Countable union
= Countable union
{title2=$\bigcup_{n=1}^\infty A_n$}
The <union> of a sequence of sets: an element belongs to it exactly when it belongs to at least one $A_n$.
= Countable union
{title2=$\bigcup_{n=1}^\infty A_n$}
The <union> of a sequence of sets: an element belongs to it exactly when it belongs to at least one $A_n$.