Uniform matroid

Matroid in which every permutation is a symmetry From Wikipedia, the free encyclopedia

Uniform matroid

In mathematics, a uniform matroid is a matroid in which the independent sets are exactly the sets containing at most r elements, for some fixed integer r. An alternative definition is that every permutation of the elements is a symmetry.

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The graphic matroid of the cycle graph C4, which is the uniform matroid . More generally, the graphic matroid of Cn is .[1]

Definition

The uniform matroid is defined over a set of elements. A subset of the elements is independent if and only if it contains at most elements. A subset is a basis if it has exactly elements, and it is a circuit if it has exactly elements. The rank of a subset is and the rank of the matroid is .[2][3]

A matroid of rank is uniform if and only if all of its circuits have exactly elements.[4]

The matroid is called the -point line.

Duality and minors

The dual matroid of the uniform matroid is another uniform matroid . A uniform matroid is self-dual if and only if .[5]

Every minor of a uniform matroid is uniform. Restricting a uniform matroid by one element (as long as ) produces the matroid and contracting it by one element (as long as ) produces the matroid .[6]

Realization

The uniform matroid may be represented as the matroid of affinely independent subsets of points in general position in -dimensional Euclidean space, or as the matroid of linearly independent subsets of vectors in general position in an -dimensional real vector space.

Every uniform matroid may also be realized in projective spaces and vector spaces over all sufficiently large finite fields.[7] However, the field must be large enough to include enough independent vectors. For instance, the -point line can be realized only over finite fields of or more elements (because otherwise the projective line over that field would have fewer than points): is not a binary matroid, is not a ternary matroid, etc. For this reason, uniform matroids play an important role in Rota's conjecture concerning the forbidden minor characterization of the matroids that can be realized over finite fields.[8]

Algorithms

The problem of finding the minimum-weight basis of a weighted uniform matroid is well-studied in computer science as the selection problem. It may be solved in linear time.[9]

Any algorithm that tests whether a given matroid is uniform, given access to the matroid via an independence oracle, must perform an exponential number of oracle queries, and therefore cannot take polynomial time.[10]

Summarize
Perspective

The free matroid over a given ground-set E is the matroid in which the independent sets are all subsets of E. It is a special case of a uniform matroid; specifically, when E has cardinality , it is the uniform matroid .[11]

Unless , a uniform matroid is connected: it is not the direct sum of two smaller matroids.[12] The direct sum of a family of uniform matroids (not necessarily all with the same parameters) is called a partition matroid.

Every uniform matroid is a paving matroid,[13] a transversal matroid[14] and a strict gammoid.[7]

Not every uniform matroid is graphic, and the uniform matroids provide the smallest example of a non-graphic matroid, . The uniform matroid is the graphic matroid of an -edge dipole graph, and the dual uniform matroid is the graphic matroid of its dual graph, the -edge cycle graph. is the graphic matroid of a graph with self-loops, and is the graphic matroid of an -edge forest. Other than these examples, every uniform matroid with contains as a minor and therefore is not graphic.[1]

The -point line provides an example of a Sylvester matroid, a matroid in which every line contains three or more points.[15]

See also

References

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