Hardness of robust graph isomorphism, Lasserre gaps, and asymmetry of random graphs

Abstract

Building on work of Cai, Fürer, and Immerman \cite{CFI92}, we show two hardness results for the Graph Isomorphism problem. First, we show that there are pairs of nonisomorphic $n$-vertex graphs $G$ and $H$ such that any sum-of-squares (SOS) proof of nonisomorphism requires degree $\Omega (n)$. In other words, we show an $\Omega (n)$-round integrality gap for the Lasserre SDP relaxation. In fact, we show this for pairs $G$ and $H$ which are not even $(1-10^{-14})$-isomorphic. (Here we say that two $n$-vertex, $m$-edge graphs $G$ and $H$ are $\alpha$-isomorphic if there is a bijection between their vertices which preserves at least $\alpha m$ edges.) Our second result is that under the {\sc R3XOR} Hypothesis \cite{Fei02} (and also any of a class of hypotheses which generalize the {\sc R3XOR} Hypothesis), the \emph{robust} Graph Isomorphism problem is hard. I.e.\ for every $\epsilon >0$, there is no efficient algorithm which can distinguish graph pairs which are $(1-\epsilon )$-isomorphic from pairs which are not even $(1-{\epsilon }_0)$-isomorphic for some universal constant ${\epsilon }_0$. Along the way we prove a robust asymmetry result for random graphs and hypergraphs which may be of independent interest.