Connecting Terminals and 2-Disjoint Connected Subgraphs

TL;DR

This paper introduces bounds and algorithms for enumerating minimal T-connecting sets in graphs, generalizing path enumeration and improving solutions for the 2-Disjoint Connected Subgraphs problem.

Contribution

It provides a new upper bound on the number of minimal T-connecting sets and an efficient enumeration algorithm, extending previous path enumeration methods.

Findings

Bound on the number of minimal T-connecting sets: ${|V ackslash T| race |T|-2} imes 3^{|V ackslash T|/3}$.

Polynomial-time enumeration within the bound.

Improved algorithm for 2-Disjoint Connected Subgraphs with runtime $O^*(1.7804^n)$.

Abstract

Given a graph $G=(V,E)$ and a set of terminal vertices $T$ we say that a superset $S$ of $T$ is $T$-connecting if $S$ induces a connected graph, and $S$ is minimal if no strict subset of $S$ is $T$-connecting. In this paper we prove that there are at most ${|V \setminus T| \choose |T|-2} \cdot 3^{\frac{|V \setminus T|}{3}}$ minimal $T$-connecting sets when $|T| \leq n/3$ and that these can be enumerated within a polynomial factor of this bound. This generalizes the algorithm for enumerating all induced paths between a pair of vertices, corresponding to the case $|T|=2$. We apply our enumeration algorithm to solve the {\sc 2-Disjoint Connected Subgraphs} problem in time $O^*(1.7804^n)$, improving on the recent $O^*(1.933^n)$ algorithm of Cygan et al. 2012 LATIN paper.