Phantom and Align Environment

Question

Is there any way to 'phantom' text within the align environment?

I have the following code:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
\phantom
{
a & = b \\
& = c
}
\end{align*}
\end{document}

Apparently, if there a tab alignment character in the argument of the phantom command, compilation will stop. I was trying to make the text within the align environment appear as blank text. Curiously, if I have a cases environment with the tab alignment character inside it, phantom will work when it surrounds the cases environment.

Thanks.

Answer 1

If you "hide" the tab alignment & from align, it will fail to work as expected. This is only because & is read as part of the argument to \phantom, which knowns nothing about & and its use. As such, you either have to spread the \phantom across the aligned components, or use an altogether different approach:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align*}
  a & = b + c \\
  \phantom{a} & \phantom{{} = b} + c \\ % Hides some components of above line
   & = d + e \\
   & \phantom{{}= d} + e % Hides some components of above line
\end{align*}
\end{document}

In the above example, some components were left untouched to indicate how some spacing correction is sometimes required when using \phantom (for example, when hiding the relation =).

The reason why \phantom around an entire cases structure works is because the tab alignment character is buried within the environment where it makes sense.

Answer 2

This is an old question, but I thought that future searchers might appreciate this alternative, which in complex situation is much easier than splitting out the phantoms over the tab stops. It uses pgf, which has \pgfsys@begininvisible and \pgfsys@endinvisible for this purpose.

\documentclass{article}
\usepackage{pgf}
\usepackage{amsmath}
\makeatletter
\newcommand\pgfinvisible{\pgfsys@begininvisible}
\newcommand\pgfshown{\pgfsys@endinvisible}
\makeatother
\begin{document}
\begin{align}
  a & = b + c \
  \pgfinvisible a & = b\pgfshown + c \ % Hides some components of above line
   & = d + e \
  \pgfinvisible  & = d\pgfshown + e % Hides some components of above line
\end{align}
\end{document}

The output:

This is the mechanism that beamer's overlay system uses.

In fact, you can put this around the entire align* environment, which doesn't work with \phantom and is what the question asker originally wanted. Then one must be careful about introducing extra vertical space.

I should add here that, apparently, this does not remove the text from the PDF (it gets offset somewhere where it won't appear on the page, as far as I understand) so this is not a suitable approach for redaction.

Answer 3

It's probably easier to use textcolor with white:

\textcolor{white}{sometext}

You need the color package.