Dimension (graph theory)

In mathematics, and particularly in graph theory, the dimension of a graph is the least integer n such that there exists a "classical representation" of the graph in the Euclidean space of dimension n with all the edges having unit length.

In a classical representation, the vertices must be distinct points, but the edges may cross one another.

For example, the Petersen graph can be drawn with unit edges in

This concept was introduced in 1965 by Paul Erdős, Frank Harary and William Tutte.

[2] It generalises the concept of unit distance graph to more than 2 dimensions.

In the worst case, every pair of vertices is connected, giving a complete graph.

with all the edges having unit length, we need the Euclidean space of dimension

In other words, the dimension of the complete graph is the same as that of the simplex having the same number of vertices.

, have dimension 2, as shown in the figure to the left.

Star graphs with m equal to 1 or 2 need only dimension 1.

The dimension of a complete bipartite graph

, can be drawn as in the figure to the right, by placing m vertices on a circle whose radius is less than a unit, and the other two vertices one each side of the plane of the circle, at a suitable distance from it.

has dimension 2, as it can be drawn as a unit rhombus in the plane.

Theorem — The dimension of a general complete bipartite graph

To show that 4-space is sufficient, as with the previous case, we use circles.

, we arrange one set of vertices arbitrarily on the circle given by

lie on a unit sphere with center

Likewise, they lie on unit spheres with centers

are not all distinct; or it does not, so its intersection with the third sphere is no more than two points, insufficient to accommodate

To summarise: Theorem — The dimension of any graph G is always less than or equal to twice its chromatic number: This proof also uses circles.

We write n for the chromatic number of G, and assign the integers

-dimensional Euclidean space, with its dimensions denoted

, we arrange all the vertices of colour n arbitrarily on the circle given by

The definition of the dimension of a graph given above says, of the minimal-n representation: This definition is rejected by some authors.

A different definition was proposed in 1991 by Alexander Soifer, for what he termed the Euclidean dimension of a graph.

[4] Previously, in 1980, Paul Erdős and Miklós Simonovits had already proposed it with the name faithful dimension.

[5] By this definition, the minimal-n representation is one such that two vertices of the graph are connected if and only if their representations are at distance 1.

The figures opposite show the difference between these definitions, in the case of a wheel graph having a central vertex and six peripheral vertices, with one spoke removed.

Its representation in the plane allows two vertices at distance 1, but they are not connected.

It is never less than the dimension defined as above: Paul Erdős and Miklós Simonovits proved the following result in 1980:[5] Theorem — The Euclidean dimension of a graph G is no more than twice its maximal degree plus one: It is NP-hard, and more specifically complete for the existential theory of the reals, to test whether the dimension or the Euclidean dimension of a given graph is at most a given value.