Unique Positive Extremals and Sharp Stability Constants for Singular Moser-Trudinger Inequalities

Abstract

We combine spectral perturbation with orthogonal transformations in $H_0^1(Ω)$ to study singular Moser-Trudinger inequalities under the energy constraint $\int_Ω|\nabla u|^2\,dx\le1$, where $Ω$ is a smooth bounded planar domain containing the origin. For the weight $|x|^{-2β}$, $0\leβ<1$, the positive maximizer is unique for small $α>0$, and the sharp global stability constant satisfies \[ C_α=\fracα{2} \left(\frac1{μ_1}-\frac1{μ_2}\right)+O(α^2), \qquad α\downarrow0, \] where $μ_1<μ_2$ are the first two weighted Dirichlet eigenvalues. We compute the second asymptotic coefficient by a weighted quartic maximization on the second eigenspace, even when the second eigenvalue is multiple. An exact reduction to the sphere orthogonal to the maximizer identifies the attaining functions and yields a limiting global-to-local ratio of $1/2$. On the disk, the energy derivative with respect to the central value characterizes radial constrained degeneracy and yields finitely many exceptional normalized parameters. Combined with known critical uniqueness and nondegeneracy, this gives uniform quadratic stability near criticality, while compactness and analytic reduction yield finite-power stability throughout the admissible range.