Permutation graph

The permutation (4,3,5,1,2) and the corresponding permutation graph

In mathematics, a permutation graph is a graph whose vertices represent the elements of a permutation, and whose edges represent pairs of elements that are reversed by the permutation. Permutation graphs may also be defined geometrically, as the intersection graphs of line segments whose endpoints lie on two parallel lines. Different permutations may give rise to the same permutation graph; a given graph has a unique representation (up to permutation symmetry) if it is prime with respect to the modular decomposition.[1]

Definition and characterization

If σ = (σ12, ..., σn) is any permutation of the numbers from 1 to n, then one may define a permutation graph from σ in which there are n vertices v1, v2, ..., vn, and in which there is an edge vivj for any two indices i and j for which i < j and σi > σj. That is, two indices i and j determine an edge in the permutation graph exactly when they determine an inversion in the permutation.

Given a permutation σ, one may also determine a set of line segments si with endpoints (i,0) and (σi,1). The endpoints of these segments lie on the two parallel lines y = 0 and y = 1, and two segments have a non-empty intersection if and only if they correspond to an inversion in the permutation. Thus, the permutation graph of σ coincides with the intersection graph of the segments. For every two parallel lines, and every finite set of line segments with endpoints on both lines, the intersection graph of the segments is a permutation graph; in the case that the segment endpoints are all distinct, a permutation for which it is the permutation graph may be given by numbering the segments on one of the two lines in consecutive order, and reading off these numbers in the order that the segment endpoints appear on the other line.

Permutation graphs have several other equivalent characterizations:

Efficient algorithms

It is possible to test whether a given graph is a permutation graph, and if so construct a permutation representing it, in linear time.[5]

As a subclass of the perfect graphs, many problems that are NP-complete for arbitrary graphs may be solved efficiently for permutation graphs. For instance:

Relation to other graph classes

Permutation graphs are a special case of circle graphs, comparability graphs, the complements of comparability graphs, and trapezoid graphs.

The subclasses of the permutation graphs include the bipartite permutation graphs (characterized in [8]) and the cographs.

Notes

References

External links

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