# 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 harvtxt|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,:$E(G)=\{\{v\_i,v\_j\}mid\; S\_icap\; S\_j\; evarnothing\}.$**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. harvtxt|Erdős|Goodman|Pósa|1966 provide a more efficient construction in which the total number of set elements is at most*n*^{2}/4. They credit the observation that all graphs are intersection graphs to harvtxt|Szpilrajn-Marczewski|1945, but say to see also harvtxt|Čulík|1963.**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.* One characterization of a

chordal graph is as the intersection graph of connected subgraphs of a tree.* A

unit disk graph is defined as the intersection graph of unit disks in the plane.* The

circle packing theorem states thatplanar graph s are exactly the intersection graphs of families of closed disks in the plane bounded by non-crossing circles.Scheinerman's conjecture states that every planar graph can also be represented as an intersection graph of line segments in the plane.* 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 strings 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 order s. 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 aposet labels every element with a set so that for any "x" and "y" in the poset, "x" ≤ "y" if and only if "f"("x") $subseteq$ "f"("y").**References***citation

last = Čulík | first = K.

contribution = Applications of graph theory to mathematical logic and linguistics

title = Proc. Symp. Graph Theory, Smolenice

year = 1963 | pages = 13–20 | id = MathSciNet | id=0176940.*citation

last1 = Erdős | first1 = Paul | authorlink1 = Paul Erdős

last2 = Goodman | first2 = A. W. | last3 = Pósa | first3 = Louis

title = The representation of a graph by set intersections

journal = Canadian Journal of Mathematics

volume = 18 | issue = 1 | year = 1966 | pages = 106–112

id = MathSciNet | id=0186575.*citation

last1 = McKee | first1 = Terry A. | last2 = McMorris | first2 = F. R.

title = Topics in Intersection Graph Theory

year = 1999

publisher = Society for Industrial and Applied Mathematics (SIAM Monographs on Discrete Mathematics and Applications, No. 2)

location = Philadelphia

isbn = 0-89871-430-3 | id = MathSciNet | id = 1672910*citation

first = E. | last = Szpilrajn-Marczewski

title = Sur deux propriétés des classes d'ensembles

journal = Fund. Math. | volume = 33 | year = 1945 | pages = 303–307

id = MathSciNet | id = 0015448.

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