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Symbol $\in$ — "is one of the members of" a set.

$x \in A$ means $x$ is a member of the set $A$ . Its negation, $x \notin A$ , means $x$ is not a member.

Examples

$3 \in \{1, 2, 3\}$ is true.
$7 \notin \{1, 2, 3\}$ is true.