Free matroid

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Free matroid

In mathematics, 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 .[1] The unique basis of this matroid is the ground-set itself, E. Among matroids on E, the free matroid on E has the most independent sets, the highest rank, and the fewest circuits.

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The graphic matroid of a forest with 4 edges, which is the free matroid with a ground set of size 4 (also the uniform matroid ). More generally, the graphic matroid of a forest with n edges is .

Every free matroid with a ground set of size n is the graphic matroid of an n-edge forest.[2]

Free extension of a matroid

The free extension of a matroid by some element , denoted , is a matroid whose elements are the elements of plus the new element , and:

  • Its circuits are the circuits of plus the sets for all bases of .[3]
  • Equivalently, its independent sets are the independent sets of plus the sets for all independent sets that are not bases.
  • Equivalently, its bases are the bases of plus the sets for all independent sets of size .

References

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