Families with infants: a general approach to solve hard partition problems

TL;DR

This paper introduces a general framework using families with infants to improve algorithms for NP-hard partition problems, leading to faster solutions for graph coloring, TSP, and perfect matching in graphs with average degree d.

Contribution

The paper presents a novel approach leveraging families with infants to accelerate algorithms for several NP-hard problems, improving bounds and simplifying proofs.

Findings

Improved algorithm for graph coloring with $O^*((2- ext{epsilon}(d))^n)$ time.

Faster TSP algorithm for graphs of average degree d with $O^*((2- ext{epsilon}(d))^n)$ time.

Efficient counting of perfect matchings with $O^*((2- ext{epsilon}(d))^{n/2})$ time and polynomial space.

Abstract

We introduce a general approach for solving partition problems where the goal is to represent a given set as a union (either disjoint or not) of subsets satisfying certain properties. Many NP-hard problems can be naturally stated as such partition problems. We show that if one can find a large enough system of so-called families with infants for a given problem, then this problem can be solved faster than by a straightforward algorithm. We use this approach to improve known bounds for several NP-hard problems as well as to simplify the proofs of several known results. For the chromatic number problem we present an algorithm with $O^*((2-\varepsilon(d))^n)$ time and exponential space for graphs of average degree $d$. This improves the algorithm by Bj\"{o}rklund et al. [Theory Comput. Syst. 2010] that works for graphs of bounded maximum (as opposed to average) degree and closes an open problem stated by Cygan and Pilipczuk [ICALP 2013]. For the traveling salesman problem we give an algorithm working in $O^*((2-\varepsilon(d))^n)$ time and polynomial space for graphs of average degree $d$. The previously known results of this kind is a polyspace algorithm by Bj\"{o}rklund et al. [ICALP 2008] for graphs of bounded maximum degree and an exponential space algorithm for bounded average degree by Cygan and Pilipczuk [ICALP 2013]. For counting perfect matching in graphs of average degree~$d$ we present an algorithm with running time $O^*((2-\varepsilon(d))^{n/2})$ and polynomial space. Recent algorithms of this kind due to Cygan, Pilipczuk [ICALP 2013] and Izumi, Wadayama [FOCS 2012] (for bipartite graphs only) use exponential space.