A simple combinatorial algorithm for restricted 2-matchings in subcubic graphs -- via half-edges

TL;DR

This paper introduces a simple combinatorial reduction using half-edges to solve maximum weight restricted 2-matchings in subcubic graphs efficiently, covering triangle-free and square-free variants.

Contribution

The authors present a straightforward reduction to maximum weight b-matching for restricted 2-matchings in subcubic graphs, improving algorithm simplicity and speed.

Findings

Algorithms run in O(n^2 log n) time

Applicable to triangle-free and square-free 2-matchings

Reduction simplifies existing complex methods

Abstract

We consider three variants of the problem of finding a maximum weight restricted $2$-matching in a subcubic graph $G$. (A $2$-matching is any subset of the edges such that each vertex is incident to at most two of its edges.) Depending on the variant a restricted $2$-matching means a $2$-matching that is either triangle-free or square-free or both triangle- and square-free. While there exist polynomial time algorithms for the first two types of $2$-matchings, they are quite complicated or use advanced methodology. For each of the three problems we present a simple reduction to the computation of a maximum weight $b$-matching. The reduction is conducted with the aid of half-edges. A half-edge of edge $e$ is, informally speaking, a half of $e$ containing exactly one of its endpoints. For a subset of triangles of $G$, we replace each edge of such a triangle with two half-edges. Two half-edges of one edge $e$ of weight $w(e)$ may get different weights, not necessarily equal to $\frac{1}{2}w(e)$. In the metric setting when the edge weights satisfy the triangle inequality, this has a geometric interpretation connected to how an incircle partitions the edges of a triangle. Our algorithms are additionally faster than those known before. The running time of each of them is $O(n^2\log{n})$, where $n$ denotes the number of vertices in the graph.