Circular-arc graph

A circular-arc graph (left) and a set of circular arcs representing it (right).

In graph theory, a circular-arc graph is the intersection graph of a set of arcs on the circle. It has one vertex for each arc in the set, and an edge between every pair of vertices corresponding to arcs that intersect.

Formally, let

$I_{1},I_{2},\ldots ,I_{n}\subset C_{1}$

be a set of arcs. Then the corresponding circular-arc graph is G = (V, E) where

$V=\{I_{1},I_{2},\ldots ,I_{n}\}$

and

$\{I_{\alpha },I_{\beta }\}\in E\iff I_{\alpha }\cap I_{\beta }\neq \varnothing .$

A family of arcs that corresponds to G is called an arc model.

Recognition

Tucker demonstrated the first polynomial recognition algorithm for circular-arc graphs, which runs in ${\mathcal {O}}(n^{3})$ time. More recently, McConnell gave the first linear (${\mathcal {O}}(n+m)$) recognition algorithm.

Relation to other graph classes

Circular-arc graphs are a natural generalization of interval graphs. If a circular-arc graph G has an arc model that leaves some point of the circle uncovered, the circle can be cut at that point and stretched to a line, which results in an interval representation. Unlike interval graphs, however, circular-arc graphs are not always perfect, as the odd chordless cycles C₅, C₇, etc., are circular-arc graphs.

Some subclasses

In the following, let $G=(V,E)$ be an arbitrary graph.

Unit circular-arc graphs

$G$ is a unit circular-arc graph if there exists a corresponding arc model such that each arc is of equal length.

Proper circular-arc graphs

$G$ is a proper circular-arc graph if there exists a corresponding arc model such that no arc properly contains another.

Helly circular-arc graphs

$G$ is a Helly circular-arc graph if there exists a corresponding arc model such that the arcs constitute a Helly family. Gavril gave a characterization of this class that implies an ${{\mathcal {O}}(n^{3})}$ recognition algorithm.

Applications

Circular-arc graphs are useful in modeling periodic resource allocation problems in operations research. Each interval represents a request for a resource for a specific period repeated in time.

References

• Golumbic, Martin Charles (1980), Algorithmic Graph Theory and Perfect Graphs, Academic Press, ISBN 0-444-51530-5 Second edition, Annals of Discrete Mathematics 57, Elsevier, 2004.

External links

• "circular arc graph". Information System on Graph Class Inclusions. {{cite web}}: External link in |work= (help)