Quantitative stability for the conformally invariant Chang-Gui inequality on the exponentiation of functions on the sphere

Abstract

In this work, we focus on a recent variant of the Trudinger-Moser-Onofri inequality introduced by S. Y. Alice Chang and Changfeng Gui \cite{CG-2023}: \begin{align*} \alpha\int_{\mathbb{S}^2}|\nabla_{\mathbb{S}^2}u|^2 {\rm d}\omega+2 \int_{\mathbb{S}^2} u {\rm d}\omega -\frac{1}{2}\ln\left[\left(\int_{\mathbb{s}^2}e^{2u}{\rm d}\omega\right)^2-\sum_{i=1}^3\left(\int_{\mathbb{s}^2}\omega_i e^{2u}{\rm d} \omega\right)^2\right] \geq 0 \end{align*} holds on $H^1(\mathbb{S}^2)$ if and only if $\alpha \geq \frac{2}{3}$. In this regime, the infimum is attained only by trivial functions when $\alpha > \frac{2}{3},$ whereas for the critical value $\alpha = \frac{2}{3}$ nontrivial extremals exist, and Chang-Gui further provided a complete classification of such solutions. Building upon their result, we found a nice conformal invariance of the associated functional. Exploiting this invariance, we were able to characterize the full family of extremals in terms of conformal maps of $\mathbb{S}^2$ and, moreover, establish a sharp quantitative stability result in the gradient norm.