optional argument for cite command

Question

It is possible to write a command as

\cite[Capter 2]{tag}

which will display in the text something as "...as stated in [tag, Chapter 2]...". No problems so far. I want to do the same while inserting a description of an environment. To be clearer, it is possible to write

\begin{Proposition}[\cite{tag}]

with no problems but when I try to write

\begin{Proposition}[\cite[Chapter 2]{tag}]

i get an error as

Argument of \@citex has an extra }

Can somebody help me?

Answer 1

In such cases you need to include the \cite command in an extra set of braces {...}. This is to help the parser.

\documentclass{article}

\newtheorem{theorem}{Theorem}

\begin{document}

\begin{theorem}[{\cite[Chapter 2]{tag}}]
  Text.
\end{theorem}

\begin{thebibliography}{9}
\bibitem{tag} A reference.
\end{thebibliography}
\end{document}