# Split graph

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 Template:Harvs, and independently introduced by Template:Harvs.[1]

A split graph may have more than one partition into a clique and an independent set; for instance, the path abc 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).[2]

## Relation to other graph families

From the definition, split graphs are clearly closed under complementation.[3] Another characterization of split graphs involves complementation: they are chordal graphs the complements of which are also chordal.[4] Just as chordal graphs are the intersection graphs of subtrees of trees, split graphs are the intersection graphs of distinct substars of star graphs.[5] 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.[6]

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 Template:Harvtxt 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.[7] The split cographs are exactly the threshold graphs, and the split permutation graphs are exactly the interval graphs that have interval graph complements.[8] Split graphs have cochromatic number 2.

## Maximum clique and maximum independent set

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:[9]

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.

## 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 d1d2 ≥ ... ≥ dn, and let m be the largest value of i such that dii - 1. Then G is a split graph if and only if

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

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.[10]

## Counting split graphs

Template:Harvtxt 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 OEIS).

This graph enumeration result was also proved earlier by Template:Harvtxt.

## Notes

1. Template:Harvtxt had a more general definition, in which the graphs they called split graphs also included bipartite graphs (that is, graphs that be partitioned into two independent sets) and the complements of bipartite graphs (that is, graphs that can be partitioned into two cliques). Template:Harvtxt use the definition given here, which has been followed in the subsequent literature; for instance, it is Template:Harvtxt, Definition 3.2.3, p.41.
2. Template:Harvtxt; Template:Harvtxt, Theorem 6.3, p. 151.
3. Template:Harvtxt, Theorem 6.1, p. 150.
4. Template:Harvtxt; Template:Harvtxt, Theorem 6.3, p. 151; Template:Harvtxt, Theorem 3.2.3, p. 41.
5. Template:Harvtxt; Template:Harvtxt; Template:Harvtxt, Theorem 4.4.2, p. 52.
6. Template:Harvtxt; Template:Harvtxt, Theorem 9.7, page 212.
7. Template:Harvtxt, Corollary 7.1.1, p. 106, and Theorem 7.1.2, p. 107.
8. Template:Harvtxt; Template:Harvtxt, Theorem 6.2, p. 150.
9. Template:Harvtxt; Template:Harvtxt; Template:Harvtxt; Template:Harvtxt, Theorem 6.7 and Corollary 6.8, p. 154; Template:Harvtxt, Theorem 13.3.2, p. 203. Template:Harvtxt further investigates the degree sequences of split graphs.

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