# The square symbol does not appear in the end of proof

\documentclass{article}
\usepackage{amsthm}
\renewenvironment{proof}{{\bfseries Proof.}}{}
\renewcommand{\qedsymbol}{$\blacksquare$}
\newtheorem{theorem}{Theorem}[section]
\begin{document}
\begin{theorem}
blablabla
\end{theorem}
\begin{proof}
abcdefghijklmnopqrstuvwxyz
\end{proof}
\end{document}


I want to make the "proof title" is bold, so I give the code \renewenvironment{proof}{{\bfseries Proof.}}{}. But why after adding this code the square symbol in the end of proof is disappear? How to appearing the square symbol in the end of proof?

There is no symbol because you left the final argument in your \renewenvironment definition blank. You need to add \qedsymbol. You also need to load amssymb to get the $\blacksquare$ symbol.

\documentclass{article}
\usepackage{amssymb,amsthm}
\renewenvironment{proof}{{\bfseries Proof.}}{\qedsymbol}
\renewcommand{\qedsymbol}{$\blacksquare$}
\newtheorem{theorem}{Theorem}[section]
\begin{document}
\begin{theorem}
blablabla
\end{theorem}
\begin{proof}
abcdefghijklmnopqrstuvwxyz
\end{proof}
\end{document}


The amsmath package should not hardwire \itshape in the definition of amsthm. Maybe a future release will provide a way to choose the font.

In the meantime, it's not difficult to modify the environment by patching.

\documentclass{article}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{xpatch}

\xpatchcmd{\proof}{\itshape}{\bfseries}{}{}
\renewcommand{\qedsymbol}{$\blacksquare$}

\newtheorem{theorem}{Theorem}[section]

\begin{document}

\begin{theorem}
blablabla
\end{theorem}
\begin{proof}
abcdefghijklmnopqrstuvwxyz
\end{proof}

\end{document}