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I've modified answer of this post for new blocks as follow but when I put an displayed equation as first line it produce error Extra }, or forgotten $. ^^I\end{frame} and when I put it after an empty line the error goes away. What exactly is the problem?

What I figured out this problem is related to Copenhagen theme!

P.s. 1 Mathematical align environment produce a different error!

P.s. 2 usual block has no such problem!

\documentclass[]{beamer}
\usetheme{Copenhagen}

\newenvironment<>{theorem}[2][Theorem]{%
\setbeamercolor{block title}{fg=white,bg= red!90!blue}%
\begin{block}{#1}#3{#2}}{\end{block}}


\begin{document}


\begin{frame}
    \begin{theorem} 
        this is OK!
    \end{theorem}


\begin{theorem}%&lt;-- error
    \[x^2=y^2+z^2\]
\end{theorem}

\begin{theorem}%&lt;-- OK

    \[x^2=y^2+z^2\]
\end{theorem}

\begin{block}{theorem}%&lt;-- OK
    \[x^2=y^2+z^2\]
\end{block}



\end{frame}


\end{document}

My goal is to have an theorem block with one optional argument when is not empty this become the block title something like as follow:

\begin{theorem}
     \[x^2=y^2+z^2\]
\end{theorem}
\begin{theorem}[MY TITLE]
     \[x^2=y^2+z^2\]
\end{theorem}

enter image description here

C.F.G
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    Doesn't your theorem env take two arguments on one optional and one mandatory. And you are not giving the mandatory one – daleif Mar 21 '23 at 10:14
  • I don't know what mandatory one is. I just copied it. – C.F.G Mar 21 '23 at 10:17
  • \newenvironment<>{theorem}[2][Theorem] the [2] says this takes two arguments. [Theorem] says the first is optional. – daleif Mar 21 '23 at 10:22
  • I know what you say but the linked post accept one argument without my optional one. (see linked post) – C.F.G Mar 21 '23 at 10:23
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    (1) The Copenhagen theme is not related. (2) if you use \begin{theorem}{test} the it compiles just fine. Your problem with not specifying the mandatory arg is that begin{theorem} then tries to use \[ as its argument and then math fails. You might just want to change [2] to [1] – daleif Mar 21 '23 at 10:24
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    And note how all those in the linked example used [1], where did that [2] come from? – daleif Mar 21 '23 at 10:25
  • I've added that extra option to be optional. Can one have optional argument without my [2] and just using linked post? – C.F.G Mar 21 '23 at 10:26
  • Not with that syntax it only supports a single optional argument. You should probably start over and rephrase your question to explain what your end goal is. – daleif Mar 21 '23 at 10:46

2 Answers2

3

Instead of reinventing the wheel, simply use the default theorem environment provided by beamer. It will accept an optional title just fine - no changes necessary.

\documentclass{beamer}
\usetheme{Copenhagen}

\begin{document}


\begin{frame}
    \begin{theorem} 
        this is OK!
    \end{theorem}


\begin{theorem}
    \[x^2=y^2+z^2\]
\end{theorem}

\begin{theorem}[test]

    \[x^2=y^2+z^2\]
\end{theorem}




\end{frame}


\end{document}

If you don't like that the optional title is shown in (), you can change the theorem begin template:

\documentclass{beamer}
\usetheme{Copenhagen}

\makeatletter
\setbeamertemplate{theorem begin}
{%
  \setbeamercolor{block title}{fg=white,bg=red!90!blue}%
  \begin{\inserttheoremblockenv}
  {%
    \ifx\inserttheoremaddition@empty
      \inserttheoremname
    \else
      \inserttheoremaddition
    \fi%
  }%
}
\makeatother


\begin{document}


\begin{frame}
    \begin{theorem} 
        this is OK!
    \end{theorem}


\begin{theorem}
    \[x^2=y^2+z^2\]
\end{theorem}

\begin{theorem}[test]

    \[x^2=y^2+z^2\]
\end{theorem}

\begin{block}{title}
content...
\end{block}




\end{frame}


\end{document}

enter image description here

samcarter_is_at_topanswers.xyz
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1
\documentclass[]{beamer}
\usetheme{Copenhagen}

\newenvironment<>{theorem}[1][Theorem]{%
    \setbeamercolor{block title}{fg=white,bg= red!90!blue}%
    \begin{block}#2{#1}
}{
    \end{block}
}

\begin{document}

\begin{frame}
    \begin{theorem}<1->
         \[x^2=y^2+z^2\]
    \end{theorem}

    \begin{theorem}<2->[MY TITLE]
         \[x^2=y^2+z^2\]
    \end{theorem}
\end{frame}

\end{document}

Here you define one optional argument which has default value Theorem. If you specify another title by [MY TITLE], this one will be used.

Thanks to your comment, it now obeys overlay specifications.

Result

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