- $\not =, \ \not>, \ \not<, \ \not \ge, \ \not \le, \ \not \in, \ \not \exists, \ \not \subseteq, \ \not \subset, \ \not \supseteq, \ \not \supset$
The above symbols all mean the opposite of the non struck through version of the symbol.
For example, $x \not\in S$ means that $x$ is not an element of $S$.
The slash $/$ through a symbol can be used to reverse the meaning of essentially any mathematical symbol (especially relations), although it is used most frequently with those listed above.
The $\LaTeX$ code for negation is
\not followed by the code for whatever symbol you want to negate.
\not \in will render $\not \in$.
Note that several of the above relations also have their own $\LaTeX$ commands for their negations, for example
\not =, and
for the definitions of the symbols above.