Elements are the objects contained in a set. A set may be defined by a common property amongst the objects. For example, the set of positive even integers is the set
The set of living people is the set
A set can also be defined by simply stating its elements. For example, one can define the set by writing its elements, as follows:
The mathematical notation for "is an element of" is . For example, to denote that is an element of the set of positive even integers, one writes . To indicate that an element, 3, is not in the set , write 3 .
The size of a set (also called its cardinality) is the number of elements in the set. For example, the size of the set is while the size of the set of positive even integers is infinity.
What is the size of the set ?
The elements in the set are , and There are of them. Hence, the size of the set is .