1

This is a follow up question to: Proof titles with ntheorem

But since the original answer-er has not been seen for a month, I'd like to ask a wider community of people.

How do you add a QED symbol to the end of the specially defined proof environment:

\usepackage{ntheorem}
\makeatletter
\newtheoremstyle{MyNonumberplain}%
  {\item[\theorem@headerfont\hskip\labelsep ##1\theorem@separator]}%
  {\item[\theorem@headerfont\hskip\labelsep ##3\theorem@separator]}
\makeatother
\theoremstyle{MyNonumberplain}
\theorembodyfont{\upshape}
\newtheorem{proof}{Proof}
1

You simply have to declare the \qedsymbol before you declare the proof environment. Sole condition for working: no blank line before \end{proof}.

This being said, I don't see why you declare a personal NoNumberPlain theorem style which is the exact copy of the already defined nonumberplain.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}

\usepackage[svgnames]{xcolor}

 \usepackage{amsmath, amssymb}
\usepackage[thmmarks, amsmath, thref]{ntheorem}
\usepackage{cleveref}

\theoremstyle{plain}
\theoremheaderfont{\bfseries}
\theoremseparator{.}
\theorembodyfont{\itshape}
\newtheorem{thm}{Theorem}

\makeatletter
\newtheoremstyle{MyNonumberplain}%
  {\item[\theorem@headerfont\hskip\labelsep ##1\theorem@separator]}%
  {\item[\theorem@headerfont\hskip\labelsep##1{ }##3\theorem@separator]}
\makeatother

\theoremstyle{MyNonumberplain}
\theoremheaderfont{\itshape}
\theoremseparator{. ---}
\theorembodyfont{\upshape}
\theoremsymbol{\ensuremath{\color{Gainsboro}\blacksquare}}
\newtheorem{proof}{Proof}

\begin{document}

\begin{thm}\label{th-a}
This theorem is awesome!
\end{thm}

\begin{proof}
Because I can change the position of qed mark correctly! Blahblah Blahblah Blahblah. 
\end{proof}

\begin{proof}[of \cref{th-a}]
This is a very important proof.
\begin{align*}
    a & = b\\ c & = d.
\end{align*}
\end{proof}

\end{document} 

enter image description here

  • The idea is that when you title the proof, say begin{proof}[Proof of Thm 1], it should look like "Proof of Theorem 1" and not "Proof (Proof of Theorem 1"), which is what your code would generate. The code in the question allows this naming of proofs. Can you provide a way to both name proofs, as above, and have the QED symbol? – user341502 Jul 24 '17 at 23:05
  • Pressed enter too early; comment edited above – user341502 Jul 24 '17 at 23:10
  • I see. I've already done the same quite some years ago. Actually, I simplified you code: you just have to type the complement. I also loaded cleverref for a smarter typing of cross-references. – Bernard Jul 24 '17 at 23:23

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