Can't use amsthm option with ntheorem, and no QED markers

Question

I'm trying to get QED markers to show up at the end of a proof only, using ntheorem but run into a couple of problems. First, using the amsthm option with ntheorem doesn't work:

\documentclass[a4paper, twosided]{article}
\usepackage{amsmath}
\usepackage{amsthm}

\usepackage[standard, amsthm, amsmath]{ntheorem}

\begin{document}
\begin{Theorem}
    A really insightful theorem.
\end{Theorem}

\begin{proof}~\\
    $\forall g \in G \ldots$
\end{proof}
\end{document}

gives the following errors:

! Package ntheorem Error: Theorem style plain already defined.
! Latex Error: Command \proof already defined.Or name \end... illegal

When I remove the \usepackage{amsthm} and amsthm from the ntheorem options it will build fine, but still no QED markers at the end of the proof.

Using the thmmarks option with ntheorem puts QED symbols at the ends of theorems as well, which I don't want.

Answer 1

The amsthm option yields compatibility with the theorem layout commands of the amsthm package, but don't use both packages. The amsthm option is not compatible with the standard option. The qed symbols are activated by the thmmarks option. Furthermore, I recommend loading the thref option to activate extended referencing features.

\documentclass[a4paper, twosided]{article}
\usepackage{amsmath}

\usepackage[standard, thref, amsmath, thmmarks]{ntheorem}%

\begin{document}

\begin{Theorem}
    A really insightful theorem.
\end{Theorem}

\begin{proof}~\\
    $\forall g \in G \ldots$
    \end{proof}

\end{document}