Solid square in a proof

Question

A similar question has been asked before but, I believe, I am trying something slightly different as I am trying to make adjustments using renewenvironment.

I am trying to obtain a solid square at the end of a proof. If I don't try to use the renewenvironment, I get a solid square. However, if I use the renewenvironment, the "Proof" title is bold (as required) but the box is not filled in.

I'm sure the solution will help me understand how things are being "over-written" at some points.

\documentclass{article}

\usepackage{amsmath}
\usepackage[most]{tcolorbox}
\usepackage{amsthm}
\usepackage{amssymb}

\renewcommand{\qedsymbol}{$\blacksquare$} % Requires amssymb
     
\newtcbtheorem[auto counter,number within=section]{theo}%
  {Theorem}{fonttitle=\bfseries\upshape, fontupper=\slshape,
     arc=0mm, colback=blue!5!white,colframe=blue!60!white,
       every box/.style={
         highlight math style={
           colback=green!10!white,
           colframe=blue
         }
       }
     }{theorem}

% % If I uncomment this, the square is not filled in!
% \renewenvironment{proof}[1][Proof]{\par\noindent\textbf{#1} }
% {\hfill$\square$\par}

\begin{document}

Why doesn't it show a solid square?

\begin{theo}{Goldbach Conjecture}{difficult}
Every even natural number greater than 2 is the sum of two prime numbers.
    \begin{proof}
        It's obvious :)
    \end{proof}
\end{theo}

\end{document}

To close voters, LateX is programming and is explicitly on-topic here. — TylerH
– TylerH, Commented Jun 19, 2025 at 13:54
@TylerH We have a whole site dedicated to it, this question should go there instead; though it would have been better if the close voters had (unanimously) voted to migrate. — Mark Rotteveel
– Mark Rotteveel, Commented Jun 19, 2025 at 15:44

samcarter_is_at_topanswers.xyz · Accepted Answer · 2025-06-18 18:22:23Z

With your renewenvironment, you replace the proof environment with an environment which does not do much other than setting the title in bold and adding an open \square at the end. If you want to have a filled square, use \blacksquare.

Or modify the original definition of the proof environment:

\documentclass{article}

\usepackage{amsmath}
\usepackage[most]{tcolorbox}
\usepackage{amsthm}
\usepackage{amssymb}

\renewcommand{\qedsymbol}{$\blacksquare$} % Requires amssymb
     
\newtcbtheorem[auto counter,number within=section]{theo}%
  {Theorem}{fonttitle=\bfseries\upshape, fontupper=\slshape,
     arc=0mm, colback=blue!5!white,colframe=blue!60!white,
       every box/.style={
         highlight math style={
           colback=green!10!white,
           colframe=blue
         }
       }
     }{theorem}
     
\usepackage{xpatch}
\xpatchcmd{\proof}{\itshape}{\itshape\bfseries}{}{}

\begin{document}

Why doesn't it show a solid square?

\begin{theo}{Goldbach Conjecture}{difficult}
Every even natural number greater than 2 is the sum of two prime numbers.
    \begin{proof}
        It's obvious :)
    \end{proof}
\end{theo}

\end{document}

Thank you. Your reply "If you want to have a filled square, use \blacksquare" immediately showed me that replacing \square with \blacksquare in the renewenvironment of my original posting code solved the problem.

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