union

given two sets $\text{[math]}$ and $\text{[math]}$ , their union $\text{[math]}$ is defined by
$\text{[math]}$

given a collection $\text{[math]}$ of sets, the union of the elements of $\text{[math]}$ is defined by the equation
$\text{[math]}$ [cite:;taken from @topology_munkres_2014 chapter 1 set theory and logic]