Uniform Central Limit Theorems for Degree Distributions of Sparse Graphon Models

Abstract

The degree distribution is among the most basic summaries of a network, yet no functional limit theorem for the empirical degree distribution of graphon models has been available, even for dense, bounded graphons, for which only finite-dimensional results were known. We study the empirical degree distribution of sparse $L^p$ graphon models, allowing unbounded kernels and hence heavy-tailed degrees. For graphons whose latent expected degrees have a density, we prove a finite-dimensional central limit theorem whenever $nρ_n/\log n\to\infty$ and $nρ_n^{2+β}\to0$, where $ρ_n$ is the sparsity level and the kernel has $2+β$ moments for some $β>0$; the limiting covariance coincides with that of the dense case. Under additional local regularity, we establish a functional central limit theorem on compact intervals in the interior of the support. Under tail regularity and a stronger sparsity condition, we prove that uniformity extends to unbounded intervals. We also show that uniform limits fail at atoms of the degree distribution, for instance in stochastic block models. The analysis rests on an exact multilinear representation of the conditional degree distribution, which identifies its Hoeffding decomposition without Edgeworth expansions, combined with a lattice version of Billingsley's tightness criterion and moment inequalities of Efron-Stein type.