Universal Set

You, the writer, decides what is in the universe, and then all elements in your universe are in the Universal Set.

We might want to define a simple universe with a few integers:

$U=\{1,2,3,4,5\}$

This is useful because we can do something like define A={1,2} and then when we say that the complement of A is the following:

$A^c = \{ 3, 4, 5 \}$

You can see that the complement of a set is everything in the Universal set that isn’t in the set.