Theorem without numbering

Question

Is it possible to have single theorems without numbering? I would like to be able to switch them off individually with asterisk-* or have a nono-theorem environment.

\newtheorem{theorem}{Theorem}
\newtheorem{nono-theorem}{Theorem}[]

Answer 1

You can switch numbering on and off for individual theorems just by defining environments with nearly identical names (as with equation environments):

\documentclass{article}
\usepackage{amsthm}
\theoremstyle{plain}

\newtheorem*{theorem*}{Theorem}
\newtheorem{theorem}{Theorem}

\begin{document}
\begin{theorem}  A numbered theorem.    \end{theorem}
\begin{theorem*} An unnumbered theorem. \end{theorem*}
\end{document}

Answer 2

(It's more of a comment.)

You can define a new theorem with a star:

\documentclass{article}

\usepackage{lipsum}
\usepackage{ntheorem}
\newtheorem{theorem}{Theorem}
\newtheorem*{theorem-non}{Theorem}

\begin{document}

\begin{theorem}
\lipsum[1]
\end{theorem}
\begin{theorem-non}
\lipsum[1]
\end{theorem-non}

\end{document}