Obstinate empty line between ntheorem’s break environment and an amsmath environment

Question

I’m using ntheorem to set my proofs, using the nonumberbreak style:

\documentclass{article}
\usepackage{amsmath}
\usepackage[amsmath]{ntheorem}
\theoremstyle{nonumberbreak}
\newtheorem{proof}{Proof}

So if I write

\begin{proof}
One line
\begin{align*}
\text{One equation}
\end{align*}
\end{proof}

I get the desired output (left one in the picture below).

But in some cases, my proof starts with some math environment right away, e.g.

\begin{proof}
\begin{align*}
\text{One equation}
\end{align*}
\end{proof}

In this case, I get exactly the same spacing as before, as if I replaced “One line” by ~ or something like that.

I can reduce the vertical space by setting \abovedisplayskip to 0px (third picture below), but there is still precisely one line left.

How can I get rid of this line?

Answer 1

You could do the following

\documentclass{article}
\usepackage{amsmath}
\usepackage[amsmath]{ntheorem}
\theoremstyle{nonumberbreak}
\newtheorem{proof}{Proof}

\begin{document}
So if I write

\begin{proof}
\mbox{aaa}\\*[-\dimexpr2\baselineskip+\abovedisplayskip\relax]\mbox{bbb}
\begin{align*}
\text{One equation}
\end{align*}
zzz
\begin{align*}
\text{One equation}
\end{align*}
zzz
\end{proof}

\end{document}

which only affects the space above one equation (you might want to omit the aaa and bbb and just use \mbox{} in a real document).

The fact that such contortions are needed might be taken as a hint that perhaps it's not the expected usage and that saying at least Consider the equation before the equation would make make egreg happier.