A Sharp Uniqueness-Nonuniqueness Dichotomy for Neutral Vortex Arrays in $2$D Euler Equations

We establish a sharp uniqueness--nonuniqueness criterion for the two-dimensional incompressible Euler equations within a class of stationary arrays of disjoint neutral vortices. Let $Θ$ be any finite nondecreasing envelope for the vorticity Lᵖ norms, and set I_Θ=∫p₀^∞ Θ(p)⁻¹\,dp. If I_Θ=∞, every array in the corresponding class admits a unique Euler evolution, even among bounded energy-admissible weak velocity solutions. If I_Θ<∞, there exists a compactly supported array in the same class admitting two distinct unforced Lagrangian Euler evolutions from the same initial datum. The criterion holds for arbitrary nondecreasing envelopes $Θ$.