What is the command for a struck out exists symbol

\exists gives you the flipped E, but what is the command for the negated symbol?

How do you cross a symbol in LaTeX?

\times is the cross product command in LaTeX.

How do I insert a pi symbol in LaTeX?

LaTeX provides almost any mathematical or technical symbol that anyone uses. For example, if you include $\pi$ in your source, you will get the pi symbol π.

The command for the negative existential quantifier is \nexists. However, I believe you should ask questions like this on the TeX site of Stack Exchange.

This is a great site that lets you draw the symbol you want, and it shows a list of possible latex commands. \nexists was the first suggestion that popped up! http://detexify.kirelabs.org/classify.html