The empty set; Cardinality
The set that has no elements is called the empty set, or null set, and
The null set is a subset of every set!
And it has only one subset --- the null set itself.
If a set S has exactly n distinct elements, where n is a non-negative
integer, then S is said to be finite, and n is said to be the cardinality of S.
“n is the cardinality of S” is denoted by |S|=n.
Examples: |{a,b}| = |{1,3,3,3}| = 2; |?| = 0
How many sets are there with cardinality 0?
How many sets are there with cardinality 1?