A $\sqrt{2}$-Approximation to the Bilu-Linial Conjecture

Abstract

Bilu and Linial conjectured that if $d\ge 2$, then every $d$-regular graph $G$ has an edge signing $σ:E(G)\to\{-1,1\}$ such that its signed adjacency matrix $A_σ$ satisfies the Ramanujan bound \begin{equation*} ρ(A_σ)\le 2\sqrt{d-1} \end{equation*} and they proved that \begin{equation*} ρ(A_σ)=O\!\left(\sqrt{d\log^3 d}\right). \end{equation*} Using a method of interlacing polynomials, Marcus, Spielman, and Srivastava confirmed one side of this conjecture that there is a signing $σ$ for which \[ λ_{\max}(A_σ)\le 2\sqrt{d-1}. \] By constructing an auxiliary bipartite graph from a balanced orientation of $G$ and applying a bipartite signing argument, we prove that every finite simple graph $G$ of maximum degree $d\ge 3$ admits an edge signing $σ$ such that \[ ρ(A_σ) \le 4\sqrt{\left\lceil \frac d2\right\rceil-1} \le 2\sqrt{2(d-1)}. \] Thus we obtain a bound within a factor at most $\sqrt2$ of the conjectured Ramanujan bound.