of
$V$,
$\mathbf{\Lambda}_{\widetilde{V}} $ and
$ \mathbf{M}_{V_*}$ are the Lorentz and Marcinkiewicz
spaces with the weights
$\widetilde{V}$ and
$V_*$ respectively,
$\mathbf{\Lambda}^0_{\widetilde{V}}=cl_{\mathbf{\Lambda}_{\widetilde{V}}}(\mathbf{L}_1\cap\mathbf{L}_\infty)$.
The space
$\mathbf{\Lambda}^0_{\widetilde{V}}$ is the minimal part of the Lorentz space
$\mathbf{\Lambda}_{\widetilde{V}}$.
It is the smallest symmetric space on
$\mathbb{R}_+$ whose fundamental function $\varphi_{\mathbf{\Lambda}^0_{\widetilde{V}}}=\widetilde{V}$ is equivalent to
$V$.
The Marcinkiewicz space
$\mathbf{M}_{V_*}$ is the largest symmetric space on
$\mathbb{R}_+$ satisfying $\varphi_{\mathbf{M}_{V_*}}= \varphi_{\mathbf{E}} = V $.
The inclusion $\mathbf{\Lambda}_{\widetilde{V}}\subseteq \mathbf{E}$ claimed in [3, II.5.4, Th. 5.5] fails in general.
Although, it is true, for example, if
$V(+\infty) = \infty$ (the space
$\mathbf{\Lambda}^0_{\widetilde{V}}$ is
minimal), or if the space
$\mathbf{E}$ itself is maximal (
$\mathbf{E}=\mathbf{E}^{11}$).
The embeddings and natural inequalities for corresponding norms are studied in detail.
Keywords:
Symmetric spaces, Lorentz and Marcinkiewicz spaces, embedding theorems.
UDC:
519.55/56
MSC: 46E30,
46E35,
26D10,
26D15,
46B70