Countable

What is countable?

Natural Numbers

Quite simple
$\mathbb{N}=\{1,2,3,4,\dots \}$
Which you can just add it up every time

Integer Numbers

$Z$ can be countable as you can go
$1\to 0$
$2\to 1$
$3\to -1$
$4\to 2$
$5\to -2$
and so on…

Here they start at 0, then go 1, -1, 2, -2, etc…

Rational Numbers

They are countable, but it is a lot more work to show