How can I temporarily change of end of a proof symbol which looks like $\square$ to $\triangle$?

Question

I want to temporarily change the end of a proof symbol which looks like $\square$ to $\triangle$, a triangle symbol for a particular proof of a particular theorem, while for other theorems in the paper I still want to use $\square$ for the end of the proof.

Is it possible and how can I achieve it?

Answer 1

I assume amsthm.

\documentclass[twocolumn]{article}
\usepackage{amsthm}
\begin{document}
\begin{proof}
This ends with a square.
\end{proof}
\begin{proof}\renewcommand{\qedsymbol}{\ensuremath{\triangle}}
This ends with a triangle.
\end{proof}
\end{document}

I use twocolumn just to make a smaller picture.

You may want to define a new environment for this.

\documentclass[twocolumn]{article}
\usepackage{amsthm}
\newenvironment{varproof}
 {\renewcommand{\qedsymbol}{\ensuremath{\triangle}}\proof}
 {\endproof}
\begin{document}
\begin{proof}
This ends with a square.
\end{proof}
\begin{varproof}
This ends with a triangle.
\end{varproof}
\end{document}

Answer 2

I assume amsthm is being used. Here is the ad-hoc solution addressing a single occurrence of a special theorem and its proof(s).

\documentclass{article}
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}
\begin{document}
\begin{theorem}
\end{theorem}
\begin{proof}
\end{proof}
{
\renewcommand{\qedsymbol}{$\triangle$}
\begin{theorem}[Special]
\end{theorem}
\begin{proof}
  Its proof.
\end{proof}
\begin{proof}
  Another one: [ a^2 + b^2 = c^2. \qedhere ]
\end{proof}
}
\begin{theorem}
\end{theorem}
\begin{proof}
\end{proof}
\end{document}