Partition matroid

In mathematics, a partition matroid or partitional matroid is a matroid formed from a direct sum of uniform matroids.

Definition

Let $B_i$ be a collection of disjoint sets, and let $d_i$ be integers with $0\le d_i\le |B_i|$. Define a set $I$ to be "independent" when, for every index $i$, $|I\cap B_i|\le d_i$. Then the sets that are independent sets in this way form the independent sets of a matroid, called a partition matroid. The sets $B_i$ are called the blocks of the partition matroid. A basis of the matroid is a set whose intersection with every block $B_i$ has size exactly $d_i$, and a circuit of the matroid is a subset of a single block $B_i$ with size exactly $d_i+1$. The rank of the matroid is $\sum d_i$.

Every uniform matroid $U{}^r_n$ is a partition matroid, with a single block $B_1$ of $n$ elements and with $d_1=r$. Every partition matroid is the direct sum of a collection of uniform matroids, one for each of its blocks.

In some publications, the notion of a partition matroid is defined more restrictively, with every $d_i=1$. The partitions that obey this more restrictive definition are the transversal matroids of the family of disjoint sets given by their blocks.

Properties

As with the uniform matroids they are formed from, the dual matroid of a partition matroid is also a partition matroid, and every minor of a partition matroid is also a partition matroid. Direct sums of partition matroids are partition matroids as well.

Matching

A maximum matching in a graph is a set of edges that is as large as possible subject to the condition that no two edges share an endpoint. In a bipartite graph with bipartition $(U,V)$, the sets of edges satisfying the condition that no two edges share an endpoint in $U$ are the independent sets of a partition matroid with one block per vertex in $U$ and with each of the numbers $d_i$ equal to one. The sets of edges satisfying the condition that no two edges share an endpoint in $V$ are the independent sets of a second partition matroid. Therefore, the bipartite maximum matching problem can be represented as a matroid intersection of these two matroids.

More generally the matchings of a graph may be represented as an intersection of two matroids if and only if every odd cycle in the graph is a triangle containing two or more degree-two vertices.

Clique complexes

A clique complex is a family of sets of vertices of a graph $G$ that induce complete subgraphs of $G$. A clique complex forms a matroid if and only if $G$ is a complete multipartite graph, and in this case the resulting matroid is a partition matroid. The clique complexes are exactly the set systems that can be formed as intersections of families of partition matroids for which every $d_i=1$.

Enumeration

The number of distinct partition matroids that can be defined over a set of $n$ labeled elements, for $n=0,1,2,\dots$, is

1, 2, 5, 16, 62, 276, 1377, 7596, 45789, 298626, 2090910, ... (sequence A005387 in OEIS).

The exponential generating function of this sequence is $f(x)=\exp(e^x(x-1)+2x+1)$.