Lower estimates for the norm and the Kuratowski measure of noncompactness of Wiener-Hopf type operators

Abstract

Let $X(\mathbb{R}^n)$ be a Banach function space and $\Omega\subseteq\mathbb{R}^n$ be a measurable set of positive measure. For a Fourier multiplier $a$ on $X(\mathbb{R}^n)$, consider the Wiener-Hopf type operator $W_\Omega(a):=r_\Omega F^{-1}aF e_\Omega$, where $F^{\pm 1}$ are the Fourier transforms, $r_\Omega$ is the operator of restriction from $\mathbb{R}^n$ to $\Omega$ and $e_\Omega$ is the operator of extension by zero from $\Omega$ to $\mathbb{R}^n$. Let $X_2(\Omega)$ be the closure of $L^2(\Omega)\cap X(\Omega)$ in $X(\Omega)$. We show that if $X(\Omega)$ satisfies the so-called weak doubling property, then \[ \|a\|_{L^\infty(\mathbb{R}^n)} \le \|W_\Omega(a)\|_{\mathcal{B}(X_2(\Omega),X(\Omega))}. \] Further, we prove that if $X(\Omega)$ satisfies the so-called separated doubling property, then the Kuratowski measure of noncompactness of $W_\Omega(a)$ admits the following lower estimate: \[ \frac{1}{2}\|a\|_{L^\infty(\mathbb{R}^n)} \le \|W_\Omega(a)\|_{\mathcal{B}(X_2(\Omega),X(\Omega)),\kappa}. \] These results are specified to the case of variable Lebesgue spaces $L^{p(\cdot)}(C,w)$ with Muckenhoupt type weights $w$ over open cones $C\subseteq\mathbb{R}^n$ with the vertex at the origin.