Kuznecov formulae for fractal measures
Yakun Xi
TL;DR
This work extends the Kuznecov formula to a broad class of singular and fractal measures by replacing smooth submanifold measures with $s$-Ahlfors regular measures admitting an averaged $s$-density $A_er$. Through a heat-kernel regularization and a Tauberian argument, the authors establish a one-term asymptotic $N_er(\lambda)\sim C_{n,s} A_er\lambda^{n-s}$ with an explicit universal constant $C_{n,s}=(2\pi)^{-(n-s)}\operatorname{vol}(B^{n-s})$, generalizing Zelditch’s results. They further connect to Hare--Roginskaya energy identities to obtain polynomial upper bounds and show these hypotheses are essentially optimal via sharpness results, including explicit slow-convergence constructions. The results provide an audible-geometric bridge for fractal measures, highlighting how fractal dimension and averaged density control high-frequency eigenfunction projections. This broadens the scope of spectral restriction phenomena beyond smooth geometry to fractal and singular settings.
Abstract
Let $(M,g)$ be a compact, connected Riemannian manifold of dimension $n\ge 2$, and let $\{e_j\}_{j=0}^\infty$ be an orthonormal basis of Laplace eigenfunctions $-Δ_g e_j=λ_j^2 e_j$. Given a finite Borel measure $μ$ on $M$, consider the Kuznecov sum \[ N_μ(λ):=\sum_{λ_j\le λ}\Bigl|\int_M e_j\,dμ\Bigr|^2. \] Assume that $μ$ is $s$-Ahlfors regular for some $s\in(0,n)$ and admits an averaged $s$-density constant $A_μ$. We prove that \[ N_μ(λ) = (2π)^{-(n-s)}\,{\rm vol}\,(B^{\,n-s})\,A_μ\,λ^{n-s} + o(λ^{n-s}) \qquad (λ\to\infty). \] The hypotheses of $s$-Ahlfors regularity and the averaged $s$-density condition are essentially optimal for such a one-term asymptotic, and in general the remainder $o(λ^{n-s})$ cannot be improved uniformly to a power-saving error term. This extends the classical Kuznecov formulae of Zelditch for smooth submanifold measures to a broad class of singular and fractal measures.