numbers 1, 2, and 3. The symbol represents set membership, for example says "1 is a member of the set " which is true by the definition of above. Here is finite, but that is not a requirement in general. The only requirement for a set is that it is well-defined. That is, for any object, , one can determine whether belongs to that set , or does not belong to that set . One example of an infinite set is the set of all natural numbers .

A function,