Definition:Set Union/Family of Sets/Subsets of General Set

Definition
Let $I$ be an indexing set. Let $\family {S_i}_{i \mathop \in I}$ be an indexed family of subsets of a set $X$.

Then the union of $\family {S_i}$ is defined as:


 * $\ds \bigcup_{i \mathop \in I} S_i := \set {x \in X: \exists i \in I: x \in S_i}$

where $i$ is a dummy variable.

Also denoted as
The set $\ds \bigcup_{i \mathop \in I} S_i$ can also be seen denoted as:


 * $\ds \bigcup_I S_i$

or, if the indexing set is clear from context:


 * $\ds \bigcup_i S_i$

The form:
 * $\ds \bigcup_{S \mathop \in X} S$

can also be seen, but this obscures the true nature of the indexing set.

On it is recommended that the full form is used.