Girth (graph theory)

In graph theory, the girth of a graph is the length of a shortest cycle contained in the graph.^[1] If the graph does not contain any cycles (i.e. it's an acyclic graph), its girth is defined to be infinity.^[2] For example, a 4-cycle (square) has girth 4. A grid has girth 4 as well, and a triangular mesh has girth 3. A graph with girth four or more is triangle-free.

Cages

A cubic graph (all vertices have degree three) of girth g that is as small as possible is known as a g-cage (or as a (3,g)-cage). The Petersen graph is the unique 5-cage (it is the smallest cubic graph of girth 5), the Heawood graph is the unique 6-cage, the McGee graph is the unique 7-cage and the Tutte eight cage is the unique 8-cage.^[3] There may exist multiple cages for a given girth. For instance there are three nonisomorphic 10-cages, each with 70 vertices : the Balaban 10-cage, the Harries graph and the Harries-Wong graph.

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  The Petersen graph has a girth of 5

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  The Heawood graph has a girth of 6

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  The McGee graph has a girth of 7

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  The Tutte–Coxeter graph (Tutte eight cage) has a girth of 8

Girth and graph coloring

For any positive integers g and χ, there exists a graph with girth at least g and chromatic number at least χ; for instance, the Grötzsch graph is triangle-free and has chromatic number 4, and repeating the Mycielskian construction used to form the Grötzsch graph produces triangle-free graphs of arbitrarily large chromatic number. Paul Erdős was the first to prove the general result, using the probabilistic method.^[4] More precisely, he showed that a random graph on n vertices, formed by choosing independently whether to include each edge with probability n^{(1 − g)/g}, has, with probability tending to 1 as n goes to infinity, at most n/2 cycles of length g or less, but has no independent set of size n/2k. Therefore, removing one vertex from each short cycle leaves a smaller graph with girth greater than g, in which each color class of a coloring must be small and which therefore requires at least k colors in any coloring.

Related concepts

The odd girth and even girth of a graph are the lengths of a shortest odd cycle and shortest even cycle respectively.

Thought of as the least length of a non-trivial cycle, the girth admits natural generalisations as the 1-systole or higher systoles in systolic geometry.

References

1. ^ R. Diestel, Graph Theory, p.8. 3rd Edition, Springer-Verlag, 2005
2. ^ Girth -- Wolfram MathWorld, http://mathworld.wolfram.com/Girth.html 
3. ^ Brouwer, Andries E., Cages, http://www.win.tue.nl/~aeb/drg/graphs/ . Electronic supplement to the book Distance-Regular Graphs (Brouwer, Cohen, and Neumaier 1989, Springer-Verlag).
4. ^ Erdős, Paul (1959), "Graph theory and probability", Canadian Journal of Mathematics 11: 34–38, doi:10.4153/CJM-1959-003-9 .