If you compile what's below, you'll observe a slight horizontal misalignment between the equations of aligned versus align. Is there a way to avoid that?
\documentclass[fleqn]{book}
\usepackage{amsmath}
\begin{document}
text
\begin{equation}
\begin{aligned}
&A\\
&B
\end{aligned}
\end{equation}
text
\begin{align}
&A\\
&B
\end{align}
text
\end{document}
This is bizarre, but fixable. The control of the aligned environment in the amsmath package is governed by the command \start@aligned. In amsmath.sty, the start of this command resembles:
\newcommand{\start@aligned}[2]{%
\RIfM@\else
\nonmatherr@{\begin{\@currenvir}}%
\fi
\savecolumn@ % Assumption: called inside a group
\null\,%
...
The problem is the use of \null\, which adds a (spurious) \thinspace after \null, causing the misalignment. Use the etoolbox package's \patchcmd to fix/patch this. Try
...
\usepackage{etoolbox}
\makeatletter
\patchcmd{\start@aligned}{\null\,}{\null}{}{}%
\makeatother
...
in your preamble after loading amsmath:
amsmath package? What about modifying it?
\patchcmd command modifies during compilation, but doesn't change it in the original file amsmath.sty. To modify this in future releases, I would suggest submitting it as a possible bug by clicking on the Technical Support address on the AMS website, or send an email to [email protected].
amsmath.sty on CTAN? Strange.