Graph isomorphisms in quasi-polynomial time
Harald Andrés Helfgott, Jitendra Bajpai, Daniele Dona · arXiv (Cornell University) · 2017
Let us be given two graphs $Γ_1$, $Γ_2$ of $n$ vertices. Are they isomorphic? If they are, the set of isomorphisms from $Γ_1$ to $Γ_2$ can be identified with a coset $H\cdotπ$ inside the symmetric group on $n$ elements. How do we find $π$ and a set of generators of $H$? The challenge of giving an always efficient algorithm answering these questions remained open for a long time. Babai has recently shown how to solve these problems -- and others linked to them -- in quasi-polynomial time, i.e. in time $\exp\left(O(\log n)^{O(1)}\right)$. His strategy is based in part on the algorithm by Luks (1980/82), who solved the case of graphs of bounded degree.