LaTeX symbol for "does not divide"

try this:

$x\nmid y$

it's been known to 'save the day'.

Detexify is amazingly helpful in finding any symbol.

Using \not with | works with plain LaTeX:

\documentclass{article}
\begin{document}
$2 \not| \;3 $
\end{document}

The symbols \\, and \: and \; create varying degrees of space if the symbols get too close.

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