Binary matroid

In matroid theory, a binary matroid is a matroid that can be represented over the finite field GF(2).^[1] That is, up to isomorphism, they are the matroids whose elements are the columns of a (0,1)-matrix and whose sets of elements are independent if and only if the corresponding columns are linearly independent in GF(2).

A matroid $M$ is binary if and only if

It is the matroid defined from a symmetric (0,1)-matrix.^[2]
For every set ${\mathcal {S}}$ of circuits of the matroid, the symmetric difference of the circuits in ${\mathcal {S}}$ can be represented as a disjoint union of circuits.^[3]^[4]
For every pair of circuits of the matroid, their symmetric difference contains another circuit.^[4]
For every pair $C,D$ where $C$ is a circuit of $M$ and $D$ is a circuit of the dual matroid of $M$ , $|C\cap D|$ is an even number.^[4]^[5]
For every pair $B,C$ where $B$ is a basis of $M$ and $C$ is a circuit of $M$ , $C$ is the symmetric difference of the fundamental circuits induced in $B$ by the elements of $C\setminus B$ .^[4]^[5]
No matroid minor of $M$ is the uniform matroid $U{}_{4}^{2}$ , the four-point line.^[6]^[7]^[8]
In the geometric lattice associated to the matroid, every interval of height two has at most five elements.^[8]

Related matroids

Every regular matroid, and every graphic matroid, is binary.^[5] A binary matroid is regular if and only if it does not contain the Fano plane (a seven-element non-regular binary matroid) or its dual as a minor.^[9] A binary matroid is graphic if and only if its minors do not include the dual of the graphic matroid of $K_{5}$ nor of $K_{3,3}$ .^[10] If every circuit of a binary matroid has odd cardinality, then its circuits must all be disjoint from each other; in this case, it may be represented as the graphic matroid of a cactus graph.^[5]

Additional properties

If $M$ is a binary matroid, then so is its dual, and so is every minor of $M$ .^[5] Additionally, the direct sum of binary matroids is binary.

Harary & Welsh (1969) define a bipartite matroid to be a matroid in which every circuit has even cardinality, and an Eulerian matroid to be a matroid in which the elements can be partitioned into disjoint circuits. Within the class of graphic matroids, these two properties describe the matroids of bipartite graphs and Eulerian graphs (not-necessarily-connected graphs in which all vertices have even degree), respectively. For planar graphs (and therefore also for the graphic matroids of planar graphs) these two properties are dual: a planar graph or its matroid is bipartite if and only if its dual is Eulerian. The same is true for binary matroids. However, there exist non-binary matroids for which this duality breaks down.^[5]^[11]

Any algorithm that tests whether a given matroid is binary, given access to the matroid via an independence oracle, must perform an exponential number of oracle queries, and therefore cannot take polynomial time.^[12]

Related matroids

Additional properties

References

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