Centre justifying equations in align environment

Question

Apologies if this has been asked before, but I couldn't find it after searching in a few places. I'm currently working with arrays of equations like the following:

\begin{align}

y_i^3=y_i^2&=z_i^2=z_i^3 &&\text{if\, $1\leq i\leq n-1$}\\

y_iz_iy_i=y_i,\quad &z_iy_iz_i=z_i &&\text{if\, $1\leq i\leq n-1$}\\

y_iz_i&=z_{i+1}y_{i+1} &&\text{if\, $1\leq i\leq n-2$}

\end{align}

But I'd really like to drop the &'s in the equations themselves, so that the equations are centre justified, the "if..." bits are left justified, and I still have labels on each line. If I drop the &'s and change the &&'s to &&&'s, then I get everything I want except that the equations are right justified.

Can anyone help?

Answer 1

The right-left alignment of align is set up in what is called an "align preamble" (stored in \align@preamble). You can update this preamble to adjust the alignment/layout of the align structure.

Below is a MWE that provides myalign, which centres the "left column" and left-aligns the "right column" within a regular align. In essence, only the "left column" alignment is altered to be centred by inserting an additional \hfil on the right side of the "left column" contents:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\makeatletter
\newenvironment{myalign}{%
  \def\align@preamble{% Definition taken from amsmath.dtx
    &\hfil
     \strut@
     \setboxz@h{\@lign$\m@th\displaystyle{####}$}%
     \ifmeasuring@\savefieldlength@\fi
     \set@field
     \hfil% <---------- Alignment added
     \tabskip\z@skip
    &\setboxz@h{\@lign$\m@th\displaystyle{{}####}$}%
     \ifmeasuring@\savefieldlength@\fi
     \set@field
     \hfil
     \tabskip\alignsep@
  }\align}{\endalign}
\begin{document}
\begin{align}
  y_i^3 = y_i^2 = z_i^2 = z_i^3 &\qquad \text{if $1 \leq i \leq n-1$} \\
  y_iz_iy_i = y_i, \quad z_iy_iz_i = z_i &\qquad \text{if $1 \leq i \leq n-1$} \\
  y_iz_i = z_{i+1}y_{i+1} &\qquad \text{if $1 \leq i \leq n-2$}
\end{align}

\begin{myalign}
  y_i^3 = y_i^2 = z_i^2 = z_i^3 &\qquad \text{if $1 \leq i \leq n-1$} \\
  y_iz_iy_i = y_i, \quad z_iy_iz_i = z_i &\qquad \text{if $1 \leq i \leq n-1$} \\
  y_iz_i = z_{i+1}y_{i+1} &\qquad \text{if $1 \leq i \leq n-2$}
\end{myalign}
\end{document}