Computing multiway cut within the given excess over the largest minimum isolating cut

TL;DR

This paper introduces an algorithm to find near-optimal multiway cuts within a specified excess over the largest minimum isolating cut, utilizing a combinatorial bound on important separators.

Contribution

The paper presents a new fixed-parameter algorithm for multiway cut that efficiently computes solutions close to the lower bound using important separator bounds.

Findings

Algorithm runs in $O(kn^{k+3})$ time.

Computes multiway cut within $k$ of the lower bound or reports none.

Bound on important separators is at most $inom{n}{i}$ for size $m+i$.

Abstract

Let $(G,T)$ be an instance of the (vertex) multiway cut problem where $G$ is a graph and $T$ is a set of terminals. For $t \in T$, a set of nonterminal vertices separating $t$ from $T \setminus \{T\}$ is called an \emph{isolating cut} of $t$. The largest among all the smallest isolating cuts is a natural lower bound for a multiway cut of $(G,T)$. Denote this lower bound by $m$ and let $k$ be an integer. In this paper we propose an $O(kn^{k+3})$ algorithm that computes a multiway cut of $(G,T)$ of size at most $m+k$ or reports that there is no such multiway cut. The core of the proposed algorithm is the following combinatorial result. Let $G$ be a graph and let $X,Y$ be two disjoint subsets of vertices of $G$. Let $m$ be the smallest size of a vertex $X-Y$ separator. Then, for the given integer $k$, the number of \emph{important} $X-Y$ separators \cite{MarxTCS} of size at most $m+k$ is at most $\sum_{i=0}^k{n \choose i}$.