Ingleton's Inequality
In mathematics, Ingleton's inequality is an inequality that is satisfied by the rank function of any representable matroid. In this sense it is a necessary condition for representability of a matroid over a finite field. Let M be a matroid and let ρ be its rank function, Ingleton inequality states that for any subsets X1, X2, X3 and X4 in the support of M, the inequality
Aubrey William Ingleton, an English mathematician, wrote an important paper in 1969 in which he surveyed the representability problem in matroids. Although the article is mainly expository, in this paper Ingleton stated and proved Ingleton's inequality, which has found interesting applications in information theory, matroid theory, and network coding.
There are interesting connections between matroids, the entropy region and group theory. Some of those connections are revealed by Ingleton's Inequality.
Perhaps, the more interesting application of Ingleton's Inequality concerns the computation of network coding capacities. Linear coding solutions are constrained by the inequality and it has an important consequence:
For definitions see, e.g.
Theorem (Ingleton's Inequality): Let M be a representable matroid with rank function ρ and let X1, X2, X3 and X4 be subsets of the support set of M, denoted by the symbol E(M). Then:
To prove the inequality we have to show the following result:
Proposition: Let V1,V2, V3 and V4 be subspaces of a vector space V, then
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