Matroid

A combinatorial abstraction of a linear algebra. A matroid is specified by a set of elements and a family of subsets of , called independent subsets, which satisfy the following axioms: 1) the empty set is independent; 2) any subset of an independent set is independent; 3) for every subset , all the independent subsets of the matroid which are contained in and are maximal with respect to inclusion relative to have the same number of elements.

Examples. 1) The set of rows of an arbitrary rectangular matrix and the family of all subsets of consisting of linearly independent rows form a matroid. 2) Let be the set of all skeleton forests (see Tree) of a graph , and let be the set of edges of the forest , . Then the set of edges of the graph and the family form a matroid. 3) Let be a bipartite graph (cf. Graph, bipartite) with parts . A subset of vertices for which there is a matching of the graph such that every vertex is incident to some edge of is called a partial transversal. The set and the set of all transversals of the graph form a so-called transversal matroid.

A matroid can be also specified by a set of elements and a family of non-empty subsets , called circuits, which satisfy the following axioms: no proper subset of a circuit is a circuit; and if , then contains a circuit. The independent subsets of this matroid are the subsets that do not contain circuits.

If is a graph, then the set of its edges and the family of its simple circuits form what is called a graphic matroid. If for the circuits of a matroid one takes the cocycles (cuts, cf. Graph, connectivity of a) of the graph , the resulting matroid is called a cographic matroid. The matroids of the last two types are also termed cyclic and cocyclic. The notion of a "matroid" is used in graph theory and combinatorics in the proof of some assertions on covering and packing of matchings.

References

[1]	H. Whitney, "On the abstract properties of linear dependence" Amer. J. Math. , 57 (1935) pp. 509–533
[2]	W.T. Tutte, "Lectures on matroids" J. Res. Nat. Bur. Standards Sec. B , 69 : 1–2 (1965) pp. 1–47

Comments

The example of the matroid consisting of the rows of a (rectangular) matrix gave rise to the name.

In matroid theory, most often the underlying set is supposed to be finite. And, in fact, it is not particularly clear what the right definitions are in the infinite case (cf. [a1], Chapt. 20). For a finite the third independence axiom (given the other 2) is equivalent to ) If and , then there is a such that .

There are many axiom systems for matroids. In addition to those based on the ideas of independent subsets and circuits there are axiom systems based on the idea of a rank function, the idea of a basis, the idea of a hyperplane, or the idea of a closure operation.

A maximal independent set is called a basis (and a minimal dependent set is called a circuit or cycle of the matroid). The maximal cardinality of an independent set contained in a subset of is called the rank . Given , the set is called the closure of ; one calls closed if and only if . A maximal (proper) closed subset of is called a hyperplane.

For a finite the "basis axiomatization" is as follows. A non-empty collection of subsets of is the set of bases of a matroid if and only if for all and there is an such that .

A closure operation on a set is a mapping of subsets of to subsets of such that , , . Such a closure operation defines a matroid if for all , , one has (the exchange axiom). The corresponding independent subsets are defined by: if and only if .

Given any matroid , there is a dual matroid , which is most simply defined in terms of bases as follows: If is the set of all bases of , then the set is the set of bases of . The duality theory has important applications; as a striking example, one could mention the following result of H. Whitney: A graph with associated graphic matroid (determined by the circuits, respectively forests, of as above) is planar if and only if the dual matroid is also graphic.

Matroids are also studied from a more geometric point of view, under the name "combinatorial geometries" (cf. also Combinatorial geometry).

Finally, it is also possible to define matroids in an algorithmic way. Let be a set system satisfying conditions 1) and 2) above and consider a weighting on , i.e. a mapping into the real numbers satisfying for all . One extends to the power set of by putting for each subset of . It is required to find a subset of maximal weight (among all subsets in ). Then is a matroid if and only if the greedy algorithm solves this problem for every weighting : One orders the elements of according to weight, say and determines (starting from the empty set) recursively as follows: In step , the element is added to unless this results in a set no longer contained in . Due to this algorithmic property, matroids are an extremely important tool in combinatorial optimization.

Matroids are also used in the stratification of Grassmann manifolds, in the analysis of higher-dimensional splines and -adic curves, and in many other areas.

An important related concept is that of an oriented matroid, which is an abstraction of a linear algebra over an ordered field, and is used in the study of convex polytopes.

The seminal paper that started the whole theory of matroids is [1] (also included in [a8]).

References

[a1]	D.J.A. Welsh, "Matroid theory" , Acad. Press (1976); reprinted (Courier Dover Publications, 2010) ISBN 9780486474397
[a2]	E.L. Lawler, "Combinatorial optimization: networks and matroids" , Holt, Rinehart & Winston (1976)
[a3]	C.H. Papadimitriou, K. Steiglitz, "Combinatorial optimization. Algorithms and complexity" , Prentice-Hall (1982)
[a4]	Neil White (ed.) , Theory of matroids , Cambridge Univ. Press (1986)
[a5]	Neil White (ed.) , Combinatorial geometries, Encyclopedia of Mathematics and its Applications 29. Cambridge Univ. Press (1987) ISBN 0-521-33339-3, Zbl 0626.00007
[a6]	Neil White (ed.) , Combinatorial geometries: Advanced theory , Cambridge Univ. Press (1986)
[a7]	M. Aigner, "Kombinatorik II. Matroide und Transversaltheorie" , Springer (1976)
[a8]	J.P.S. Kung, "A source book in matroid theory" , Birkhäuser (1986)
[a9]	Oxley, James G. Matroid theory. Oxford Science Publications. Oxford: Oxford University Press (1992). ISBN 0-19-853563-5, Zbl 0784.05002

Greedoid

A generalisation of the concept of matroid. A greedoid on a set $V$ is a set system $\mathcal{F}$ of subset of $V$, called "feasible" sets, with the properties: 1) the empty set is feasible, $\emptyset \in \mathcal{F}$; 2) if $F \in \mathcal{F}$ is non-empty, then there is $x \in F$ such that $F \setminus \{x\} \in \mathcal{F}$; 3) if $X, Y \in \mathcal{F}$ with $|X| > |Y|$ then there is $x \in X$ such that $Y \cup \{x\} \in \mathcal{F}$.

Axiom (3), the exchange property, implies that all maximal feasible sets have the same number of elements. The independent sets of a matroid form a greedoid, and the feasible sets of a greedoid form a matroid if they are hereditary, that is, axiom (2) holds in the strong form that for every $x \in F$, $F \setminus \{x\} \in \mathcal{F}$.

References

• Björner, Anders; Ziegler, Günter M. "Introduction to greedoids", Matroid applications, ed Neil White, Encycl. Math. Appl. 40, Cambridge University Press (1992) 284-357. ISBN 0-521-38165-7 Zbl 0772.05026