Tcolorbox : create a proof environnement with a QED symbol at the very end

Question

I have created a tcolorbox environment to write proofs.

Here is my code:

\usepackage{tcolorbox}
  \tcbuselibrary{most}
\newtcolorbox{proof}{
  fonttitle=\bfseries,
  enhanced,
  top=0mm,
  boxrule=0pt,frame empty,
  borderline west={4pt}{0pt}{blue},
  coltitle=blue,
  colback=white,
  sharp corners,
  title=Proof
}
\begin{proof}
  Let's show that girls are Evil.
  \begin{adjustwidth}{5mm}{0mm}
    Girls require time and money
    \begin{equation}
      girls = time \times money 
    \end{equation}
    Time is money
    \begin{equation}
      time = money 
    \end{equation}
    So girls are money squared:
    \begin{equation}
      girls = money^2 
    \end{equation}
    Money is the root of all evil
    \begin{equation}
      money = \sqrt{evil}
    \end{equation}
    So, girls are evil:
    \begin{equation}
      girls = \left(\sqrt{evil}\right)^2 = evil
    \end{equation}
  \end{adjustwidth}
  We have shown that girls are Evil.
\end{proof}
% From https://www.onlinemathlearning.com/funny-math-proofs.html

Now, I would like that a blue square is automatically added at the end of the proof (at the very end of the last line) like here. I do not know how to specify this in the proof environment definition.

Try the examples given in doc of tcolorbox v4.42, sec. 17.4 Using other theorem environments with tcolorbox. — muzimuzhi Z, Dec 25 '20 at 10:57
I think adding the after upper={\qed} option to \newtcolorbox with the \usepackage{amsthm} package will do what you want. I would also suggest you not use the name proof as your envuronment name. Also, while code snippets are useful in explanations, it is always best to compose a fully compilable MWE that sets up the problem including the \documentclass and the appropriate packages so that those trying to help don't have to recreate it. — Peter Grill, Dec 25 '20 at 10:58

gernot · Answer 1 · 2020-12-27T11:47:21.023

Add the lines

\usepackage{amssymb}% defines \blacksquare
\AtEndEnvironment{proof}{\null\hfill\textcolor{blue}{$\blacksquare$}}

to your preamble; replace \blacksquare by \square if the symbol is too massive.

For LaTeX versions older than 2020/10/01, you need the etoolbox package before using \AtEndEnvironment.

\documentclass{article}
\usepackage{amssymb}
\AtEndEnvironment{proof}{\null\hfill\textcolor{blue}{$\blacksquare$}}%
\usepackage{changepage}
\usepackage{tcolorbox}
  \tcbuselibrary{most}
\newtcolorbox{proof}{
  fonttitle=\bfseries,
  enhanced,
  top=0mm,
  boxrule=0pt,frame empty,
  borderline west={4pt}{0pt}{blue},
  coltitle=blue,
  colback=white,
  sharp corners,
  title=Proof
}
\begin{document}
\begin{proof}
  Let's show that girls are Evil.
  \begin{adjustwidth}{5mm}{0mm}
    So, girls are evil:
    \begin{equation}
      girls = \left(\sqrt{evil}\right)^2 = evil
    \end{equation}
  \end{adjustwidth}
  We have shown that girls are Evil.
\end{proof}
\end{document}

Adapted from this answer

Thanks. However, when I compile your code, I got the error ! Undefined control sequence. <recently read> \AtEndEnvironment l.3 \AtEndEnvironment — Julien M., Dec 25 '20 at 14:28
@J.Martin \AtEndEnvironment is provided by etoolbox package. — muzimuzhi Z, Dec 25 '20 at 18:23
With a recent version of LaTeX, the \AtEndEnvironment command is already builtin. — gernot, Dec 26 '20 at 12:36

Bernard · Accepted Answer · 2020-12-25T15:53:21.060

Using the ntheorem package, you have the automatic placement of the end-of-proof symbol at the end of the last line, including the cases when the last line is an equation:

\documentclass{article}
\usepackage{fourier} 
 \usepackage{amsmath, amssymb}
\usepackage[thmmarks, amsmath, thref]{ntheorem}
\usepackage{changepage} 
\usepackage{tcolorbox}
\tcbuselibrary{most}
\theoremstyle{nonumberbreakn}
\theoremheaderfont{\scshape\color{blue}}
\theoremseparator{:\smallskip}
\theorembodyfont{\upshape}
\theoremsymbol{\ensuremath{\color{blue}\blacksquare}}
\newtheorem{proof}{Proof}
\tcolorboxenvironment{proof}{
  fonttitle=\bfseries,
  enhanced,
  top=0mm,
  boxrule=0pt,frame empty,
  borderline west={4pt}{0pt}{blue},
  coltitle=blue,
  colback=white,
  sharp corners
}
\begin{document}
\begin{proof}
  Let's show that girls are Evil.
  \begin{adjustwidth}{5mm}{0mm}
    Girls require time and money
    \begin{equation}
      girls = time \times money
    \end{equation}
    Time is money
    \begin{equation}
      time = money
    \end{equation}
    So girls are money squared:
    \begin{equation}
      girls = money^2
    \end{equation}
    Money is the root of all evil
    \begin{equation}
      money = \sqrt{evil}
    \end{equation}
    So, girls are evil:
    \begin{equation}
      girls = \left(\sqrt{evil}\right)^2 = evil
    \end{equation}
  \end{adjustwidth}
% We have shown that girls are Evil.
\end{proof}
\end{document}

Thanks. In the code given in my first message, I have chosen to have a line break after the "proof" label. To keep this line break using your code, one has to use the nonumberbreak theoremstyle in place of nonumberplain. — Julien M., Dec 25 '20 at 14:59
It seems I misunderstood the exact layout. I'll fix that in a moment. — Bernard, Dec 25 '20 at 15:47

Tcolorbox : create a proof environnement with a QED symbol at the very end

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