# Partition of a set

In mathematics, a **partition of a set** is a grouping of its elements into non-empty subsets, in such a way that every element is included in exactly one subset.

Every equivalence relation on a set defines a partition of this set, and every partition defines an equivalence relation. A set equipped with an equivalence relation or a partition is sometimes called a setoid, typically in type theory and proof theory.

A partition of a set *X* is a set of non-empty subsets of *X* such that every element *x* in *X* is in exactly one of these subsets^{[2]} (i.e., *X* is a disjoint union of the subsets).

Equivalently, a family of sets *P* is a partition of *X* if and only if all of the following conditions hold:^{[3]}

For any equivalence relation on a set *X*, the set of its equivalence classes is a partition of *X*. Conversely, from any partition *P* of *X*, we can define an equivalence relation on *X* by setting *x* ~ *y* precisely when *x* and *y* are in the same part in *P*. Thus the notions of equivalence relation and partition are essentially equivalent.^{[5]}

The axiom of choice guarantees for any partition of a set *X* the existence of a subset of *X* containing exactly one element from each part of the partition. This implies that given an equivalence relation on a set one can select a canonical representative element from every equivalence class.

A partition *α* of a set *X* is a **refinement** of a partition *ρ* of *X*—and we say that *α* is *finer* than *ρ* and that *ρ* is *coarser* than *α*—if every element of *α* is a subset of some element of *ρ*. Informally, this means that *α* is a further fragmentation of *ρ*. In that case, it is written that *α* ≤ *ρ*.

This *finer-than* relation on the set of partitions of *X* is a partial order (so the notation "≤" is appropriate). Each set of elements has a least upper bound and a greatest lower bound, so that it forms a lattice, and more specifically (for partitions of a finite set) it is a geometric lattice.^{[6]} The *partition lattice* of a 4-element set has 15 elements and is depicted in the Hasse diagram on the left.

The lattice of noncrossing partitions of a finite set has recently taken on importance because of its role in free probability theory. These form a subset of the lattice of all partitions, but not a sublattice, since the join operations of the two lattices do not agree.

The total number of partitions of an *n*-element set is the Bell number *B _{n}*. The first several Bell numbers are

*B*

_{0}= 1,

*B*

_{1}= 1,

*B*

_{2}= 2,

*B*

_{3}= 5,

*B*

_{4}= 15,

*B*

_{5}= 52, and

*B*

_{6}= 203 (sequence in the OEIS). Bell numbers satisfy the recursion

The Bell numbers may also be computed using the Bell triangle in which the first value in each row is copied from the end of the previous row, and subsequent values are computed by adding two numbers, the number to the left and the number to the above left of the position. The Bell numbers are repeated along both sides of this triangle. The numbers within the triangle count partitions in which a given element is the largest singleton.

The number of partitions of an *n*-element set into exactly *k* (non-empty) parts is the Stirling number of the second kind *S*(*n*, *k*).

The number of noncrossing partitions of an *n*-element set is the Catalan number