In mathematics, a basis of a matroid is a maximal independent set of the matroid—that is, an independent set that is not contained in any other independent set.
In a transversal matroid, where the independent sets are endpoints of matchings in a given bipartite graph, the bases are called transversals.
In a linear matroid, where the independent sets are the linearly-independent sets of vectors in a given vector-space, the bases are just called bases of the vector space. Hence, the concept of basis of a matroid generalizes the concept of basis from linear algebra.
In a uniform matroid, where the independent sets are all sets with cardinality at most k (for some integer k), the bases are all sets with cardinality exactly k.
In a partition matroid, where elements are partitioned into categories and the independent sets are all sets containing at most kc elements from each category c, the bases are all sets which contain exactly kc elements from category c.
In a free matroid, where all subsets of the ground-set E are independent, the unique basis is E.
All matroids satisfy the following properties, for any two distinct bases and :[2][3]
Basis-exchange property: if , then there exists an element such that is a basis.
Symmetric basis-exchange property: if , then there exists an element such that both and are bases. Brualdi[4] showed that it is in fact equivalent to the basis-exchange property.
Multiple symmetric basis-exchange property: if , then there exists a subset such that both and are bases. Brylawski, Greene, and Woodall, showed (independently) that it is in fact equivalent to the basis-exchange property.
Bijective basis-exchange property: There is a bijection from to , such that for every , is a basis. Brualdi[4] showed that it is equivalent to the basis-exchange property.
Partition basis-exchange property: For each partition of into m parts, there exists a partition of into m parts, such that for every , is a basis.[5]
However, a basis-exchange property that is both symmetric and bijective is not satisfied by all matroids: it is satisfied only by base-orderable matroids.
In general, in the symmetric basis-exchange property, the element need not be unique. Regular matroids have the unique exchange property, meaning that for some, the corresponding b is unique.[6]
It follows from the basis exchange property that no member of can be a proper subset of another.
Moreover, all bases of a given matroid have the same cardinality. In a linear matroid, the cardinality of all bases is called the dimension of the vector space.
It is conjectured that all matroids satisfy the following property:[2] For every integer t ≥ 1, If B and B' are two t-tuples of bases with the same multi-set union, then there is a sequence of symmetric exchanges that transforms B to B'.
The bases of a matroid characterize the matroid completely: a set is independent if and only if it is a subset of a basis. Moreover, one may define a matroid to be a pair , where is the ground-set and is a collection of subsets of , called "bases", with the following properties:[7][8]
(B1) There is at least one base -- is nonempty;
(B2) If and are distinct bases, and , then there exists an element such that is a basis (this is the basis-exchange property).
(B2) implies that, given any two bases A and B, we can transform A into B by a sequence of exchanges of a single element. In particular, this implies that all bases must have the same cardinality.
If is a finite matroid, we can define the orthogonal or dual matroid by calling a set a basis in if and only if its complement is in . It can be verified that is indeed a matroid. The definition immediately implies that the dual of is .[9]: 32 [10]
Using duality, one can prove that the property (B2) can be replaced by the following:
(B2*) If and are distinct bases, and , then there exists an element such that is a basis.
A dual notion to a basis is a circuit. A circuit in a matroid is a minimal dependent set—that is, a dependent set whose proper subsets are all independent. The terminology arises because the circuits of graphic matroids are cycles in the corresponding graphs.
One may define a matroid to be a pair , where is the ground-set and is a collection of subsets of , called "circuits", with the following properties:[8]
(C1) The empty set is not a circuit;
(C2) A proper subset of a circuit is not a circuit;
(C3) If C1 and C2 are distinct circuits, and x is an element in their intersection, then contains a circuit.
Another property of circuits is that, if a set is independent, and the set is dependent (i.e., adding the element makes it dependent), then contains a unique circuit , and it contains . This circuit is called the fundamental circuit of w.r.t. . It is analogous to the linear algebra fact, that if adding a vector to an independent vector set makes it dependent, then there is a unique linear combination of elements of that equals .[10]
Matroid polytope - a polytope in Rn (where n is the number of elements in the matroid), whose vertices are indicator vectors of the bases of the matroid.
^Welsh, D. J. A. (1976), Matroid Theory, L.M.S. Monographs, vol. 8, Academic Press, ISBN978-0-12-744050-7, Zbl0343.05002. Section 1.2, "Axiom Systems for a Matroid", pp. 7–9.