\documentclass{article}
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\begin{document}
\section{Known Negations}

\[ \not=, \not<,\not\le,\not\ll,
   \not\prec,\not\preceq,
   \not\subset,\not\subseteq,\not\sqsubseteq,
   \not\lhd,\not\unlhd,
   \not\asymp \]
\[ \not\equiv, \not>,\not\ge,\not\gg,
   \not\succ,\not\succeq,
   \not\supset,\not\supseteq,\not\sqsupseteq,
   \not\rhd,\not\unrhd,
   \not\approx, \]

\section{Unknown Negations of Characters}

\[ \not a,  \not\alpha \]

\[ \not+, \not 4 \]

\section{Random Negations}

\[ \not\sin \]
\[ \not\operatorname{R} \]
\[ \not\sqrt{x} \]
\[ \not\frac{1}{2} \]

\section{Falling off the End}
\[ a/b c \not \]
\[ \frac{a\not}{b} \]

\end{document}
