Number of Set Partitions by Number of Components/Examples
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Examples of Number of Set Partitions by Number of Components
$4$-Sets into $2$ Subsets
A set with $4$ elements $\set {1, 2, 3, 4}$ can be partitioned into $2$ subsets in $\ds {4 \brace 2} = 7$ ways:
Let $T$ be the set with $3$ elements $\set {1, 2, 3}$.
The partition of $T$ into $1$ set is:
- $\set {1, 2, 3}$
to which $\set 4$ can be added in one way to achieve:
- $\set {1, 2, 3 \mid 4}$
The partitions of $T$ into $2$ sets are:
- $\set {1, 2 \mid 3}$
- $\set {1, 3 \mid 2}$
- $\set {2, 3 \mid 1}$
to which the element $4$ can be added in $2$ ways each:
- $\set {1, 2, 4 \mid 3}$
- $\set {1, 2 \mid 3, 4}$
- $\set {1, 3, 4 \mid 2}$
- $\set {1, 3 \mid 2, 4}$
- $\set {2, 3, 4 \mid 1}$
- $\set {2, 3 \mid 1, 4}$
Thus we have a total of $7$ ways of partitioning a $4$-element set into $2$ components.