Finite inverse nodal problems for singular weighted Sturm--Liouville equations: variational selection and spectral matching

Abstract

The recent work \cite{HeWuXiaZhang2025} introduced a finite-data inverse nodal framework for regular one-dimensional Sturm-Liouville operators. Nevertheless, formulating a multidimensional inverse nodal theory for Schr\"odinger operators remains a challenging open problem. This paper addresses this gap by establishing a finite radial inverse nodal theory for the radial spectral branch of Schr\"odinger operators in balls. In contrast with the interval case, the nodal data are the radii of spherical nodal hypersurfaces of radial eigenfunctions. The exact radial reduction preserves this PDE nodal geometry and leads naturally to a singular weighted Sturm--Liouville problem with weight $w(r)=r^{n-1}$, rather than to a regular interval model. We minimize the PDE distance $\|Q-Q_0\|_{L^p(B_R)}$ , equivalently the weighted distance $\|q-q_0\|_{L^p_w(0,R)}$, among all radial potentials matching finitely many prescribed spherical nodal radii. For the associated nodal constraint sets, we prove spectral regularity, nodal differentiability, weak closedness, exact realization, and a finite-codimensional $C^1$ manifold structure. Crucially, without the variational selection imposed by \(q_0\), the prescribed finite nodal radii fail to determine the potential uniquely: the admissible potentials are invariant under constant shifts, and the finite nodal constraints yield locally infinite-dimensional level sets at regular points.