Graph Alignment via Birkhoff Relaxation
This work provides state-of-the-art theoretical guarantees for graph alignment, which is important for applications like network analysis, but it is incremental as it builds on existing relaxation methods.
The paper tackles the graph alignment problem, a quadratic assignment problem, by analyzing the Birkhoff relaxation and showing that its optimal solution closely approximates the true alignment for small noise levels, with theoretical guarantees under the Gaussian Wigner Model, achieving correct alignment of 1-o(1) fraction of vertices when σ = o(n^{-1.25}).
We consider the graph alignment problem, wherein the objective is to find a vertex correspondence between two graphs that maximizes the edge overlap. The graph alignment problem is an instance of the quadratic assignment problem (QAP), known to be NP-hard in the worst case even to approximately solve. In this paper, we analyze Birkhoff relaxation, a tight convex relaxation of QAP, and present theoretical guarantees on its performance when the inputs follow the Gaussian Wigner Model. More specifically, the weighted adjacency matrices are correlated Gaussian Orthogonal Ensemble with correlation $1/\sqrt{1+σ^2}$. Denote the optimal solutions of the QAP and Birkhoff relaxation by $Π^\star$ and $X^\star$ respectively. We show that $\|X^\star-Π^\star\|_F^2 = o(n)$ when $σ= o(n^{-1.25})$ and $\|X^\star-Π^\star\|_F^2 = Ω(n)$ when $σ= Ω(n^{-0.5})$. Thus, the optimal solution $X^\star$ transitions from a small perturbation of $Π^\star$ for small $σ$ to being well separated from $Π^\star$ as $σ$ becomes larger than $n^{-0.5}$. This result allows us to guarantee that simple rounding procedures on $X^\star$ align $1-o(1)$ fraction of vertices correctly whenever $σ= o(n^{-1.25})$. This condition on $σ$ to ensure the success of the Birkhoff relaxation is state-of-the-art.