The LaTeX command for notin (∉) is \notin (not an element of).
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Write \notin inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
As a relation, \notin gets a thick space on each side, and a long inline formula may break after it. With unicode-math under XeLaTeX or LuaLaTeX, typing ∉ in a formula gives the same symbol.
$a \notin b$Open this example in the live preview: it renders as you type, with no compiler and no signup.
\supseteq\subset\supset\subsetneq\sqsubseteq\sqsupseteq\in\ni\vdash\models\asymp\doteq\triangleq\lessapproxRelation symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \notin in math mode; it needs no extra package. For example, $a \notin b$.
None: \notin is built into LaTeX, so no extra package is required.