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In graph theory, a factor of a graph G is a spanning subgraph, i.e., a subgraph that has the same vertex set as G. A k-factor of a graph is a spanning k-regular subgraph, and a k-factorization partitions the edges of the graph into disjoint k-factors. A graph G is said to be k-factorable if it admits a k-factorization. In particular, a 1-factor is a perfect matching, and a 1-factorization of a k-regular graph is an edge coloring with k colors. A 2-factor is a collection of cycles that spans all vertices of the graph.

1-factorization

1-factorization of Desargues graph: each color class is a 1-factor.

If a graph is 1-factorable, then it has to be a regular graph. However, not all regular graphs are 1-factorable. A k-regular graph is 1-factorable if it has chromatic index k; examples of such graphs include:

Any regular bipartite graph.^[1] Hall's marriage theorem can be used to show that a k-regular bipartite graph contains a perfect matching. One can then remove the perfect matching to obtain a (k − 1)-regular bipartite graph, and apply the same reasoning repeatedly.
Any complete graph with an even number of nodes (see below).^[2]

However, there are also k-regular graphs that have chromatic index k + 1, and these graphs are not 1-factorable; examples of such graphs include:

Any regular graph with an odd number of nodes.
The Petersen graph.

Complete graphs

A 1-factorization of a complete graph corresponds to pairings in a round-robin tournament. The 1-factorization of complete graphs is a special case of Baranyai's theorem concerning the 1-factorization of complete hypergraphs.

Petersen graph can be partitioned into a 1-factor (red) and a 2-factor (blue). However, the graph is not 1-factorable.

One method for constructing a 1-factorization of a complete graph involves placing all but one of the vertices on a circle, forming a regular polygon, with the remaining vertex at the center of the circle. With this arrangement of vertices, one way of constructing a 1-factor of the graph is to choose an edge e from the center to a single polygon vertex together with all possible edges that lie on lines perpendicular to e. The 1-factors that can be constructed in this way form a 1-factorization of the graph.

1-factorization of K₈ in which each 1-factor consists of an edge from the center to a vertex of a heptagon together with all possible perpendicular edges

The number of distinct 1-factorizations of K₂, K₄, K₆, K₈, ... is 1, 1, 6, 6240, 1225566720, 252282619805368320, 98758655816833727741338583040, ... OEIS A000438.

1-factorization conjectureThe overfull conjecture implies the 1-factorization conjecture.

Let G be a k-regular graph with 2n nodes. If k is sufficiently large, it is known that G has to be 1-factorable:

If k = 2n − 1, then G is the complete graph K_2n, and hence 1-factorable (see above).
If k = 2n − 2, then G can be constructed by removing a perfect matching from K_2n. Again, G is 1-factorable.
Chetwynd & Hilton (1985) show that if k ≥ 12n/7, then G is 1-factorable.

The 1-factorization conjecture^[3] is a long-standing conjecture that states that k ≈ n is sufficient. In precise terms, the conjecture is:

If n is odd and k ≥ n, then G is 1-factorable. If n is even and k ≥ n − 1 then G is 1-factorable.

The overfull conjecture implies the 1-factorization conjecture.

Perfect 1-factorization

A perfect pair from a 1-factorization is a pair of 1-factors whose union induces a Hamiltonian cycle.

A perfect 1-factorization (P1F) of a graph is a 1-factorization having the property that every pair of 1-factors is a perfect pair. A perfect 1-factorization should not be confused with a perfect matching (also called a 1-factor).

In 1964, Anton Kotzig conjectured that every complete graph K_2n where n ≥ 2 has a perfect 1-factorization. So far, it is known that the following graphs have a perfect 1-factorization:^[4]

the infinite family of complete graphs K_2p where p is an odd prime (by Anderson and also Nakamura, independently),
the infinite family of complete graphs K_{p + 1} where p is an odd prime,
and sporadic additional results, including K_2n where 2n ∈ {16, 28, 36, 40, 50, 126, 170, 244, 344, 730, 1332, 1370, 1850, 2198, 3126, 6860, 12168, 16808, 29792}. Some newer results are collected here.

If the complete graph K_{n + 1} has a perfect 1-factorization, then the complete bipartite graph K_n,n also has a perfect 1-factorization.^[5]

2-factorization

If a graph is 2-factorable, then it has to be 2k-regular for some integer k. Julius Petersen showed in 1891 that this necessary condition is also sufficient: any 2k-regular graph is 2-factorable.^[6]

If a connected graph is 2k-regular and has an even number of edges it may also be k-factored, by choosing each of the two factors to be an alternating subset of the edges of an Euler tour.^[7] This applies only to connected graphs; disconnected counterexamples include disjoint unions of odd cycles, or of copies of K_{2k +1}.

Notes

Harary (1969), Theorem 9.2, p. 85. Diestel (2005), Corollary 2.1.3, p. 37.
Harary (1969), Theorem 9.1, p. 85.
Chetwynd & Hilton (1985). Niessen (1994). Perkovic & Reed (1997). West.
Wallis, W. D. (1997), "16. Perfect Factorizations", One-factorizations, Mathematics and Its Applications 390 (1 ed.), Springer US, p. 125, doi:10.1007/978-1-4757-2564-3_16, ISBN 978-0-7923-4323-3
Bryant, Darryn; Maenhaut, Barbara M.; Wanless, Ian M. (May 2002), "A Family of Perfect Factorisations of Complete Bipartite Graphs", Journal of Combinatorial Theory, A 98 (2): 328–342, doi:10.1006/jcta.2001.3240, ISSN 0097-3165
Petersen (1891), §9, p. 200. Harary (1969), Theorem 9.9, p. 90. See Diestel (2005), Corollary 2.1.5, p. 39 for a proof.

Petersen (1891), §6, p. 198.

References

Bondy, John Adrian; Murty, U. S. R. (1976), Graph Theory with Applications, North-Holland, ISBN 0-444-19451-7, Section 5.1: "Matchings".
Chetwynd, A. G.; Hilton, A. J. W. (1985), "Regular graphs of high degree are 1-factorizable", Proceedings of the London Mathematical Society 50 (2): 193–206, doi:10.1112/plms/s3-50.2.193.
Diestel, Reinhard (2005), Graph Theory (3rd ed.), Springer, ISBN 3-540-26182-6, Chapter 2: "Matching, covering and packing". Electronic edition.
Harary, Frank (1969), Graph Theory, Addison-Wesley, ISBN 0-201-02787-9, Chapter 9: "Factorization".
Hazewinkel, Michiel, ed. (2001), "One-factorization", Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
Niessen, Thomas (1994), "How to find overfull subgraphs in graphs with large maximum degree", Discrete Applied Mathematics 51 (1–2): 117–125, doi:10.1016/0166-218X(94)90101-5.
Perkovic, L.; Reed, B. (1997), "Edge coloring regular graphs of high degree", Discrete Mathematics, 165–166: 567–578, doi:10.1016/S0012-365X(96)00202-6.
Petersen, Julius (1891), "Die Theorie der regulären graphs", Acta Mathematica 15: 193–220, doi:10.1007/BF02392606.
West, Douglas B. "1-Factorization Conjecture (1985?)". Open Problems – Graph Theory and Combinatorics. Retrieved 2010-01-09.
Weisstein, Eric W., "Graph Factor", MathWorld.
Weisstein, Eric W., "k-Factor", MathWorld.
Weisstein, Eric W., "k-Factorable Graph", MathWorld.

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Graph factorization