Getting items above and below a compound math symbol

Question

I'm trying to get information to appear above and below a math symbol such as \sum or \leftrightarrows. I've found information detailing \mathop and the syntax behind it but when I try to use it, it appears my tex editor doesn't recognize it as a command. Is mathop not in the ams math package?

I'm a relatively new latex user so any help would be appreciated.

The tutorial I'm following is http://truonglatex.wordpress.com/2010/04/13/arrows-and-stack-math-symbols/

My output looks as the attached image. The code is identical to the one on the website.

Thanks any help would be appreciated.

Answer 1

Without MWE (Minimal Working Example) we can guess, how you used the code, but we cannot, how you want to, hence two cases.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

You have

$
a\mathop{\sum\sum\sum}_{a=\frac12}^{\text{some text}}b 
$

You want to

\[
a\mathop{\sum\sum\sum}_{a=\frac12}^{\text{some text}}b 
\]

If you want it in inline mode, use \verb+\limits+
$
a\mathop{\sum\sum\sum}\limits_{a=\frac12}^{\text{some text}}b 
$


\end{document}