- $\ne, \not >, \not <, \not \ge, \not \le$
The above symbols all mean the opposite of the non struck through version of the symbol.
For example, $x \ne y$ means that $x$ is not equal to $y$.
The $\LaTeX$ code for negation is
\not followed by the code for whatever symbol you want to negate.
\not \ge will render $\not \ge$.
Note that some of the above relations also have their own $\LaTeX$ commands for their negations, for example