What does the letter U mean in the following expression: $$ \bigcup_{\alpha} A_\alpha \;? $$
It doesn't look like logical OR.
Link to original screen shot.
$\endgroup$ 11 Answer
$\begingroup$It represents the union of all the sets $A_\alpha$. By the "union", what it basically means that:
$$ a \in \bigcup_{\alpha} A_\alpha \iff \exists \alpha :a \in A_\alpha $$
or in words - it is the set of elements which are contained in any of the $A_\alpha$.
$\endgroup$ 7
