I have included the package amsmath as required to defined a new theorem/corollary (without number), but I keep having the following error:

! LaTeX Error: Command \* already defined.

Now, these are the packages I am including, if that can help:

\usepackage{fullpage}
\usepackage{parskip}
\usepackage{color}
\usepackage{amsfonts}
\usepackage{graphicx}
\usepackage{amssymb}
\usepackage{amsmath}
\newtheorem*{corollary}{Corollary}


And then I am tring to use the new command with:

\begin{corollary}
...
\end{corollary}

• You need \usepackage{amsthm} in order to use \newtheorem* – egreg May 19 '15 at 21:31
• Oh, sorry, I thought I needed amsmath... I am really tired.. – nbro May 19 '15 at 21:33
• You do need amsmath if you're writing a mathematical document. It's just not sufficient. – egreg May 19 '15 at 21:33

The package you need is amsthm (besides the recommended amsmath).

Add \usepackage{amsthm} and all will go flawlessly.