# Set Union/Set of Sets/Examples

## Examples of Unions of Sets of Sets

### Set of Arbitrary Sets

Let:

 $\displaystyle A$ $=$ $\displaystyle \set {1, 2, 3, 4}$ $\displaystyle B$ $=$ $\displaystyle \set {a, 3, 4}$ $\displaystyle C$ $=$ $\displaystyle \set {2, a}$

Let $\mathscr S = \set {A, B, C}$.

Then:

$\displaystyle \bigcup \mathscr S = \set {1, 2, 3, 4, a}$

### Set of Initial Segments

Let $\Z$ denote the set of integers.

Let $\map \Z n$ denote the initial segment of $\Z_{> 0}$:

$\map \Z n = \set {1, 2, \ldots, n}$

Let $\mathscr S := \set {\map \Z n: n \in \Z_{> 0} }$

That is, $\mathscr S$ is the set of all initial segments of $\Z_{> 0}$.

Then:

$\displaystyle \bigcup \mathscr S = \Z_{> 0}$

that is, the set of strictly positive integers.