The LaTeX command for complement (∁) is ^\complement (set complement (superscript)).
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Write ^\complement inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. This symbol needs the amssymb package, so add \usepackage{amssymb} to your preamble.
Set notation lives in math mode: write $^\complement$, even in the middle of a sentence. With unicode-math under XeLaTeX or LuaLaTeX, typing ∁ in a formula gives the same symbol.
$A ^\complement B$Open this example in the live preview: it renders as you type, with no compiler and no signup.
\emptyset\varnothing\mathbb{N}\mathbb{Z}\mathbb{Q}\mathbb{R}\mathbb{C}\mathbb{F}\bigcap\bigcup\bigsqcup\uplus\biguplus\mathcal{P}Set symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use ^\complement in math mode after adding \usepackage{amssymb} to your preamble. For example, $A ^\complement B$.
It comes from the amssymb package: add \usepackage{amssymb} to your preamble.