Element

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An element, also called a member, is an object contained within a set or class.

$A=\{1,\,2,\,3,\,4\}$ means set $A$ contains the elements 1, 2, 3 and 4.

To show that an element is contained within a set, the $\in$ symbol is used. The opposite of $\in$ is $\notin$ , which means the element is not contained within the set.

Sets as Elements

Elements can also be sets. For example, $B = \{1,\,2,\,\{3,\,4\}\}$ . The elements of $B$ are $1$ , $2$ , and $\{3,\,4\}$ .

Element

From AoPSWiki

Sets as Elements

See Also