Custom alignment of columns in align environment

Question

I need to typeset two columns of equations, each row typeset with an equation number to the right as with an ordinary equation. The first column must be centered, and the second column must be left-aligned. There will be many rows, so it must automatically break at a page break as appropriate.

There are plenty of related questions, but I can't find any existing solutions that will let me do exactly what I want. (It's a real pain the align environment can't take optional column alignment specifiers.)

Answer 1

Making align break across pages is simple: add \allowdisplaybreaks[1] to your preamble.

And since your first column is text, it's also trivial to center it within align: just wrap it in a \makebox larger than all the images (which centers its contents) and use align as normal.

\documentclass{article}
\usepackage{amsmath}

\allowdisplaybreaks[1]
\newcommand{\centerdia}[1]{\makebox[2in]{\includegraphics{#1}}}
\begin{document}
\begin{align}
  \centerdia{dia1} & c=20x^2+5x-10+\frac{x^3-4x^2+500x-f(240)}{50}  \\
  \centerdia{dia2} & \mu=10*\epsilon\\
  \centerdia{dia3} & \mu=10*\epsilon
\end{align}
\end{document}

Answer 2

Use a modified version of align; change 2\tabcolsep into the separation you prefer.

\documentclass{article}
\usepackage{amsmath,environ}
\makeatletter
\NewEnviron{specialalign}
  {\def\align@preamble{%
     &\hfil
      \strut@
      \setboxz@h{\@lign$\m@th\displaystyle{####}$}%
      \ifmeasuring@\savefieldlength@\fi
      \set@field
      \hfil
      \tabskip2\tabcolsep
     &\setboxz@h{\@lign$\m@th\displaystyle{{}####}$}%
      \ifmeasuring@\savefieldlength@\fi
      \set@field
      \hfil
      \tabskip\alignsep@
  }%
  \begin{align}\BODY\end{align}}
\makeatother

\begin{document}
\begin{specialalign}
x & a=b \\
yyy & c=d+e+f \\
zzzzzz & 1\ne0
\end{specialalign}
\end{document}