| Folded cube graph | ||||
| Vertices: | 2n-1 | |||
| Edges: | 2n-2n | |||
| Diameter: | \left\lfloor
\right\rfloor | |||
| Chromatic Number: | \begin{cases}2&evenn\ 4&oddn\end{cases} | |||
| Properties: | Regular Hamiltonian Distance-transitive. | |||
In graph theory, a folded cube graph is an undirected graph formed from a hypercube graph by adding to it a perfect matching that connects opposite pairs of hypercube vertices.
The folded cube graph of dimension k (containing 2k - 1 vertices) may be formed by adding edges between opposite pairs of vertices in a hypercube graph of dimension k - 1. (In a hypercube with 2n vertices, a pair of vertices are opposite if the shortest path between them has length n.) It can, equivalently, be formed from a hypercube graph (also) of dimension k, which has twice as many vertices, by identifying together (or contracting) every opposite pair of vertices.
A dimension-k folded cube graph is a k-regular with 2k - 1 vertices and 2k - 2k edges.
The chromatic number of the dimension-k folded cube graph is two when k is even (that is, in this case, the graph is bipartite) and four when k is odd.[1] The odd girth of a folded cube of odd dimension is k, so for odd k greater than three the folded cube graphs provide a class of triangle-free graphs with chromatic number four and arbitrarily large odd girth. As a distance-regular graph with odd girth k and diameter (k - 1)/2, the folded cubes of odd dimension are examples of generalized odd graphs.[2]
When k is odd, the bipartite double cover of the dimension-k folded cube is the dimension-k cube from which it was formed. However, when k is even, the dimension-k cube is a double cover but not the bipartite double cover. In this case, the folded cube is itself already bipartite. Folded cube graphs inherit from their hypercube subgraphs the property of having a Hamiltonian cycle, and from the hypercubes that double cover them the property of being a distance-transitive graph.[3]
When k is odd, the dimension-k folded cube contains as a subgraph a complete binary tree with 2k - 1 nodes. However, when k is even, this is not possible, because in this case the folded cube is a bipartite graph with equal numbers of vertices on each side of the bipartition, very different from the nearly two-to-one ratio for the bipartition of a complete binary tree.[4]
In parallel computing, folded cube graphs have been studied as a potential network topology, as an alternative to the hypercube. Compared to a hypercube, a folded cube with the same number of nodes has nearly the same vertex degree but only half the diameter. Efficient distributed algorithms (relative to those for a hypercube) are known for broadcasting information in a folded cube.[5]