Set

Definition

A set is a collection of objects, called the elements of the set. For an object $x$ and a set $A$, either $x$ is an element of $A$, written $x \in A$, or it is not, written $x \notin A$. Two sets $A$ and $B$ are equal when they have the same elements: for each $x$, $x \in A$ if and only if $x \in B$. A finite set can be written by a list of its elements in braces, for example $\{1, 2, 3\}$.