On the boundedness of dilation operators in the context of Triebel-Lizorkin-Morrey spaces

Abstract

In this paper we study the behavior of dilation operators $ D_\lambda \colon f \mapsto f(\lambda\,\cdot) $ with $ \lambda > 1 $ in the context of Triebel-Lizorkin-Morrey spaces $\mathcal{E}^{s}_{u,p,q}(\mathbb{R}^d)$. For that purpose we prove upper and lower bounds for the operator (quasi-)norm $\| D_\lambda \,|\, \mathcal{L}\big(\mathcal{E}^s_{u,p,q}(\mathbb{R}^d)\big) \| $. We show that for $s>\sigma_p $ the operator (quasi-)norm $\| D_\lambda \,|\, \mathcal{L}\big(\mathcal{E}^s_{u,p,q}(\mathbb{R}^d)\big) \| $ up to constants behaves as $\lambda^{s - \frac{d}{u}} $. For the borderline case $ s = \sigma_{p} $ we observe a behavior of the form $\lambda^{\sigma_p- \frac{d}{u}}$, multiplied with logarithmic terms of $\lambda$ that also depend on the fine index $q$. For $s < \sigma_{p}$ and $p \geq 1$ we find the relation $\| D_\lambda \,|\, \mathcal{L}\big(\mathcal{E}^s_{u,p,q}(\mathbb{R}^d)\big) \| \sim \lambda^{ - \frac{d}{u}}$. The case $s < \sigma_{p}$ and $p < 1$ is investigated as well. Our proofs are mainly based on the Fourier analytic approach to Triebel-Lizorkin-Morrey spaces. As byproducts we show an advanced Fourier multiplier theorem for band-limited functions in the context of Morrey spaces and derive some new equivalent (quasi-)norms and characterizations of $\mathcal{E}^{s}_{u,p,q}(\mathbb{R}^d)$. Keywords: Dilation Operator, Morrey space, Triebel-Lizorkin-Morrey space, Fourier multiplier