Critical Optimizers for the Finite-Rank Fractional Lieb--Thirring Inequality

Abstract

We study optimal potentials for the finite-rank fractional Lieb--Thirring problem at the eigenvalue-counting endpoint, for \(0 2s\). Using the zero-energy Birman--Schwinger formulation, we prove a profile decomposition of normalized optimizing sequences with exact splitting of rank and potential mass, and strong approximation in \(L^{d/(2s)}\). Strict subadditivity yields existence and compactness modulo translations and dilations. At rank two, the optimal constant equals the rank-one constant, the supremum is not attained, and every normalized optimizing sequence splits into two Sobolev potentials. We also prove strict increase of the minimal potential masses and a uniform spectral gap for optimizing sequences. For each optimizer, we derive an Euler--Lagrange system and establish boundedness, local Hölder regularity and \(V(x)=|x|^{-4s}(c_\infty+o(1))\) as \(|x|\to\infty\), with \(c_\infty\geq0\). The conformal orbit of every optimizer contains a representative with a strictly positive asymptotic coefficient.