Uniqueness and nonuniqueness for mean field equations with multiple singularities on flat tori

Abstract

We study mean field equations with multiple positive singularities on flat tori. We establish a uniform uniqueness criterion in terms of the total singular mass and the scale-invariant spectral quantity $λ_1(\mathbb T)|\mathbb T|$, where $λ_1(\mathbb T)$ denotes the first positive eigenvalue of the Laplacian. Combining nodal-set analysis for Jacobi fields with a cylindrical Alexandrov--Bol type inequality and sharp small-capacity asymptotics, we obtain the optimal leading coefficient $8/π^2$ in the total-mass uniqueness bound as $λ_1(\mathbb T)|\mathbb T|\to0$. Sharpness is demonstrated by examples with two symmetric singularities on degenerating rectangular tori that admit at least three distinct solutions. For the case of two singularities with total mass $2ρ$, we further derive uniqueness and multiplicity results near $ρ=4π$ from the critical-point structure of the associated symmetrized Green function. In particular, when its only critical points are the four nondegenerate two-torsion points, the equation admits a unique solution for $ρ$ just below $4π$ and exactly three solutions for $ρ$ just above $4π$.