Expander Decomposition with Almost Optimal Overhead

We present the first polynomial-time algorithm for computing a near-optimal \emph{flow}-expander decomposition. Given a graph $G$ and a parameter $ϕ$, our algorithm removes at most a $ϕ\log^{1+o(1)}n$ fraction of edges so that every remaining connected component is a $ϕ$-\emph{flow}-expander (a stronger guarantee than being a $ϕ$-\emph{cut}-expander). This achieves overhead $\log^{1+o(1)}n$, nearly matching the $Ω(\log n)$ graph-theoretic lower bound that already holds for cut-expander decompositions, up to a $\log^{o(1)}n$ factor. Prior polynomial-time algorithms required removing $O(ϕ\log^{1.5}n)$ and $O(ϕ\log^{2}n)$ fractions of edges to guarantee $ϕ$-cut-expander and $ϕ$-flow-expander components, respectively.