The LaTeX command for nmid (∤) is \nmid (does not divide).
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Write \nmid 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.
As a relation, \nmid 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.
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Use \nmid in math mode after adding \usepackage{amssymb} to your preamble. For example, $a \nmid b$.
It comes from the amssymb package: add \usepackage{amssymb} to your preamble.