Approximating Multicut and the Demand Graph

In the minimum Multicut problem, the input is an edge-weighted supply graph $G=(V,E)$ and a simple demand graph $H=(V,F)$. Either $G$ and $H$ are directed (DMulC) or both are undirected (UMulC). The goal is to remove a minimum weight set of edges in $G$ such that there is no path from $s$ to $t$ in the remaining graph for any $(s,t) \in F$. UMulC admits an $O(\log k)$-approximation where $k$ is the vertex cover size of $H$ while the best known approximation for DMulC is $\min\{k, \tilde{O}(n^{11/23})\}$. These approximations are obtained by proving corresponding results on the multicommodity flow-cut gap. In contrast to these results some special cases of Multicut, such as the well-studied Multiway Cut problem, admit a constant factor approximation in both undirected and directed graphs. Motivated by both concrete instances from applications and abstract considerations, we consider the role that the structure of the demand graph $H$ plays in determining the approximability of Multicut. In undirected graphs our main result is a $2$-approximation in $n^{O(t)}$ time when the demand graph $H$ excludes an induced matching of size $t$. This gives a constant factor approximation for a specific demand graph that motivated this work. In contrast to undirected graphs, we prove that in directed graphs such approximation algorithms can not exist. Assuming the Unique Games Conjecture (UGC), for a large class of fixed demand graphs DMulC cannot be approximated to a factor better than worst-case flow-cut gap. As a consequence we prove that for any fixed $k$, assuming UGC, DMulC with $k$ demand pairs is hard to approximate to within a factor better than $k$. On the positive side, we prove an approximation of $k$ when the demand graph excludes certain graphs as an induced subgraph. This generalizes the Multiway Cut result to a much larger class of demand graphs.