T is an index set. And for each [tex]t \in T[/tex] [tex] A_t [/tex] is a set(adsbygoogle = window.adsbygoogle || []).push({});

[tex]\bigcup_{t \in T} A_t = \{x : \exists t \in T with x \in A_t \}[/tex]

What means this [tex]\bigcup[/tex] symbol and entire expression?

And question on index set: is it used just for orderring any other set?

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# Mathematical notation

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