# Perfect graph

Last updated The Paley graph of order 9, colored with three colors and showing a clique of three vertices. In this graph and each of its induced subgraphs the chromatic number equals the clique number, so it is a perfect graph.

In graph theory, a perfect graph is a graph in which the chromatic number of every induced subgraph equals the order of the largest clique of that subgraph (clique number). Equivalently stated in symbolic terms an arbitrary graph $G=(V,E)$ is perfect if and only if for all $S\subseteq V$ we have $\chi (G[S])=\omega (G[S])$ .

## Contents

The perfect graphs include many important families of graphs and serve to unify results relating colorings and cliques in those families. For instance, in all perfect graphs, the graph coloring problem, maximum clique problem, and maximum independent set problem can all be solved in polynomial time. In addition, several important min-max theorems in combinatorics, such as Dilworth's theorem, can be expressed in terms of the perfection of certain associated graphs.

## Properties

• By the perfect graph theorem, a graph $G$ is perfect if and only if its complement ${\bar {G}}$ is perfect.
• By the strong perfect graph theorem, perfect graphs are the same as Berge graphs, which are graphs $G$ where neither $G$ nor ${\bar {G}}$ contains an induced cycle of odd length 5 or more.

See below section for more details.

## History

The theory of perfect graphs developed from a 1958 result of Tibor Gallai that in modern language can be interpreted as stating that the complement of a bipartite graph is perfect; this result can also be viewed as a simple equivalent of Kőnig's theorem, a much earlier result relating matchings and vertex covers in bipartite graphs. The first use of the phrase "perfect graph" appears to be in a 1963 paper of Claude Berge, after whom Berge graphs are named. In this paper he unified Gallai's result with several similar results by defining perfect graphs, and he conjectured the equivalence of the perfect graph and Berge graph definitions; his conjecture was proved in 2002 as the strong perfect graph theorem.

## Families of graphs that are perfect

Some of the more well-known perfect graphs are: 

• Bipartite graphs, which are graphs that can be colored with two colors, including forests (graphs without cycles).
• Line graphs of bipartite graphs (see Kőnig's theorem). Rook's graphs (line graphs of complete bipartite graphs) are a special case.
• Chordal graphs, the graphs in which every cycle of four or more vertices has a chord, an edge between two vertices that are not consecutive in the cycle. These include
• Comparability graphs formed from partially ordered sets by connecting pairs of elements by an edge whenever they are related in the partial order. These include:
• bipartite graphs, complements of interval graphs, trivially perfect graphs, threshold graphs, windmill graphs,
• permutation graphs (graphs in which the edges represent pairs of elements that are reversed by a permutation),
• and cographs (graphs formed by recursive operations of disjoint union and complementation).
• Perfectly orderable graphs, which are graphs that can be ordered in such a way that a greedy coloring algorithm is optimal on every induced subgraph. These include the bipartite graphs, the chordal graphs, the comparability graphs,
• Trapezoid graphs, which are intersection graphs of trapezoids whose parallel pairs of edges lie on two parallel lines. These include the interval graphs, trivially perfect graphs, threshold graphs, windmill graphs, and permutation graphs; their complements are a subset of the comparability graphs.

## Relation to min-max theorems

In all graphs, the clique number provides a lower bound for the chromatic number, as all vertices in a clique must be assigned distinct colors in any proper coloring. The perfect graphs are those for which this lower bound is tight, not just in the graph itself but in all of its induced subgraphs. For graphs that are not perfect, the chromatic number and clique number can differ; for instance, a cycle of length five requires three colors in any proper coloring but its largest clique has size two.

A proof that a class of graphs is perfect can be seen as a min-max theorem: the minimum number of colors needed for these graphs equals the maximum size of a clique. Many important min-max theorems in combinatorics can be expressed in these terms. For instance, Dilworth's theorem states that the minimum number of chains in a partition of a partially ordered set into chains equals the maximum size of an antichain, and can be rephrased as stating that the complements of comparability graphs are perfect. Mirsky's theorem states that the minimum number of antichains into a partition into antichains equals the maximum size of a chain, and corresponds in the same way to the perfection of comparability graphs.

The perfection of permutation graphs is equivalent to the statement that, in every sequence of ordered elements, the length of the longest decreasing subsequence equals the minimum number of sequences in a partition into increasing subsequences. The Erdős–Szekeres theorem is an easy consequence of this statement.

Kőnig's theorem in graph theory states that a minimum vertex cover in a bipartite graph corresponds to a maximum matching, and vice versa; it can be interpreted as the perfection of the complements of bipartite graphs. Another theorem about bipartite graphs, that their chromatic index equals their maximum degree, is equivalent to the perfection of the line graphs of bipartite graphs.

## Characterizations and the perfect graph theorems

In his initial work on perfect graphs, Berge made two important conjectures on their structure that were only proved later.

The first of these two theorems was the perfect graph theorem of Lovász (1972), stating that a graph is perfect if and only if its complement is perfect. Thus, perfection (defined as the equality of maximum clique size and chromatic number in every induced subgraph) is equivalent to the equality of maximum independent set size and clique cover number. A seven-vertex cycle and its complement, showing in each case an optimal coloring and a maximum clique (shown with heavy edges). Since neither graph uses a number of colors equal to its clique size, neither is perfect.

The second theorem, conjectured by Berge, provided a forbidden graph characterization of perfect graphs. An induced cycle of odd length at least 5 is called an odd hole. An induced subgraph that is the complement of an odd hole is called an odd antihole. An odd cycle of length greater than 3 cannot be perfect, because its chromatic number is three and its clique number is two. Similarly, the complement of an odd cycle of length 2k + 1 cannot be perfect, because its chromatic number is k + 1 and its clique number is k. (Alternatively, the imperfection of this graph follows from the perfect graph theorem and the imperfection of the complementary odd cycle). Because these graphs are not perfect, every perfect graph must be a Berge graph, a graph with no odd holes and no odd antiholes. Berge conjectured the converse, that every Berge graph is perfect. This was finally proven as the strong perfect graph theorem of Chudnovsky, Robertson, Seymour, and Thomas (2006). It trivially implies the perfect graph theorem, hence the name.

The perfect graph theorem has a short proof, but the proof of the strong perfect graph theorem is long and technical, based on a deep structural decomposition of Berge graphs. Related decomposition techniques have also borne fruit in the study of other graph classes, and in particular for the claw-free graphs.

There is a third theorem, again due to Lovász, which was originally suggested by Hajnal. It states that a graph is perfect if the sizes of the largest clique, and the largest independent set, when multiplied together, equal or exceed the number of vertices of the graph, and the same is true for any induced subgraph. It is an easy consequence of the strong perfect graph theorem, while the perfect graph theorem is an easy consequence of it.

The Hajnal characterization is not met by odd n-cycles or their complements for n > 3: the odd cycle on n > 3 vertices has clique number 2 and independence number (n 1)/2. The reverse is true for the complement, so in both cases the product is n 1.

## Algorithms on perfect graphs

In all perfect graphs, the graph coloring problem, maximum clique problem, and maximum independent set problem can all be solved in polynomial time ( Grötschel, Lovász & Schrijver 1988 ). The algorithm for the general case involves the Lovász number of these graphs, which (for the complement of a given graph) is sandwiched between the chromatic number and clique number. Calculating the Lovász number can be formulated as a semidefinite program and approximated numerically in polynomial time using the ellipsoid method for linear programming. For perfect graphs, rounding this approximation to an integer gives the chromatic number and clique number in polynomial time; the maximum independent set can be found by applying the same approach to the complement of the graph. However, this method is complicated and has a high polynomial exponent. More efficient combinatorial algorithms are known for many special cases.

For many years the complexity of recognizing Berge graphs and perfect graphs remained open. From the definition of Berge graphs, it follows immediately that their recognition is in co-NP (Lovász 1983). Finally, subsequent to the proof of the strong perfect graph theorem, a polynomial time algorithm was discovered by Chudnovsky, Cornuéjols, Liu, Seymour, and Vušković.

## Related Research Articles In the mathematical field of graph theory, a bipartite graph is a graph whose vertices can be divided into two disjoint and independent sets and such that every edge connects a vertex in to one in . Vertex sets and are usually called the parts of the graph. Equivalently, a bipartite graph is a graph that does not contain any odd-length cycles.

This is a glossary of graph theory terms. Graph theory is the study of graphs, systems of nodes or vertices connected in pairs by lines or edges. In graph theory, graph coloring is a special case of graph labeling; it is an assignment of labels traditionally called "colors" to elements of a graph subject to certain constraints. In its simplest form, it is a way of coloring the vertices of a graph such that no two adjacent vertices are of the same color; this is called a vertex coloring. Similarly, an edge coloring assigns a color to each edge so that no two adjacent edges are of the same color, and a face coloring of a planar graph assigns a color to each face or region so that no two faces that share a boundary have the same color. In the mathematical area of graph theory, a clique is a subset of vertices of an undirected graph such that every two distinct vertices in the clique are adjacent; that is, its induced subgraph is complete. Cliques are one of the basic concepts of graph theory and are used in many other mathematical problems and constructions on graphs. Cliques have also been studied in computer science: the task of finding whether there is a clique of a given size in a graph is NP-complete, but despite this hardness result, many algorithms for finding cliques have been studied.

In graph theory, the perfect graph theorem of László Lovász states that an undirected graph is perfect if and only if its complement graph is also perfect. This result had been conjectured by Berge, and it is sometimes called the weak perfect graph theorem to distinguish it from the strong perfect graph theorem characterizing perfect graphs by their forbidden induced subgraphs. In graph theory, an edge coloring of a graph is an assignment of "colors" to the edges of the graph so that no two incident edges have the same color. For example, the figure to the right shows an edge coloring of a graph by the colors red, blue, and green. Edge colorings are one of several different types of graph coloring. The edge-coloring problem asks whether it is possible to color the edges of a given graph using at most k different colors, for a given value of k, or with the fewest possible colors. The minimum required number of colors for the edges of a given graph is called the chromatic index of the graph. For example, the edges of the graph in the illustration can be colored by three colors but cannot be colored by two colors, so the graph shown has chromatic index three. In the mathematical area of graph theory, a chordal graph is one in which all cycles of four or more vertices have a chord, which is an edge that is not part of the cycle but connects two vertices of the cycle. Equivalently, every induced cycle in the graph should have exactly three vertices. The chordal graphs may also be characterized as the graphs that have perfect elimination orderings, as the graphs in which each minimal separator is a clique, and as the intersection graphs of subtrees of a tree. They are sometimes also called rigid circuit graphs or triangulated graphs. In graph theory, a Meyniel graph is a graph in which every odd cycle of length five or more has at least two chords, edges connecting non-consecutive vertices of the cycle. The chords may be uncrossed or they may cross each other, as long as there are at least two of them.

In graph theory, the strong perfect graph theorem is a forbidden graph characterization of the perfect graphs as being exactly the graphs that have neither odd holes nor odd antiholes. It was conjectured by Claude Berge in 1961. A proof by Maria Chudnovsky, Neil Robertson, Paul Seymour, and Robin Thomas was announced in 2002 and published by them in 2006.

In mathematics, in the areas of order theory and combinatorics, Dilworth's theorem characterizes the width of any finite partially ordered set in terms of a partition of the order into a minimum number of chains. It is named for the mathematician Robert P. Dilworth (1950). In graph theory, the complement or inverse of a graph G is a graph H on the same vertices such that two distinct vertices of H are adjacent if and only if they are not adjacent in G. That is, to generate the complement of a graph, one fills in all the missing edges required to form a complete graph, and removes all the edges that were previously there. It is not, however, the set complement of the graph; only the edges are complemented. In the mathematical area of graph theory, Kőnig's theorem, proved by Dénes Kőnig (1931), describes an equivalence between the maximum matching problem and the minimum vertex cover problem in bipartite graphs. It was discovered independently, also in 1931, by Jenő Egerváry in the more general case of weighted graphs. In graph theory, a rook's graph is a graph that represents all legal moves of the rook chess piece on a chessboard. Each vertex of a rook's graph represents a square on a chessboard, and each edge represents a legal move from one square to another. The same graphs can be defined mathematically as the Cartesian products of two complete graphs or as the line graphs of complete bipartite graphs. In graph theory, an area of mathematics, a claw-free graph is a graph that does not have a claw as an induced subgraph.

In graph theory, a clique cover or partition into cliques of a given undirected graph is a partition of the vertices of the graph into cliques, subsets of vertices within which every two vertices are adjacent. A minimum clique cover is a clique cover that uses as few cliques as possible. The minimum k for which a clique cover exists is called the clique cover number of the given graph. In graph theory, a branch of discrete mathematics, a distance-hereditary graph is a graph in which the distances in any connected induced subgraph are the same as they are in the original graph. Thus, any induced subgraph inherits the distances of the larger graph.

In graph theory, an area of mathematics, an equitable coloring is an assignment of colors to the vertices of an undirected graph, in such a way that In the mathematical field of graph theory, the bull graph is a planar undirected graph with 5 vertices and 5 edges, in the form of a triangle with two disjoint pendant edges. In graph theory, a skew partition of a graph is a partition of its vertices into two subsets, such that the induced subgraph formed by one of the two subsets is disconnected and the induced subgraph formed by the other subset is the complement of a disconnected graph. Skew partitions play an important role in the theory of perfect graphs.

In graph theory, a branch of mathematics, the Erdős–Hajnal conjecture states that families of graphs defined by forbidden induced subgraphs have either large cliques or large independent sets. It is named for Paul Erdős and András Hajnal.

1. West, Douglas Brent, author. (2017-02-14). Introduction to graph theory. ISBN   9780131437371. OCLC   966410137.CS1 maint: multiple names: authors list (link)