Set Union/Examples/Set of Initial Segments

Example of Union of Set of Sets
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:
 * $\ds \bigcup \mathscr S = \Z_{>0}$

that is, the set of strictly positive integers.