ECE 21
ABSTACTFEB
19920Two of the most well-known problems in graph theory are: D (a) Find a maximum matching (or perfect matching, if one exists), and (b) Find a maximum independent set of vertices.The first problem-usually called the matching problem-is known to have a polynomial algorithm; the second-often called the vertex packing problem-is known to be NP-complete. However, many graph theorists-especially those who do not deal much with complexity of algorithms-know little more about the complexity issues associated with these two problems than these two basic facts.What is not so widely known within the graph theory community is that these two problems have motivated a great deal of recent activity in the area of algorithms and their complexity. Of course it is not known whether or not P = NP, but most workers in the area currently believe that equality is unlikely to hold. Motivated by this belief, a number of people have studied variations of both matching and vertex packing with the general theme being two-fold. On the one hand, one can add various side conditions to the matching problem and study the complexity-both sequential and parallel-of the resulting --problems. On the other hand, one can investigate certain large and interesting classes of graphs trying to prove that for these classes the vertex packing problem has a polynomial solution.Each branch of this two-pronged attack has yielded both interesting theorems and 0 perplexing unsolved problems. This paper will survey this work.
I Introduction and Background: The two fundamental ProblemsIn this paper, graphs will be assumed to be connected, undirected and will have no multiple edges or loops. A matching is any set of independent edges; i.e., no two have a vertex in common. A maximal matching is a matching not properly contained in any other matching. A maximum matching is one of largest cardinality. A perfect matching (sometimes called a 1-factor) in a graph G is a matching which covers all vertices of G. A set of vertices S C V(G) is independent if no two vertices of S are adjacent. An * work supported by ONR Contract #N00014-85-K-0488, #N00014-91-J-1142 and Laboratoire de Recherche en Informatique, CNRS, Univ. Paris Sud ___ 1 ir P'zblic release and sale; its ) I di-ziutIon is unlimited. independent set S is maximal if it is not a proper subset of any other independent set and maximum if it is an independent set of largest cardinality.So far, then, maximal and maximum matchings and independent sets are quite analogous; matchings corresponding to sets of edges and independent sets corresponding to sets of vertices. To be sure, they are related. A (maximal, maximum) matching in a graph G corresponds to a (maximal, maximum) independent set in the line graph L(G). But we shall soon see that the concepts quickly diverge in difficulty.First, however, let us note that matching and vertex packing remain closely related, at least in the computational sense, if one considers the class of bipartite graphs. Historically speaking, it was this class of gra...