A simple proof of reverse Sobolev inequalities on the sphere and Sobolev trace inequalities on the unit ball

Frank et al. (J. Funct. Anal., 2022) stated that there is no relation between the reversed Hardy-Littlewood-Sobolev (HLS) inequalities and reverse Sobolev inequalities. However, we demonstrate that reverse Sobolev inequalities of order $\gamma\in(\frac{n}{2},\frac{n}{2}+1)$ on the $n$-sphere can be readily derived from the reversed HLS inequalities. For the case $\gamma\in(\frac{n}{2}+1,\frac{n}{2}+2)$, we present a simple proof of reverse Sobolev inequalities by using the center of mass condition introduced by Hang. In addition, applying this approach, we establish the quantitative stability of reverse Sobolev inequalities of order $\gamma\in(\frac{n}{2}+1,\frac{n}{2}+2)$ with explicit lower bounds. Finally, by using conformally covariant boundary operators and reverse Sobolev inequalities, we derive Sobolev trace inequalities on the unit ball.