Mathematician Gian-Carlo Rota conjectured in 1971 that, for every finite field, the family of matroids that can be represented over that field has finitely many forbidden minors. Rota's conjecture is now known to be true for fields of four or fewer elements, but remains unproven for larger fields.
Statement of the conjecture
If is a set of points in a vector space defined over a field , then the linearly independent subsets of form the independent sets of a matroid ; is said to be a representation of any matroid isomorphic to . Not every matroid has a representation over every field; for instance, the Fano plane (a matroid with seven elements in which a set of up to three points is independent if it does not form one of seven three-point lines) is representable only over fields of characteristic two. Therefore, the matroids that are representable over a field form a proper subclass of all matroids.
A minor of a matroid is another matroid formed by two operations: restriction and contraction. In the case of points from a vector space, restriction is simply the removal of some of the points from ; contraction is a dual operation in which points are removed and the remaining space is projected into a space complementary to the span of the removed points. Thus, minors preserve representability. A matroid that is not representable over , and is minor-minimal with that property, is called a "forbidden minor"; a matroid is representable over if and only if it does not contain one of the forbidden minors.
W. T. Tutte proved that the binary matroids (matroids representable over the field of two elements) have a single forbidden minor, the uniform matroid (geometrically, a line with four points on it).
A matroid is representable over the ternary field GF(3) if and only if it does not have one or more of the following four matroids as minors: a five-point line , its dual matroid (five points in general position in three dimensions), the Fano plane, or the dual of the Fano plane. Thus, Rota's conjecture is true in this case as well. As a consequence of this result and of the forbidden minor characterization by Tutte (1958) of the regular matroids (matroids that can be represented over all fields) it follows that a matroid is regular if and only if it is both binary and ternary.
There are seven forbidden minors for the matroids representable over GF(4). They are:
- The six-point line .
- The dual to the six-point line, six points in general position in four dimensions.
- A self-dual six-point rank-three matroid with a single three-point line.
- The non-Fano matroid formed by the seven points at the vertices, edge midpoints, and centroid of an equilateral triangle in the Euclidean plane. This configuration is one of two known sets of planar points with fewer than two-point lines.
- The dual of the non-Fano matroid,
- The eight-point matroid of a square antiprism, and the dual of the antiprism matroid.
For GF(5), several forbidden minors on up to 12 elements are known, but it is not known whether the list is complete.
Geoff Whittle announced recently during a visit to the UK that he, Jim Geelen, and Bert Gerards have solved Rota's Conjecture. It will take them some years to write up their research in its entirety and publish it. 
- Rota's basis conjecture, a different conjecture by Rota about linear algebra and matroids
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- Rota's Conjecture: Researcher solves 40-year-old math problem PhysOrg, August 15, 2013.
- CWI researcher proves famous Rota’s Conjecture CWI, August 22, 2013.