Logarithmic fractional Hardy inequalities

Abstract

We establish sharp fractional Hardy inequalities in the critical regimes $sp=d$ and $sp=1$, with explicit logarithmic weight functions. In the whole-space setting, for $p>d\ge 1$, $sp=d$, we prove that \begin{equation*} \int_{\mathbb R^d} \int_{\mathbb R^d} \frac{|u(x)-u(y)|^p}{|x-y|^{2d}}\,dx\,dy \geq \left(\frac{p-1}{p}\right)^p \int_{B_R}|u(x)|^p W_{p,d,R}(x)\,dx, \end{equation*} for $u\in C_c(B_R)$, where $W_{p,d,R}$ is a logarithmic-type weight and the constant is sharp. We also show that the corresponding inequality with the simpler weight $|x|^{-d}(\log(R/|x|))^{-p}$ cannot hold with any positive constant. Furthermore, we derive two-sided estimates and asymptotic formulae for $W_{p,d,R}$, including its limiting connection with the classical Hardy inequality. In the half-space $\mathbb{R}^{d}_{+}$, we establish an analogous sharp fractional Hardy inequality in the critical case $sp=1$, with an explicit logarithmic weight depending on the distance to the boundary. We also investigate the asymptotic behaviour of this weight, obtaining estimates that describe its behaviour near the boundary. These results elucidate the role of logarithmic weights in critical fractional Hardy inequalities in both whole-space and half-space settings.