Intersection graph

In the mathematical area of graph theory, an intersection graph is a graph that represents the pattern of intersections of a family of sets. Any graph may be represented as an intersection graph, but some important special classes of graphs may be defined by the types of sets that are used to form an intersection representation of them.

For an overview of the theory of intersection graphs, and of important special classes of intersection graphs, see McKee & McMorris (1999).

Formal definition

Formally, an intersection graph is an undirected graph formed from a family of sets

S_i, i = 0, 1, 2, ...

by creating one vertex v_i for each set S_i, and connecting two vertices v_i and v_j by an edge whenever the corresponding two sets have a nonempty intersection, that is,

E(G) = {{v_i, v_j} | S_i ∩ S_j ≠ ∅}.

All graphs are intersection graphs

Any undirected graph G may be represented as an intersection graph: for each vertex v_i of G, form a set S_i consisting of the edges incident to v_i; then two such sets have a nonempty intersection if and only if the corresponding vertices share an edge. Erdős, Goodman & Pósa (1966) provide a construction that is more efficient (which is to say requires a smaller total number of elements in all of the sets S_i combined) in which the total number of set elements is at most n²/4 where n is the number of vertices in the graph. They credit the observation that all graphs are intersection graphs to Szpilrajn-Marczewski (1945), but say to see also Čulík (1964). The intersection number of a graph is the minimum total number of elements in any intersection representation of the graph.

Classes of intersection graphs

Many important graph families can be described as intersection graphs of more restricted types of set families, for instance sets derived from some kind of geometric configuration:

• An interval graph is defined as the intersection graph of intervals on the real line, or of connected subgraphs of a path graph.
• A circular arc graph is defined as the intersection graph of arcs on a circle.
• A polygon-circle graph is defined as the intersection of polygons with corners on a circle.
• One characterization of a chordal graph is as the intersection graph of connected subgraphs of a tree.
• A trapezoid graph is defined as the intersection graph of trapezoids formed from two parallel lines. They are a generalization of the notion of permutation graph, in turn they are a special case of the family of the complements of comparability graphs known as cocomparability graphs.
• A unit disk graph is defined as the intersection graph of unit disks in the plane.
• The circle packing theorem states that planar graphs are exactly the intersection graphs of families of closed disks in the plane bounded by non-crossing circles.
• Scheinerman's conjecture (now a theorem) states that every planar graph can also be represented as an intersection graph of line segments in the plane. However, intersection graphs of line segments may be nonplanar as well, and recognizing intersection graphs of line segments is complete for the existential theory of the reals (Schaefer 2010).
• The line graph of a graph G is defined as the intersection graph of the edges of G, where we represent each edge as the set of its two endpoints.
• A string graph is the intersection graph of curves on a plane.
• A graph has boxicity k if it is the intersection graph of multidimensional boxes of dimension k, but not of any smaller dimension.

Related concepts

An order-theoretic analog to the intersection graphs are the containment orders. In the same way that an intersection representation of a graph labels every vertex with a set so that vertices are adjacent if and only if their sets have nonempty intersection, so a containment representation f of a poset labels every element with a set so that for any x and y in the poset, x ≤ y if and only if f(x) ⊆ f(y).

References

• Čulík, K. (1964), "Applications of graph theory to mathematical logic and linguistics", Theory of Graphs and its Applications (Proc. Sympos. Smolenice, 1963), Prague: Publ. House Czechoslovak Acad. Sci., pp. 13–20, MR0176940 .
• Erdős, Paul; Goodman, A. W.; Pósa, Louis (1966), "The representation of a graph by set intersections", Canadian Journal of Mathematics 18 (1): 106–112, doi:10.4153/CJM-1966-014-3, MR0186575, http://www.renyi.hu/~p_erdos/1966-21.pdf .
• Golumbic, Martin Charles (1980), Algorithmic Graph Theory and Perfect Graphs, Academic Press, ISBN 0-12-289260-7 .
• McKee, Terry A.; McMorris, F. R. (1999), Topics in Intersection Graph Theory, SIAM Monographs on Discrete Mathematics and Applications, 2, Philadelphia: Society for Industrial and Applied Mathematics, ISBN 0-89871-430-3, MR1672910 
• Szpilrajn-Marczewski, E. (1945), "Sur deux propriétés des classes d'ensembles", Fund. Math. 33: 303–307, MR0015448 .
• Schaefer, Marcus (2010), "Complexity of Some Geometric and Topological Problems", Graph Drawing, 17th International Symposium, GS 2009, Chicago, IL, USA, September 2009, Revised Papers, Lecture Notes in Computer Science, 5849, Springer-Verlag, pp. 334–344, doi:10.1007/978-3-642-11805-0_32, ISBN 978-3-642-11804-3, http://ovid.cs.depaul.edu/documents/convex.pdf .

External links

• Jan Kratochvíl, A video lecture on intersection graphs (June 2007)

• E. Prisner, A Journey through Intersection Graph County