With ntheorem, thmmarks and no amsthm, defining the proof environment right

Question

I'm using ntheorem with the thmmarks option but without the amsthm option (for reasons I won't go into). I would like to define the proof environment à la amsthm. Specifically, I want:

• To be able to use the proofSymbol command to affect all proofs, as described in this answer.
• To be able to specify that, by default, proofs will have no QED symbol.
• For the proof symbol, when it appears, to appear by default at the end of the line (i.e. after an \hfil).

What I did was 'lift' the definition from ntheorem.sty:

\newcommand{\openbox}{\leavevmode
  \hbox to.77778em{%
  \hfil\vrule
  \vbox to.675em{\hrule width.6em\vfil\hrule}%
  \vrule\hfil}}
\gdef\proofSymbol{\openbox}
\newcommand{\proofname}{Proof}
\newenvironment{proof}[1][\proofname]{
  \th@nonumberplain
  \def\theorem@headerfont{\itshape}%
  \normalfont
  \theoremsymbol{\ensuremath{_\blacksquare}}
  \@thm{proof}{proof}{#1}}%
  {\@endtheorem}

And add a counter definition. Here's the result in an working example:

\documentclass{article}
\usepackage[thmmarks]{ntheorem}
\usepackage{amssymb}
\usepackage{lipsum}

\makeatletter
\newcommand{\openbox}{\leavevmode
  \hbox to.77778em{%
  \hfil\vrule
  \vbox to.675em{\hrule width.6em\vfil\hrule}%
  \vrule\hfil}}
\gdef\proofSymbol{\openbox}
\newcommand{\proofname}{Proof}
\newcounter{proof}\newcounter{currproofctr}\newcounter{endproofctr}
\newenvironment{proof}[1][\proofname]{
  \th@nonumberplain
  \def\theorem@headerfont{\itshape}%
  \normalfont
  \theoremsymbol{\ensuremath{_\blacksquare}}
  \@thm{proof}{proof}{#1}}%
  {\@endtheorem}
\makeatother

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

\begin{document}
\begin{proof}
Without \verb|\qed| in this one.
\end{proof}
\begin{proof}
With \verb|\qed| in this one. \qed
\end{proof}
\end{document}

(Edit: Dropped the \lipsum 's from the MWE) What happens is that I get a black square when not specifying \qed, but no black square when I do specify it. So,

• How do I get \qed to work now?
• To avoid a symbol by default now, I'll just set \theoremsymbol{\relax}, right?
• Can I overcome the "no \par before the end" limitation?

Answer 1

Here is a way to do it in detail: I define two different environments, proof(with a qed symbol) and proof-wo without qed symbol.

However, note it is enough to define only a proof environment (with qed symbol by default), and add \NoEndMark if you do not want this symbol.

Also I do not see the point of numbering proofs. Nevertheless, should you really want to, just change the theorem style to plain and replace \newtheorem{proof-wo}{Proof} with \newtheorem{proof-wo}[proof]{Proof}if you want to have only one counter for both environments.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{xcolor}
\usepackage{mathtools}
\usepackage[amsmath, thmmarks]{ntheorem}
\usepackage{amssymb}
\usepackage{lipsum}
 \theoremstyle{nonumberplain}
 \theoremheaderfont{\itshape}
 \theorembodyfont{\normalfont}
 \theoremseparator{.\,—}
 \theoremsymbol{}
 \newtheorem{proof-wo}{Proof}
 \theoremsymbol{\ensuremath{\color{lightgray}\blacksquare}}
 \newtheorem{proof}{Proof}

\begin{document}

\begin{proof-wo}
Without \verb|\qed| Nam dui ligula, fringilla a, euismod sodales, sollicitudin
vel, wisi. Morbi auctor lorem non justo. Nam lacus libero, pretium at, lobortis
vitae, ultricies et, tellus. Donec aliquet, tortor sed accumsan bibendum, erat
ligula aliquet magna, vitae ornare odio metus a mi. Morbi ac orci et nisl hen-
drerit mollis. Suspendisse ut massa. Cras nec ante. Pellentesque a nulla. Cum
sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus
mus. Aliquam tincidunt urna. Nulla ullamcorper vestibulum turpis. Pellen-
tesque cursus luctus mauris.
\end{proof-wo}

%
\begin{proof}
With \verb|\qed| Nam dui ligula, fringilla a, euismod sodales, sollicitudin
vel, wisi. Morbi auctor lorem non justo. Nam lacus libero, pretium at, lobortis
vitae, ultricies et, tellus. Donec aliquet, tortor sed accumsan bibendum, erat
ligula aliquet magna, vitae ornare odio metus a mi. Morbi ac orci et nisl hen-
drerit mollis. Suspendisse ut massa. Cras nec ante. Pellentesque a nulla. Cum
sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus
mus. Aliquam tincidunt urna. Nulla ullamcorper vestibulum turpis. Pellen-
tesque cursus luctus mauris.
\end{proof}
%
\begin{proof}\NoEndMark
Without \verb|\qed| Nam dui ligula, fringilla a, euismod sodales, sollicitudin
vel, wisi. Morbi auctor lorem non justo. Nam lacus libero, pretium at, lobortis
vitae, ultricies et, tellus. Donec aliquet, tortor sed accumsan bibendum, erat
ligula aliquet magna, vitae ornare odio metus a mi. Morbi ac orci et nisl hen-
drerit mollis. Suspendisse ut massa. Cras nec ante. Pellentesque a nulla. Cum
sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus
mus. Aliquam tincidunt urna. Nulla ullamcorper vestibulum turpis. Pellen-
tesque cursus luctus mauris.
\end{proof}

\end{document} 

Answer 2

Too long for a comment

The standard method of making an ntheorem proof env is something similar to (untested, in a hurry)

\usepackage[amsmath,thmmarks]{ntheorem}
\theoremstyle{nonumberplain}
\theoremheaderfont{\itshape}
\theorembodyfont{\normalfont}
\theoresymbol{\ensuremath\square}
\newtheorem{proof}{Proof}

I do not understand why you are using all that code.

I would then also make a variant that has an empty \theoremsymbol and call it say proof*

Remember that ntheorem and amsthm are very different beasts and the former does not support \qed