Put math at the end of display

Question

I'm trying to emulate the following behaviour i.e. the $(r \in \mathcal N)$ at the end of the line.

I'd like to know how to do this in \[ \], equation and align environments. I'm using amsart.

EDIT: It is posible to do the dual for non amsart? put something in the begin of the line?

Thanks.

Answer 1

It's a little bit hacky and overrides all the nice measuring mechanisms from amsmath, but it should be prety relialble in most of the cases. Note that the solution is sensible to the leqno and reqno options.

\documentclass[leqno]{amsart}

\makeatletter
\newenvironment{tagedequation}[1]{%
  \gdef\@tempa{#1}
  \refstepcounter{equation}
  (\iftagsleft@\theequation\else\(\@tempa\)\fi)\hfill\(
}{%
  \)\hfill(\iftagsleft@\(\@tempa\)\else\theequation\fi)
}
\makeatother

\begin{document}
\begin{tagedequation}{r\in\mathfrak{N}}
  \theta' = \gamma\theta,\quad r'=r
\end{tagedequation}
\end{document}

Answer 2

This might be a start. I've shown two varieties depending on your desires for centering the content.

\documentclass{amsart}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\begin{flalign}
\phantom{(r\in\mathfrak{N})}&&\theta' = \gamma\theta,
  \quad r'=r&&(r\in\mathfrak{N})
\end{flalign}
\begin{flalign}
&&\theta' = \gamma\theta,
  \quad r'=r&&(r\in\mathfrak{N})
\end{flalign}
\end{document}