Matching (graph theory)
Matching (graph theory)
In the mathematical discipline of graph theory, a matching or independent edge set in a graph is a set of edges without common vertices. Finding a matching in a bipartite graph can be treated as a network flow problem.
Definition
Given a graph G = (V,E), a matching M in G is a set of pairwise non-adjacent edges, none of which are loops; that is, no two edges share a common vertex.
A vertex is matched (or saturated) if it is an endpoint of one of the edges in the matching. Otherwise the vertex is unmatched.
A maximal matching is a matching M of a graph G with the property that if any edge not in M is added to M, it is no longer a matching, that is, M is maximal if it is not a subset of any other matching in graph G. In other words, a matching M of a graph G is maximal if every edge in G has a non-empty intersection with at least one edge in M. The following figure shows examples of maximal matchings (red) in three graphs.
A perfect matching (a.k.a. 1-factor) is a matching which matches all vertices of the graph. That is, every vertex of the graph is incident to exactly one edge of the matching. Every perfect matching is maximum and hence maximal. In some literature, the term complete matching is used. In the above figure, only part (b) shows a perfect matching. A perfect matching is also a minimum-size edge cover. Thus, ν(G) ≤ ρ(G) , that is, the size of a maximum matching is no larger than the size of a minimum edge cover. A perfect matching can only occur when the graph has an even number of vertices.
A near-perfect matching is one in which exactly one vertex is unmatched. This can only occur when the graph has an odd number of vertices, and such a matching must be maximum. In the above figure, part (c) shows a near-perfect matching. If, for every vertex in a graph, there is a near-perfect matching that omits only that vertex, the graph is also called factor-critical.
Given a matching M,
an alternating path is a path that begins with an unmatched vertex and[2] whose edges belong alternately to the matching and not to the matching.
an augmenting path is an alternating path that starts from and ends on free (unmatched) vertices.
One can prove that a matching is maximum if and only if it does not have any augmenting path. (This result is sometimes called Berge's lemma.)
An induced matching is a matching that is an induced subgraph.[3]
Properties
In any graph without isolated vertices, the sum of the matching number and the edge covering number equals the number of vertices.[4] If there is a perfect matching, then both the matching number and the edge cover number are |V | / 2.
If A and B are two maximal matchings, then |A| ≤ 2|B| and |B| ≤ 2|A|. To see this, observe that each edge in B \ A can be adjacent to at most two edges in A \ B because A is a matching; moreover each edge in A \ B is adjacent to an edge in B \ A by maximality of B, hence
Further we deduce that
In particular, this shows that any maximal matching is a 2-approximation of a maximum matching and also a 2-approximation of a minimum maximal matching. This inequality is tight: for example, if G is a path with 3 edges and 4 vertices, the size of a minimum maximal matching is 1 and the size of a maximum matching is 2.
Matching polynomials
A generating function of the number of k-edge matchings in a graph is called a matching polynomial. Let G be a graph and mk be the number of k-edge matchings. One matching polynomial of G is
Another definition gives the matching polynomial as
where n is the number of vertices in the graph. Each type has its uses; for more information see the article on matching polynomials.
Algorithms and computational complexity
Maximum matching
A fundamental problem in combinatorial optimization is finding a maximum matching. This problem has various algorithms for different classes of graphs:
In an unweighted bipartite graph, the optimization problem is to find a maximum cardinality matching[5]. This problem is often called maximum bipartite matching[5] or maximum cardinality bipartite matching. Micali and Vazirani's matching algorithm, the fastest general algorithm known so far, runs in time O(√VE) time.[6] A randomised algorithm by Mucha and Sankowski,[7] based on the fast matrix multiplication algorithm, gives complexity. For the special case of planar graphs the problem can be solved in time .[8]
In a weighted bipartite graph, the optimization problem is to find a maximum-weight matching; a dual problem is to find a minimum-weight matching. This problem is often called maximum weighted bipartite matching, or the assignment problem. The Hungarian algorithm solves the assignment problem and it was one of the beginnings of combinatorial optimization algorithms. It uses a modified shortest path search in the augmenting path algorithm. If the Bellman–Ford algorithm is used for this step, the running time of the Hungarian algorithm becomes , or the edge cost can be shifted with a potential to achieve running time with the Dijkstra algorithm and Fibonacci heap.[9]
In a non-bipartite weighted graph, the problem of maximum weight matching can be solved in time using simply Edmonds' blossom algorithm.
Maximal matchings
A maximal matching can be found with a simple greedy algorithm. A maximum matching is also a maximal matching, and hence it is possible to find a largest maximal matching in polynomial time. However, no polynomial-time algorithm is known for finding a minimum maximal matching, that is, a maximal matching that contains the smallest possible number of edges.
A maximal matching with k edges is an edge dominating set with k edges. Conversely, if we are given a minimum edge dominating set with k edges, we can construct a maximal matching with k edges in polynomial time. Therefore, the problem of finding a minimum maximal matching is essentially equal to the problem of finding a minimum edge dominating set.[10] Both of these two optimization problems are known to be NP-hard; the decision versions of these problems are classical examples of NP-complete problems.[11] Both problems can be approximated within factor 2 in polynomial time: simply find an arbitrary maximal matching M.[12]
Counting problems
The number of matchings in a graph is known as the Hosoya index of the graph. It is #P-complete to compute this quantity, even for bipartite graphs.[13] It is also #P-complete to count perfect matchings, even in bipartite graphs, because computing the permanent of an arbitrary 0–1 matrix (another #P-complete problem) is the same as computing the number of perfect matchings in the bipartite graph having the given matrix as its biadjacency matrix. However, there exists a fully polynomial time randomized approximation scheme for counting the number of bipartite matchings.[14] A remarkable theorem of Kasteleyn states that the number of perfect matchings in a planar graph can be computed exactly in polynomial time via the FKT algorithm.
Finding all maximally-matchable edges
Online bipartite matching
Characterizations and notes
Kőnig's theorem states that, in bipartite graphs, the maximum matching is equal in size to the minimum vertex cover. Via this result, the minimum vertex cover, maximum independent set, and maximum vertex biclique problems may be solved in polynomial time for bipartite graphs.
Hall's marriage theorem provides a characterization of bipartite graphs which have a perfect matching and the Tutte theorem provides a characterization for arbitrary graphs.
A perfect matching is a spanning 1-regular subgraph, a.k.a. a 1-factor. In general, a spanning k-regular subgraph is a k-factor.
Applications
Matching in general graphs
A Kekulé structure of an aromatic compound consists of a perfect matching of its carbon skeleton, showing the locations of double bonds in the chemical structure. These structures are named after Friedrich August Kekulé von Stradonitz, who showed that benzene (in graph theoretical terms, a 6-vertex cycle) can be given such a structure.[24]
The Hosoya index is the number of non-empty matchings plus one; it is used in computational chemistry and mathematical chemistry investigations for organic compounds.
Matching in bipartite graphs
Graduation problem [27] is about choosing minimum set of classes from given requirements for graduation.
Hitchcock transport problem involves bipartite matching as sub-problem.
Subtree isomorphism problem involves bipartite matching as sub-problem.
See also
Dulmage–Mendelsohn decomposition, a partition of the vertices of a bipartite graph into subsets such that each edge belongs to a perfect matching if and only if its endpoints belong to the same subset
Edge coloring, a partition of the edges of a graph into matchings
Matching preclusion, the minimum number of edges to delete to prevent a perfect matching from existing
Rainbow matching, a matching in an edge-colored bipartite graph with no repeated colors
Skew-symmetric graph, a type of graph that can be used to model alternating path searches for matchings
Stable matching, a matching in which no two elements prefer each other to their matched partners
Vertex independent set, a set of vertices (rather than edges) no two of which are adjacent to each other
Stable marriage problem (also known as stable matching problem)