# Partition matroid

In mathematics, a partition matroid or partitional matroid is a matroid formed from a direct sum of uniform matroids.[1]

## 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 $\sum d_i$.[2]

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.[3]

## 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.[4]

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.[5]

## 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 $d_i=1$.[6]

## 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 $f(x)=\exp(e^x(x-1)+2x+1)$.[7]

## References

1. ^ Recski, A. (1975), "On partitional matroids with applications", Infinite and finite sets (Colloq., Keszthely, 1973; dedicated to P. Erdős on his 60th birthday), Vol. III, Colloq. Math. Soc. Janós Bolyai 10, Amsterdam: North-Holland, pp. 1169–1179, MR 0389630.
2. ^ Lawler, Eugene L. (1976), Combinatorial Optimization: Networks and Matroids, Rinehart and Winston, New York: Holt, p. 272, MR 0439106.
3. ^ E.g., see Kashiwabara, Okamoto & Uno (2007). Lawler (1976) uses the broader definition but notes that the $d_i=1$ restriction is useful in many applications.
4. ^ Papadimitriou, Christos H.; Steiglitz, Kenneth (1982), Combinatorial Optimization: Algorithms and Complexity, Englewood Cliffs, N.J.: Prentice-Hall Inc., pp. 289–290, ISBN 0-13-152462-3, MR 663728.
5. ^ Fekete, Sándor P.; Firla, Robert T.; Spille, Bianca (2003), "Characterizing matchings as the intersection of matroids", Mathematical Methods of Operations Research 58 (2): 319–329, arXiv:math/0212235, doi:10.1007/s001860300301, MR 2015015.
6. ^ Kashiwabara, Kenji; Okamoto, Yoshio; Uno, Takeaki (2007), "Matroid representation of clique complexes", Discrete Applied Mathematics 155 (15): 1910–1929, doi:10.1016/j.dam.2007.05.004, MR 2351976. For the same results in a complementary form using independent sets in place of cliques, see Tyshkevich, R. I.; Urbanovich, O. P.; Zverovich, I. È. (1989), "Matroidal decomposition of a graph", Combinatorics and graph theory (Warsaw, 1987), Banach Center Publ. 25, Warsaw: PWN, pp. 195–205, MR 1097648.
7. ^ Recski, A. (1974), "Enumerating partitional matroids", Studia Scientiarum Mathematicarum Hungarica 9: 247–249 (1975), MR 0379248.