Definition:Set Partition

Definition
Let $S$ be a set.

A partition of $S$ is a set of subsets $\Bbb S$ of $S$ such that:


 * $(1): \quad$ All sets in $\Bbb S$ are pairwise disjoint: $\forall S_1, S_2 \in \Bbb S: S_1 \cap S_2 = \varnothing$;
 * $(2): \quad$ The union of all the sets forms the whole set $S$: $\displaystyle \bigcup \Bbb S = S$;
 * $(3): \quad$ None of the sets in $\Bbb S$ is null: $\forall T \in \Bbb S: T \ne \varnothing$.

A partition is sometimes called a decomposition.

This same definition is sometimes also encountered in combinatorics.

Finite Expansion
If $S_1, S_2, \ldots, S_n$ form a partition of $S$, the notations:
 * $S = S_1 | S_2 | \cdots | S_n$

or:
 * $S = \left\{{S_1 | S_2 | \cdots | S_n}\right\}$

are sometimes seen.

The representation by such a partition $\displaystyle \bigcup_{k=1}^n S_k = S$ is also called a finite expansion of $S$.

Note
The definition of a partition in the field of topology is slightly more specialized.