On the sharp constants in curl-Sobolev inequalities on ${\mathbb{S}}^n$

Abstract

Let $n\equiv 3(\mathrm{mod}4)$ and set $p=\frac{n-1}{2}$. On an oriented Riemannian $n$-manifold we consider the (middle-degree) curl operator, $\mathrm{curl}:\ast \mathrm{d}:\hat{I}{\text{c◯}}^p\to \hat{I}{\text{c◯}}^p$, and the associated conformally invariant Sobolev quotients on $({\mathbb{S}}^n,g_{\mathrm{st}})$, \[ J_1(α)=\frac{\big(\int|\mathrm{curl}α|^{\frac{2n}{n+1}}\,\mathrm{dV}\big)^{\frac{n+1}{n}}}{\int\langle\mathrm{curl}α,α\rangle\,\mathrm{dV}}, \qquad J_2(α)=\frac{\big(\int|\mathrm{curl}α|^{\frac{2n}{n+1}}\,\mathrm{dV}\big)^{\frac{n+1}{n}}}{\inf_ϕ\big(\int|α-\mathrm{d}ϕ|^{\frac{2n}{n-1}}\,\mathrm{dV}\big)^{\frac{n-1}{n}}}. \] Killing $p$-forms and their conformal images form a natural family of critical points for both functionals, analogous to the Aubin-Talenti family in the classical Sobolev inequality. We prove a quantitative local stability estimate for $J_1$ around this family, which in particular implies that every such form is a strict local minimizer in the conformally invariant space $W^{1,\frac{2n}{n+1}}$. In contrast, we show that these critical points are unstable for $J_2$ (and for related conformally invariant quotients), yielding a strict upper bound for the sharp constant of the $J_2$ inequality. By conformal invariance, the results on ${\mathbb{S}}^n$ transfer naturally to ${\mathbb{R}}^n$.

32 pages