The resonant boundary $Q$-curvature problem and 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.