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Uniform Matroid


The uniform matroid U_(r,n) is the matroid on an n-element ground set whose independent sets are exactly the subsets of size at most r, where 0<=r<=n. Its matroid rank is r, and its matroid bases are all the r-element subsets. In particular it has (n; r) matroid bases.

A parallel q-thickening replaces each ground set element by a parallel class of q elements. Chen et al. (2026) proved that every such thickening of U_(r,n) with 0<r<n and q>=3 satisfies the multiplicative Merino-Welsh conjecture. For q=2, they found that 62 is the smallest number of elements in a counterexample within this family, attained by doubling each element of U_(21,31). This minimum is not asserted over all matroids.


See also

Matroid, Matroid Basis, Matroid Rank, Merino-Welsh Conjecture

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References

Chen, J.; Jin, X.; and Ma, T. "On Minimum-Size Matroid Counterexamples to the Multiplicative Merino-Welsh Inequality." 20 Oct 2026. https://arxiv.org/abs/2610.27706.Oxley, J. Matroid Theory, 2nd ed. Oxford, England: Oxford University Press, 2011. https://doi.org/10.1093/acprof:oso/9780198566946.001.0001.

Cite this as:

Weisstein, Eric W. "Uniform Matroid." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/UniformMatroid.html

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