# Examples and counterexamples in mathematics/Sets

- Set at Wikipedia.

## Set without members[edit]

- Empty set at Wikipedia.

The empty set, denoted by (or sometimes ) contains no members. If you find it strange and disturbing, think about the number zero (denoted 0); it was a strange and disturbing idea, but now is generally accepted. The number of members in is 0.

The empty set is a set, not "absence of set". Likewise, an empty box is a box, not "absence of box"; and 0 is a number, not "absence of number". Substituting 0 into a function *f* we get another number *f*(0), generally not 0. For example, . Also, The latter fact has a set-theoretic counterpart, see the next item.

## The powerset of the empty set is not empty[edit]

- Power set at Wikipedia.

The power set (or "powerset") of any set *S* is the set of all subsets of *S*, including the empty set and *S* itself. If then its power set contains and nothing else; it is that is, Likewise a box that contains only an empty box is a non-empty box. The number of elements in this power set is 1. Generally, if *S* contains *n* elements, then its power set contains elements. In particular,