Critical GJMS Equations on $\mathbb{H}^n \times \mathbb{S}^m$

Abstract

Let $M=\mathbb{H}^n\times\mathbb{S}^m$, where $n\geq 2$, $m\geq 1$, and $N=n+m$. Let $P_k$ be the order-$2k$ GJMS operator, with $1\leq k<N/2$, and assume that $\Lambda_0=\inf\sigma_{L^2(M)}(P_k)>0$. We study$$P_kU-\lambda U=|U|^{q-2}U,\qquad q=\frac{2N}{N-2k},\qquad 0<\lambda\leq\Lambda_0,$$and attainment of the associated critical quotient $S_{\lambda,k}(M)$. Let $S_{N,k}$ be the Euclidean best Sobolev constant. For $0<\lambda<\Lambda_0$, the inequality $S_{\lambda,k}(M)<S_{N,k}$ implies attainment and a nontrivial weak solution. Localized Euclidean extremals establish this inequality when $N\geq4k$, or when $2k+2\leq N<4k$ and $\lambda>\Lambda_{\mathrm{loc}}$, where $\Lambda_{\mathrm{loc}}$ is explicit. If $N\geq2k+2$ and $S_{\Lambda_0,k}(M)<S_{N,k}$, attainment also holds at $\lambda=\Lambda_0$ in the threshold form completion. At the threshold, $L^2$-coercivity fails precisely on the constant spherical eigenspace. We combine cocompactness for its hyperbolic coefficient with a profile decomposition relative to the critical transformations preserving $\mathcal{A}=\mathbb{R}^{n-1}\times{0}$. Under the threshold hypotheses above, the strict Euclidean inequality excludes concentration escaping $\mathcal{A}$ from normalized minimizing sequences. If $r_j^{(J)}$ denotes the remainder after the first $J$ extracted profiles, then$$\lim_{J\to\infty}\limsup_{j\to\infty}|r_j^{(J)}|_{L^q(\mathbb{R}^N)}=0,$$which yields compactness modulo the axis-preserving transformations.