Definition:Set Union/Family of Sets/Universal Set
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Definition
Let $\mathbb U$ be a universal set.
Let $I$ be an indexing set.
Let $\family {S_i}_{i \mathop \in I}$ be an indexed family of subsets of $\mathbb U$.
Then the union of $\family {S_i}$ is defined and denoted as:
- $\ds \bigcup_{i \mathop \in I} S_i := \set {x \in \mathbb U: \exists i \in I: x \in S_i}$
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:
- $\ds \bigcup_i S_i$
However, on this website it is recommended that the full form is used.
Sources
- 1982: P.M. Cohn: Algebra Volume 1 (2nd ed.) ... (previous) ... (next): Chapter $1$: Sets and mappings: $\S 1.2$: Sets