Published online by Cambridge University Press: 21 July 2017
Abstract
This article concerns a class of techniques, herein referred to as edge switching techniques, that enable a new edge decomposition to be obtained from an existing one by interchanging edges between the subgraphs in the decomposition. These techniques can be viewed as generalisations of classical path switching methods for proper edge colourings. Their use in other edge decomposition settings dates back at least to 1980, but the last ten years have seen them rapidly developed and employed to resolve Lindner's conjecture on embedding partial Steiner triple systems, Alspach's cycle decomposition problem, and numerous other questions. Here we aim to give the reader a gentle introduction to these techniques and to some of their most significant applications beyond edge colouring.
Overview
An edge decomposition (hereafter simply a decomposition) of a graph G is a set of subgraphs of G such that each edge of G occurs in exactly one of the subgraphs. Questions concerning edge decompositions of graphs form a major theme in graph theory, combinatorial design theory, and finite geometry. Significantly for our purposes here, proper edge colourings of graphs are simply edge decompositions into matchings.
The edge switching techniques that we discuss here are methods that enable a new decomposition to be obtained from an existing one by interchanging edges between the graphs in the decomposition. They can be seen as an extension of the classical switching methods in graph colouring used by, for example, Kempe [66] and Vizing [92]. In 1980 Andersen, Hilton and Mendelsohn [4] employed a seminal example of edge switching in the setting of triangle decompositions. This result was eventually extended to 4- and 5-cycle decompositions by Raines and Szaniszló [81] in 1999 and again to decompositions into cycles of arbitrary lengths in 2005 [27]. Since then, edge switching techniques have been rapidly developed and applied to new settings and problems. This article aims to survey these techniques and some of their major applications beyond edge colouring.
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