Solid black box in the proof environment

Question

I am using

\documentclass[12pt]{article}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}

\begin{document}

\begin{proof}
A = B
\end{proof}

\end{document}

and an empty square box will be added automatically at the end of the proof. I am wondering how to add a solid black box instead, in an automatic way.

Answer 1

The documentation of amsthm says:

A QED symbol, □, is automatically appended at the end of a proof environment. To substitute a different end-of-proof symbol, use \renewcommand to redefine the command \qedsymbol.

Thus, one might proceed as follows:

\renewcommand{\qedsymbol}{$\blacksquare$}

A mwe to play with:

\documentclass[a4paper,11pt]{article}
\usepackage{amsmath,amssymb,amsthm}

\renewcommand{\qedsymbol}{$\blacksquare$}

\begin{document}
\begin{proof}[Proof of the Main Theorem]
  \begin{equation*}
    G(t)=L\gamma!\,t^{-\gamma}+t^{-\delta}\eta(t) \qedhere
  \end{equation*}
\end{proof}
\end{document}

If you're looking for beamer, it is quite the same: you have to re-define the template qed symbol (almost similar to my previous answer in How to customise (e.g. make thicker) the end-proof symbol in Beamer?).

A mwe:

\documentclass{beamer}

\begin{document}

\begin{frame}
\setbeamertemplate{qed symbol}{$\blacksquare$}
\begin{proof}
\begin{itemize}
\item First item.
\item Second item.
\item Third item.
\end{itemize}
\end{proof}
\end{frame}

\end{document}

Notice that in this case the symbol will assume the colors of the Beamer color theme currently in use.

Answer 2

If you are using the amsthm package (or one of the ams documentclasses), then the symbol used is given by the command \qedsymbol. It can be redefined as follows:

\documentclass{article}

\usepackage{amsthm,amssymb}

\renewcommand{\qedsymbol}{\rule{0.7em}{0.7em}}

\begin{document}
\begin{proof}
  Clear.
\end{proof}
\end{document}