Split graph

This article is about graphs that can be partitioned into a clique and an independent set. For cuts that form complete bipartite graphs, see split (graph theory).

A split graph, partitioned into a clique and an independent set.

In graph theory, a branch of mathematics, a split graph is a graph in which the vertices can be partitioned into a clique and an independent set. Split graphs were first studied by Földes and Hammer (1977a, 1977b), and independently introduced by Tyshkevich and Chernyak (1979).

A split graph may have more than one partition into a clique and an independent set; for instance, the path a–b–c is a split graph, the vertices of which can be partitioned in three different ways:

1. the clique {a,b} and the independent set {c}
2. the clique {b,c} and the independent set {a}
3. the clique {b} and the independent set {a,c}

Split graphs can be characterized in terms of their forbidden induced subgraphs: a graph is split if and only if no induced subgraph is a cycle on four or five vertices, or a pair of disjoint edges (the complement of a 4-cycle).

Relation to other graph families

From the definition, split graphs are clearly closed under complementation. Another characterization of split graphs involves complementation: they are chordal graphs the complements of which are also chordal. Just as chordal graphs are the intersection graphs of subtrees of trees, split graphs are the intersection graphs of distinct substars of star graphs. Almost all chordal graphs are split graphs; that is, in the limit as n goes to infinity, the fraction of n-vertex chordal graphs that are split approaches one.

Because chordal graphs are perfect, so are the split graphs. The double split graphs, a family of graphs derived from split graphs by doubling every vertex (so the clique comes to induce an antimatching and the independent set comes to induce a matching), figure prominently as one of five basic classes of perfect graphs from which all others can be formed in the proof by Chudnovsky et al. (2006) of the Strong Perfect Graph Theorem.

If a graph is both a split graph and an interval graph, then its complement is both a split graph and a comparability graph, and vice versa. The split comparability graphs, and therefore also the split interval graphs, can be characterized in terms of a set of three forbidden induced subgraphs. The split cographs are exactly the threshold graphs, and the split permutation graphs are exactly the interval graphs that have interval graph complements. Split graphs have cochromatic number 2.

Algorithmic problems

Let G be a split graph, partitioned into a clique C and an independent set I. Then every maximal clique in a split graph is either C itself, or the neighborhood of a vertex in I. Thus, it is easy to identify the maximum clique, and complementarily the maximum independent set in a split graph. In any split graph, one of the following three possibilities must be true:

1. There exists a vertex x in I such that C ∪ {x} is complete. In this case, C ∪ {x} is a maximum clique and I is a maximum independent set.
2. There exists a vertex x in C such that I ∪ {x} is independent. In this case, I ∪ {x} is a maximum independent set and C is a maximum clique.
3. C is a maximal clique and I is a maximal independent set. In this case, G has a unique partition (C,I) into a clique and an independent set, C is the maximum clique, and I is the maximum independent set.

Some other optimization problems that are NP-complete on more general graph families, including graph coloring, are similarly straightforward on split graphs. Finding a Hamiltonian cycle remains NP-complete even for split graphs which are strongly chordal graphs. It is also well known that the Minimum Dominating Set problem remains NP-complete for split graphs.

Degree sequences

One remarkable property of split graphs is that they can be recognized solely from their degree sequences. Let the degree sequence of a graph G be d₁ ≥ d₂ ≥ ... ≥ d_n, and let m be the largest value of i such that d_i ≥ i - 1. Then G is a split graph if and only if

$\sum _{i=1}^{m}d_{i}=m(m-1)+\sum _{i=m+1}^{n}d_{i}.$

If this is the case, then the m vertices with the largest degrees form a maximum clique in G, and the remaining vertices constitute an independent set.

Counting split graphs

Royle (2000) showed that n-vertex split graphs with n are in one-to-one correspondence with certain Sperner families. Using this fact, he determined a formula for the number of (nonisomorphic) split graphs on n vertices. For small values of n, starting from n = 1, these numbers are

1, 2, 4, 9, 21, 56, 164, 557, 2223, 10766, 64956, 501696, ... (sequence A048194 in the OEIS).

This graph enumeration result was also proved earlier by Tyshkevich & Chernyak (1990).

References

• Bender, E. A.; Richmond, L. B.; Wormald, N. C. (1985), "Almost all chordal graphs split", J. Austral. Math. Soc., A, 38 (2): 214–221, doi:10.1017/S1446788700023077, MR 0770128.
• Bertossi, Alan A. (1984), "Dominating sets for split and bipartite graphs", Information Processing Letters, 19: 37–40, doi:10.1016/0020-0190(84)90126-1.
• Brandstädt, Andreas; Le, Van Bang; Spinrad, Jeremy (1999), Graph Classes: A Survey, SIAM Monographs on Discrete Mathematics and Applications, ISBN 0-89871-432-X.
• Chudnovsky, Maria; Robertson, Neil; Seymour, Paul; Thomas, Robin (2006), "The strong perfect graph theorem", Annals of Mathematics, 164 (1): 51–229, doi:10.4007/annals.2006.164.51, MR 2233847.
• Földes, Stéphane; Hammer, Peter Ladislaw (1977a), "Split graphs", Proceedings of the Eighth Southeastern Conference on Combinatorics, Graph Theory and Computing (Louisiana State Univ., Baton Rouge, La., 1977), Congressus Numerantium, vol. XIX, Winnipeg: Utilitas Math., pp. 311–315, MR 0505860.
• Földes, Stéphane; Hammer, Peter Ladislaw (1977b), "Split graphs having Dilworth number two", Canadian Journal of Mathematics, 29 (3): 666–672, doi:10.4153/CJM-1977-069-1, MR 0463041.
• Golumbic, Martin Charles (1980), Algorithmic Graph Theory and Perfect Graphs, Academic Press, ISBN 0-12-289260-7, MR 0562306.
• Hammer, Peter Ladislaw; Simeone, Bruno (1981), "The splittance of a graph", Combinatorica, 1 (3): 275–284, doi:10.1007/BF02579333, MR 0637832.
• McMorris, F. R.; Shier, D. R. (1983), "Representing chordal graphs on K_1,n", Commentationes Mathematicae Universitatis Carolinae, 24: 489–494, MR 0730144.
• Merris, Russell (2003), "Split graphs", European Journal of Combinatorics, 24 (4): 413–430, doi:10.1016/S0195-6698(03)00030-1, MR 1975945.
• Müller, Haiko (1996), "Hamiltonian Circuits in Chordal Bipartite Graphs", Discrete Mathematics, 156: 291–298, doi:10.1016/0012-365x(95)00057-4.
• Royle, Gordon F. (2000), "Counting set covers and split graphs" (PDF), Journal of Integer Sequences, 3 (2): 00.2.6, MR 1778996.
• Tyshkevich, Regina I. (1980), "[The canonical decomposition of a graph]", Doklady Akademii Nauk SSSR (in Russian), 24: 677–679, MR 0587712
• Tyshkevich, Regina I.; Chernyak, A. A. (1979), "Canonical partition of a graph defined by the degrees of its vertices", Isv. Akad. Nauk BSSR, Ser. Fiz.-Mat. Nauk (in Russian), 5: 14–26, MR 0554162.
• Tyshkevich, Regina I.; Chernyak, A. A. (1990), Еще один метод перечисления непомеченных комбинаторных объектов, Matem. Zametki (in Russian), 48 (6): 98–105, MR 1102626. Translated as "Yet another method of enumerating unmarked combinatorial objects" (1990), Mathematical notes of the Academy of Sciences of the USSR 48 (6): 1239–1245, doi:10.1007/BF01240267.
• Tyshkevich, Regina I.; Melnikow, O. I.; Kotov, V. M. (1981), "On graphs and degree sequences: the canonical decomposition", Kibernetika (in Russian), 6: 5–8, MR 0689420.
• Voss, H.-J. (1985), "Note on a paper of McMorris and Shier", Commentationes Mathematicae Universitatis Carolinae, 26: 319–322, MR 0803929.

Further reading

• A chapter on split graphs appears in the book by Martin Charles Golumbic, "Algorithmic Graph Theory and Perfect Graphs".