2-factor theorem explained

In the mathematical discipline of graph theory, the 2-factor theorem, discovered by Julius Petersen, is one of the earliest works in graph theory. It can be stated as follows:

Let

G

be a regular graph whose degree is an even number,
2k

. Then the edges of
G

can be partitioned into
k

edge-disjoint 2-factors.

Here, a 2-factor is a subgraph of

in which all vertices have degree two; that is, it is a collection of cycles that together touch each vertex exactly once.

Proof

In order to prove this generalized form of the theorem, Petersen first proved that a 4-regular graph can be factorized into two 2-factors by taking alternate edges in a Eulerian trail. He noted that the same technique used for the 4-regular graph yields a factorization of a

-regular graph into two

-factors.

such that every point has indegree and outdegree

. Next, replace every vertex

v\inV(D)

by two vertices

and

v''

, and replace every directed edge

of the oriented graph by an undirected edge from

v''

. Since

has in- and outdegrees equal to

the resulting bipartite graph

-regular. The edges of

can be partitioned into

perfect matchings by a theorem of Kőnig. Now merging

with

v''

for every

recovers the graph

, and maps the

perfect matchings of

onto

2-factors of

which partition its edges.

History

The theorem was discovered by Julius Petersen, a Danish mathematician. It is one of the first results ever discovered in the field of graph theory. The theorem appears first in the 1891 article "Die Theorie der regulären graphs". To prove the theorem, Petersen's fundamental idea was to 'colour' the edges of a trail or a path alternatively red and blue, and then to use the edges of one or both colours for the construction of other paths or trials.

2-factor theorem explained

Proof

History

See also