Conflict between ntheorem and amsthm

Question

I am using etoolbox and ntheorem packages to have a custom numbering of theorems and equations. Everything works but these side errors occur:

• \qedhere command is unknown (EDIT: following some suggestions I worked something out, so the \qedhere issue is somehow solved)
• theorems title (the optional argument) is typeset in bold

As a MWE, the following code

\documentclass{book}  
\usepackage{amsmath}  
\usepackage{etoolbox}  
\usepackage[amsmath,amsthm,framed,thmmarks]{ntheorem}  
%  
\renewcommand{\theequation}{\thechapter.\arabic{equation}}  
\setcounter{equation}{0}  
\newcounter{tempcounter}  
\newtheorem{thm}{Theorem}[chapter]  
%  
\BeforeBeginEnvironment{thm}{\setcounter{tempcounter}{\arabic{equation}}}  
\AtBeginEnvironment{thm}  
{\setcounter{thm}{\thetempcounter}\subequations}  
\AtEndEnvironment{thm}{\endsubequations}  
%  
\begin{document}  
\chapter{This is a chapter}  
\begin{equation}\label{Eq:A}  
2+2=4  
\end{equation}  
\begin{thm}[First Theorem]\label{Thm:First}  
\begin{equation}\label{Eq:B}  
1+1=2  
\end{equation}  
\begin{proof}  
If equation \ref{Eq:A} was trivial, equation \ref{Eq:B} in Theorem \ref{Thm:First} is even more \qedhere \\  
trivial.  
\end{proof}  
\end{thm}  
\end{document}

results in this output:

The \qedhere command is unknown and therefore ignored. I think that the problem is a conflict between the amsthm option in ntheorem package and the amsthm package. However, if I add \usepackage{amsthm} in the preamble I get errors like

! LaTeX Error: Command \theoremstyle already defined.

and many more.

I would like either to solve the two problems listed above, or to have an alternative code in order to have:

• theorems and equations following the same numbering, resetting at each chapter
• equations in theorem environments subnumbered as shown in the above MWE's output.

Any other suggestion is highly appreciated.

Answer 1

The ntheorem package covers the \qedhere in different ways. Here's an example on which you can elaborate. The "normal font attribution" is easily obtained by (re)defining the theorem style.

\documentclass{book}  
\usepackage{amsmath,amssymb}  
\usepackage{etoolbox}  
\usepackage[amsmath,framed,thmmarks]{ntheorem}  
%  
\numberwithin{equation}{chapter}
\newcommand{\qedhere}{\ifmmode\qed\else\hfill\proofSymbol\fi}

\makeatletter
\renewtheoremstyle{plain}
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2\theorem@separator]}%
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2\ {\normalfont(##3)}\theorem@separator]}
\makeatother
\theoremstyle{plain}

\newtheorem{thm}{Theorem}[chapter]

\theoremstyle{nonumberplain}
\theoremheaderfont{\itshape}
\theorembodyfont{\normalfont}
\theoremsymbol{\ensuremath{\square}}
\newtheorem{proof}{Proof}
\qedsymbol{\ensuremath{\square}}

\newcounter{tempcounter}
\BeforeBeginEnvironment{thm}{\setcounter{tempcounter}{\arabic{equation}}}  
\AtBeginEnvironment{thm}  
{\setcounter{thm}{\thetempcounter}\subequations}  
\AtEndEnvironment{thm}{\endsubequations}  
%  
\begin{document}  
\chapter{This is a chapter}  
\begin{equation}\label{Eq:A}  
2+2=4  
\end{equation}  
\begin{thm}[First Theorem]\label{Thm:First}  
\begin{equation}\label{Eq:B}  
1+1=2  
\end{equation}  
\begin{proof}  
If equation \ref{Eq:A} was trivial, equation \ref{Eq:B} in Theorem \ref{Thm:First} is even more
trivial.
\[ 0+0=0 \qedhere\]
\end{proof}  
\end{thm}  

\begin{thm}[Second Theorem]\label{Thm:Second}  
\begin{equation}\label{Eq:C}  
1+1=2  
\end{equation}  
\begin{proof}  
If equation \ref{Eq:A} was trivial, equation \ref{Eq:C} in Theorem \ref{Thm:Second} is even more
trivial.
\end{proof}  
\end{thm}  

Let's see another proof.
\begin{proof}
\begin{itemize}
\item Fact one
\item Fact two
\item Fact three\qedhere
\end{itemize}
\end{proof}

\begin{proof}
Here the proof ends with an \texttt{align*} environment.
\begin{align*}
0&=0+0\\
1&=0+1\qedhere
\end{align*}
\end{proof}

\end{document}