ntheorem: Manually placing end-of-proof symbol in an equation

Question

I would like to use ntheorem to produce theorems and proofs. I've got an end-of-proof symbol that works ok but, in the case of a trivial result, I'd like to place the symbol in the theorem itself. I've got something that works ok, except when the theorem ends with an equation.

I'm not 100% sure that I'm using the package correctly. The following is my attempt at a MWE:

\documentclass{article}
\usepackage{amssymb}
\usepackage[thmmarks]{ntheorem}
\newtheorem{thm}{Theorem}
\theoremstyle{nonumberplain}
\theorembodyfont{\normalfont}
\theoremsymbol{\ensuremath\square}
\newtheorem{proof}{Proof:}
\begin{document}
\begin{thm}
Some theorems need a proof.
\end{thm}
\begin{proof}
And the end of proof symbol works even with an equation such as
[
P = NP.
]
\end{proof}
\begin{thm}
Some are trivial and don't need a proof. \hfill\proofSymbol
\end{thm}
\begin{thm}
But how do you place the symbol when the trivial result ends with an equation
[
x^{2}-1 = (x-1)(x+1)? \hfill\proofSymbol
]
\end{thm}
\end{document}

Any corrections to what I've done? And, most importantly, how do I place the end-of-proof symbol correcly in Theorem 3?

Answer 1

You can define a trivthm environment, which shares the counter of thm, and declare \theoremsymbol just before this environment:

\documentclass{article}
\usepackage{amssymb}
\usepackage[thmmarks]{ntheorem}
\newtheorem{thm}{Theorem}
\theoremsymbol{\ensuremath\square}
\newtheorem{trivthm}[thm]{Theorem}
\theoremstyle{nonumberplain}
\theorembodyfont{\normalfont}
\newtheorem{proof}{Proof:}
\begin{document}
\begin{thm}
Some theorems need a proof.
\end{thm}
\begin{proof}
And the end of proof symbol works even with an equation such as
[
P = NP.
]
\end{proof}
\begin{trivthm}
Some are trivial and don't need a proof.
\end{trivthm}
\begin{trivthm}
But how do you place the symbol when the trivial result ends with an equation
[
x^{2}-1 = (x-1)(x+1)?
]
\end{trivthm}
\end{document}