Remainder terms and sharp quantitative stability for a nonlocal Sobolev inequality on the Heisenberg group

TL;DR

The work develops gradient-type remainder terms for a nonlocal Hardy–Littlewood–Sobolev inequality on the Heisenberg group $\mathbb{H}^{n}$ and proves quantitative stability of critical points in multi-bubble configurations, particularly in the critical regime $Q=4$, $\mu\in(2,4)$. It combines a concentration-compactness analysis with nondegeneracy of Heisenberg bubbles and a projection onto the bubble manifold $\mathfrak{M}$ to bound the energy defect above and below by the squared distance to $\mathfrak{M}$. Corollaries on bounded domains using weak-$L^q$ norms extend the remainder estimate to local settings. Altogether, the results generalize classical stability theory to a nonlocal, sub-Riemannian context, providing sharp, structural insights into extremals and their stability for nonlocal Sobolev inequalities on $\mathbb{H}^{n}$.

Abstract

In this paper, we study the following nonlocal Sobolev inequality on the Heisenberg group \begin{equation}\label{eq:HLS} S_{HL}(Q,μ) \left(\int_{\mathbb{H}^{n}}\int_{\mathbb{H}^{n}}\frac{|u(ξ)|^{Q^{\ast}_μ}|u(η)|^{Q^{\ast}_μ}}{|η^{-1}ξ|^μ}{d}ξ{d}η\right)^{\frac{1}{Q^{\ast}_μ}}\leq \int_{\mathbb{H}^{n}}|\nabla_{\mathbb{H}}u|^{2}dξ,\quad \forall \, u\in S^{1,2}(\mathbb{H}^{n}), \end{equation} where $Q=2n+2$ is the homogeneous dimension of the Heisenberg group $\mathbb{H}^{n}$, $n\geq1$, $μ\in(0,Q)$, $Q^{\ast}_μ=\frac{2Q-μ}{Q-2}$ is the upper critical exponent in the sense of the Hardy-Littlewood-Sobolev inequality and the Folland-Stein-Sobolev inequality on the Heisenberg group, $S_{HL}(Q,μ)$ is the sharp constant of \eqref{eq:HLS}, and $S^{1,2}(\mathbb{H}^{n})$ is the Folland-Stein-Sobolev space. %of the nonlocal-Sobolev inequality. It is well-known that, up to a translation and suitable scaling, \begin{equation}\label{eq:abs} -Δ_{\mathbb{H}} u=\left(\int_{\mathbb{H}^{n}}\frac{|u(η)| ^{Q^{\ast}_μ}}{|η^{-1}ξ|^μ}{d}η\right)|u|^{Q_μ^*-2}u,~~u\in S^{1,2}(\mathbb{H}^{n}) \end{equation} is the Euler-Lagrange equation corresponding to the associated minimization problem. On the one hand, we show the existence of a gradient-type remainder term for inequality \eqref{eq:HLS} when $Q\geq4$, $μ\in (0,4]$, and as a corollary, derive the existence of a remainder term in the weak $L^{\frac{Q}{Q-2}}$-norm on bounded domains. On the other hand, we establish the quantitative stability of critical points for equation \eqref{eq:abs} in the multi-bubble case when $Q=4$ and $μ\in (2,4)$.

Theorems & Definitions (36)

• Proposition 1.1
• Proposition 1.2
• Lemma 1.1
• Lemma 1.2
• Theorem 1.1
• Remark 1.1
• Theorem 1.2
• Remark 1.2
• Theorem 1.3
• Theorem 1.4