Consider the following LaTeX2e manuscript
\documentclass{scrartcl}
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}
\begin{document}
\begin{theorem*}[Hello World]
Hello, world!
\end{theorem*}
\end{document}
When I compile it with pdflatex, the compilation fails with the following error message:
! LaTeX Error: Environment theorem* undefined.
See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
...
l.6 \begin{theorem*}
[Hello World]
?
What is the problem? How can I fix it?
\newtheoremstyle*{lem}{Lemma}, but the documentation never states that a suddenlem*would work.\newtheorem*{theorem*}{Theorem}to your preamble.