Cardinality

To work out the cardinality of a set, simply count the amount of different values in a set. To write cardinality, simply put it around two pipe symbols:

$$|\{a,b,c\}|$$

So:

$$|\{a,b,c\}|=3$$

and:

$$|\{a,a\}|=1$$

and:

$$|\{\varnothing \}|=1$$