# Transversal (combinatorics)

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In combinatorial mathematics, given a collection C of sets, a transversal is a set containing exactly one element from each member of the collection. When the sets of the collection are mutually disjoint, each element of the transversal corresponds to exactly one member of C (the set it is a member of). If the original sets are not disjoint, there are two possibilities for the definition of a transversal. One variation, the one that mimics the situation when the sets are mutually disjoint, is that there is a bijection f from the transversal to C such that x is an element of f(x) for each x in the transversal. In this case, the transversal is also called a system of distinct representatives. The other, less commonly used, possibility does not require a one-to-one relation between the elements of the transversal and the sets of C. Loosely speaking, in this situation the members of the system of representatives are not necessarily distinct.

A partial transversal is a set containing at most one element from each member of the collection, or (in the stricter form of the concept) a set with an injection from the set to C.

The transversals of a finite collection C of finite sets form the basis sets of a matroid, the "transversal matroid" of C. The independent sets of the transversal matroid are the partial transversals of C.

A generalization of the concept of a transversal would be a set that just has a non-empty intersection with each member of C. An example of this would be a Bernstein set, which is defined as a set that has a non-empty intersection with each set of C, but contains no set of C, where C is the collection of all perfect sets of a topological Polish space. As another example, let C consist of all the lines of a projective plane, then a blocking set in this plane is a set of points which intersects each line but contains no line.

## Examples

In group theory, given a subgroup H of a group G, a right (respectively left) transversal is a set containing exactly one element from each right (respectively left) coset of H. In this case, the "sets" (cosets) are mutually disjoint.

Given a direct product of groups $G=H\times K$ , then H is a transversal for the cosets of K.

Hall's marriage theorem gives necessary and sufficient conditions for a finite collection of not necessarily distinct, but non-empty sets to have a transversal.

## Systems of distinct representatives

A refinement, due to H. J. Ryser, of Hall's marriage theorem gives lower bounds on the number of systems of distinct representatives (SDRs) of a collection of sets.

Theorem. Let S1, S2, ..., Sm be a collection of sets such that $S_{i_{1}}\cup S_{i_{2}}\cup \dots \cup S_{i_{k}}$ contains at least k elements for k = 1,2,...,m and for all k-combinations {$i_{1},i_{2},\ldots ,i_{k}$ } of the integers 1,2,...,m and suppose that each of these sets contains at least t elements. If tm then the collection has at least t ! SDRs, and if t > m then the collection has at least t ! / (t - m)! SDRs.

## Category theory

In the language of category theory, a transversal of a collection of mutually disjoint sets is a section of the quotient map induced by the collection.