Can we remove the ( ) bracket from Theorem 1.2.1 ( [7])

Question

Can we remove the ( ) bracket from the environment like Theorem 1.2.1 ( [7]).

Answer 1

this can be accomplished by defining a \newtheoremstyle when using amsthm.

here's an example:

\documentclass{article}
\usepackage{amsthm}
%% goal is to omit parens from note, place cite as part of theorem number
\newtheoremstyle{cited}%
  {3pt}% (space above)
  {3pt}% (space below)
  {\itshape}% (body font)
  {}% (indent amount)
  {\bfseries}% {theorem head font}
  {.}% {punctuation after theorem head}
  {.5em}% {space after theorem head}
  {\thmname{#1} \thmnumber{#2} \thmnote{\normalfont#3}}% {theorem head spec}

\theoremstyle{cited}
\newtheorem{citedthm}{Theorem}

\begin{document}

\begin{citedthm}[\cite{xxx}]
this theorem is borrowed from somewhere else.
\end{citedthm}

\begin{thebibliography}{99}

\bibitem{xxx}
this is a bibitem.

\end{thebibliography}

\end{document}

(i'm sure a very similar question was asked within the past few days, because i had already experimented with this problem, but i can't find it. i would also like to thank @mafp, who provided a similar answer in the question How to remove parentheses when I refer to some lemma or proposition.)

edit: in a comment, it has been requested that there be no space between the theorem number and the cited reference. to accomplish that, omit the space from the "theorem spec" line (the last line of the \newtheoremstyle definition):

  {\thmname{#1} \thmnumber{#2}\thmnote{\normalfont#3}}% {theorem head spec}

in this situation, when sometimes there is a "cited" theorem and sometimes a "non-cited" theorem, the same numbering system can be used like this:

\theoremstyle{plain}
\newtheorem{thm}{Theorem}
\theoremstyle{cited}
\newtheorem{citedthm}[thm]{Theorem}

specifying the optional argument [thm] between the name of the new theorem environment and its heading text will cause the same counter to be used as already defined when the {thm} environment is set up.