Definition:Set Union/Family of Sets/Universal Set

Definition
Let $\mathbb U$ be a universal set.

Let $I$ be an indexing set.

Let $\left \langle {S_i} \right \rangle_{i \mathop \in I}$ be a family of subsets of $\mathbb U$.

Then the union of $\left \langle {S_i} \right \rangle$ is defined as:


 * $\displaystyle \bigcup_{i \mathop \in I} S_i := \left\{{x \in \mathbb U: \exists i \in I: x \in S_i}\right\}$

Thus $S \cup T$ can be defined as:
 * $S \cup T := \left\{{x \in \mathbb U: x \in S \lor x \in T}\right\}$

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


 * $\displaystyle \bigcup_I S_i$

or, if the indexing set is clear from context:


 * $\displaystyle \bigcup_i S_i$

However, on this website it is recommended that the full form is used.