[Tex/LaTex] how to insert an upside-down triangle

Question

I tried inserting an upside-down triangle with \triangledown but even when I put the two $ signs around it, I keep receiving an error.
Does anyone know how to insert such special characters?

Answer 1

If the error message is:

! Undefined control sequence.
<recently read> \triangledown

l.3 $\triangledown
                  $
?

then a package is missing, which provides the symbol. For example, package amssymb provides the symbol:

\documentclass{article}
\usepackage{amssymb}
\begin{document}
$\triangledown$
\end{document}

If you want to have the nabla symbol, then this symbol is available without additional packages:

\documentclass{article}
\begin{document}
$\nabla$
\end{document}