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Q: Is there an easy way to change the default proof environment endmark (it's normally a funny open square and I'd like to change it to \blacksquare) when using the ntheorem package with the amsthm and thmmarks options? I'd like to continue using the ntheorem package without migrating over to amsthm

You can see below that I added a \square endmark to the different theorem environments. I'm just stuck on changing the proof endmark. You can see below I tried the code

\renewcommand{\qedsymbol}{\ensuremath{\blacksquare}}

It didn't work. A number of other approaches didn't work either.

\documentclass{article}
\usepackage[amsthm,thmmarks]{ntheorem}
\usepackage{amssymb}

\theoremstyle{break}
\theoremsymbol{\ensuremath{\square}}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{definition}[theorem]{Definition}
%\renewcommand{\qedsymbol}{\ensuremath{\blacksquare}}

\begin{document}

\begin{theorem}[My Theorem]
  Lorem ipsum. 
\end{theorem}
\begin{proof}
  Lorem ipsum.
\end{proof}

\begin{definition}[My Definition]
  Lorem ipsum.
\end{definition}

\end{document} 

endmarks

11

I think you are looking for

\renewcommand\proofSymbol{\ensuremath{\blacksquare}}

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