set

In mathematics, a set is a collection of distinct objects. Each object, or element, can occur at most once. Ordering is not significant: sets do not impose any ordering on their elements.

Sets are usually named using capital letters, and brackets, {}, are used to enumerate a set’s elements. For example:

$A = \{2, 5, 1, 3 \}$
$B = \{Homer, Marge, Bart, Lisa\}$

Two sets are considered equal if and only if they contain exactly the same elements.

Symbols:

$\in$ denotes set membership. i.e., $2 \in A$ . We can say that $2$ is a member of $A$ or $2$ belongs to $A$ . We can also say that $A$ contains $2$ .