The resonant boundary $Q$-curvature problem and boundary-weighted barycenters

TL;DR

This paper investigates the existence of conformal metrics with prescribed Q-curvature on 4D manifolds with boundary, using advanced Morse theory and topological methods to handle noncompact variational problems.

Contribution

It extends Morse theory to a noncompact boundary value problem involving the Paneitz operator, and analyzes boundary-weighted barycenters to establish existence results.

Findings

Established Morse inequalities for the problem.

Derived existence criteria for prescribed Q-curvature.

Analyzed the topology of boundary-weighted barycenters.

Abstract

Given a compact four-dimensional Riemannian manifold $(M, g)$ with boundary, we study the problem of existence of Riemannian metrics on $M$ conformal to $g$ with prescribed $Q$-curvature in the interior $\mathring{M}$ of $M$, and zero $T$-curvature and mean curvature on the boundary $\partial M$ of $M$. This geometric problem is equivalent to solving a fourth-order elliptic boundary value problem (BVP) involving the Paneitz operator with boundary conditions of Chang-Qing and Neumann operators. The corresponding BVP has a variational formulation but the corresponding variational problem, in the case under study, is not compact. To overcome such a difficulty we perform a systematic study, \`a la Bahri, of the so called {\it critical points at infinity}, compute their Morse indices, determine their contribution to the difference of topology between the sublevel sets of associated Euler-Lagrange functional and hence extend the full Morse Theory to this noncompact variational problem. To establish Morse inequalities we were led to investigate from the topological viewpoint the space of boundary-weighted barycenters of the underlying manifold, which arise in the description of the topology of very negative sublevel sets of the related functional. As an application of our approach we derive various existence results and provide a Poincar\'e-Hopf type criterium for the prescribed $Q$-curvature problem on compact four dimensional Riemannian manifolds with boundary.