Spacing problem with aligned nested in an align environement

Question

This minimal example

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
  A 
  &\leq
    \begin{aligned}[t]
      &\int \text{first part of a long expression}
      \\& \times \text{second part of the long expression}
    \end{aligned}
  \\&\leq \int \text{shorther expression}
\end{align*}
\end{document}

gives

The two integral signs are not aligned because there is too much space before the first ones (or equivalently, before the aligned environment).

How can I get the correct horizontal space before the first integral sign?

Answer 1

In general whenever aligned is used when there is "something" is front for it to align with, then always use

\! \begin{aligned} ....

This removes the spacing that is inserted at the start of aligned

Edit: there seems to be another issue at play, if the contents of aligned starts with & then there is also spacing issues. The problem and a fix can be seen in this MWE:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
  A
  &\leq\rlap{\raisebox{-7em}[0pt][0pt]{\rule{0.4pt}{8em}}} 
  \!
  \begin{aligned}[t]
    &\int
  \end{aligned}
  \\
  &\leq
  \!\begin{aligned}[t]
    \! &\int
  \end{aligned}
  \\
  &\leq \int
\end{align*}
\end{document}

Answer 2

You can avoid the aligned, in this case, by using split:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
\begin{split}
A&\leq \int \text{first part of a long expression} \\
 & \qquad\times \text{second part of the long expression}
\end{split}
\\
&\leq \int \text{shorther expression}
\end{align*}
\end{document}

The alternative with aligned is

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
A&\leq
  \!\begin{aligned}[t]
    &\!\int \text{first part of a long expression} \\
    &\quad\times \text{second part of the long expression}
  \end{aligned}
\\
&\leq \int \text{shorther expression}
\end{align*}
\end{document}

One \! is necessary in front of aligned, see Why is there a \, space at the beginning of the "aligned" environment?; another one is needed between & and \int, because after & there is an implicit {} that forces a thin spaces between it and \int, which is an operator atom.