A Tight Analysis of Bethe Approximation for Permanent

We prove that the permanent of nonnegative matrices can be deterministically approximated within a factor of $\sqrt{2}^n$ in polynomial time, improving upon the previous deterministic approximations. We show this by proving that the Bethe approximation of the permanent, a quantity computable in polynomial time, is at least as large as the permanent divided by $\sqrt{2}^{n}$. This resolves a conjecture of Gurvits. Our bound is tight, and when combined with previously known inequalities lower bounding the permanent, fully resolves the quality of Bethe approximation for permanent. As an additional corollary of our methods, we resolve a conjecture of Chertkov and Yedidia, proving that fractional belief propagation with fractional parameter $γ=-1/2$ yields an upper bound on the permanent.