Convexity of inverse spectral Green kernels and nondegeneracy of Robin centers in convex domains

Abstract

Let $Ω\subset\mathbb{R}^N$ be a bounded convex domain and let $A=-Δ_D$ be the positive Dirichlet Laplacian. For every real $s>0$ with $N>2s$, we prove that the function \[ (x,y)\longmapsto K_{s,Ω}(x,y)^{-1/(N-2s)}, \] where $K_{s,Ω}$ is the Green kernel of $A^{-s}$, extends continuously by zero to the diagonal and is jointly convex on $Ω\timesΩ$. We also prove that the associated regular part extends real analytically across the diagonal and that the corresponding Robin function is strictly convex and diverges at the boundary. Consequently, for every real $s>0$ satisfying $N>2s$, there is a unique Robin center. This applies in particular to the spectral fractional Dirichlet Laplacian and to all integer-order Navier polyharmonic operators. If $Ω$ is of class $C^{2,\vartheta}$, $0<\vartheta<1$, we further establish second-order rigidity. For $0 0$ throughout $Ω$. For integer orders, we introduce a finite-part doubling principle across real spectral orders. The argument is based on a two-term expansion of truncated square energies together with the spectral identity $A^{-σ}A^{-σ}=A^{-2σ}$. It follows that, for every $p\in\mathbb N$ with $N>2p$, the unique Navier polyharmonic Robin center is nondegenerate; if $N>3p$, the Hessian is positive definite throughout the domain. The boundary regularity required by these second-order results is independent of the polyharmonic order.