Morse theory and topological degree for resonant mean field equations on Riemann surfaces with boundary

Abstract

In this paper, we establish Morse theory for the Neumann mean field equation on a compact Riemann surface at $ρ=4πm$, $m\in\mathbb N$. To address the loss of compactness associated with bubbling, we use Bahri's theory of critical points at infinity. Assume that the reduced functionals are Morse and that the leading scale coefficient does not vanish at their critical points. We classify the critical points at infinity arising from interior, boundary, and mixed concentration, compute their Morse indices and local relative homology, and establish deformation lemmas at resonance. We prove that, for $m\geq 2$, every sufficiently negative sublevel is homotopy equivalent to the boundary-weighted barycenter space of order $m-1$. The Euler--Poincaré identity gives the Leray--Schauder degree at resonance without assuming nondegeneracy of the solutions. If all solutions are nondegenerate, we further obtain strong Morse inequalities with explicit Betti numbers over $\mathbb Z_2$. As a consequence, we obtain existence and multiplicity results, as well as criteria for solutions with prescribed Morse index.