Newbie LaTex symbol question

1. Feb 22, 2012

MorningGlory

Hello, everyone! First time poster. Quick question: I am working in TeXShop on a Macbook. When I type $\nexists$, I get an error message saying "Undefined control sequence" even though it is in the list of short course symbol tables available under the Help menu in TeXShop. $\not \exists$ works fine, but I would prefer the cleaner look that I should get with the $\nexists$ symbol. How can I fix this?

2. Feb 22, 2012

jhae2.718

Are you using the ams packages? I believe $\nexists$ is defined in amssymb. If not, try:
Code (Text):

\newcommand{\nexists}{\not\exists}