Placing equation number on left with reqno option in amsart document class

Question

Is there a way to force individual equations to place their equation numbers (or more specifically the result of a \tag*{blah}) on a specific side of the page regardless of the options to amsart.

I sometimes wish to use the equation enviornment to define 'requirements' in my papers which should look like:

R_e: equation stuff

Where R_e is the equation tag. R_e should be on the left even when reqno is passed to the documentclass. How can I accomplish this?

Answer 1

If you just want to do this for the equation environment, define a command to be used in the equation environment:

\newcommand{\LeftEqNo}{\let\veqno\@@leqno}

For instance,

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\newcommand{\LeftEqNo}{\let\veqno\@@leqno}
\makeatother
\begin{document}
  \begin{equation}
    x^2 + y^2 = 1,
  \end{equation}
  \begin{equation}\LeftEqNo
    x^2 + y^2 = 1.
  \end{equation}
\end{document}

The align environment is implemented quite differently, and you will need the environ package used like

\usepackage{environ}
\makeatletter
\NewEnviron{Lalign}{\tagsleft@true\begin{align}\BODY\end{align}}
\makeatother

Then use \begin{Lalign} ... \end{Lalign} for left-numbered align environments.

I'm afraid that various hacks of the same kind will be necessary for the other ams environments, but I can't list everything without knowing which ones you need :).