Growth envelopes of some variable and mixed function spaces

TL;DR

This paper investigates the unboundedness properties of functions in variable and mixed Lebesgue and Lorentz spaces using growth envelopes, extending classical results and identifying key unboundedness directions.

Contribution

It provides explicit growth envelope characterizations for variable and mixed Lebesgue and Lorentz spaces, highlighting the importance of the minimal exponent in unboundedness analysis.

Findings

Growth envelopes for mixed Lebesgue spaces depend on the smallest p_i.

Variable Lebesgue spaces' envelopes depend on the essential infimum p_-.

Results inform Hardy inequalities and optimal embedding spaces.

Abstract

We study unboundedness properties of functions belonging Lebesgue and Lorentz spaces with variable and mixed norms using growth envelopes. Our results extend the ones for the corresponding classical spaces in a natural way. In the case of spaces with mixed norms it turns out that the unboundedness in the worst direction, i.e., in the direction where $p_{i}$ is the smallest, is crucial. More precisely, the growth envelope is given by $E_G(L_{\vec{p}}(\Omega)) = (t^{-1/\min\{p_{1}, \ldots, p_{d} \}},\min\{p_{1}, \ldots, p_{d} \})$ for mixed Lebesgue and $E_G(L_{\vec{p},q}(\Omega)) = (t^{-1/\min\{p_{1}, \ldots, p_{d} \}},q)$ for mixed Lorentz spaces, respectively. For the variable Lebesgue spaces we obtain $E_G(L_{p(\cdot)}(\Omega)) = (t^{-1/p_{-}},p_{-})$, where $p_{-}$ is the essential infimum of $p(\cdot)$, subject to some further assumptions. Similarly, for the variable Lorentz space it holds $E_G(L_{p(\cdot),q}(\Omega)) = (t^{-1/p_{-}},q)$. The growth envelope is used for Hardy-type inequalities and limiting embeddings. In particular, as a by-product we determine the smallest classical Lebesgue (Lorentz) space which contains a fixed mixed or variable Lebesgue (Lorentz) space, respectively.