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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.

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you could \phantom each individual 'cell' –  cmhughes Aug 27 '12 at 1:20
    
you could pack the alignment in a minipage with the \phantom wrapped around that, but unfortunately it would foul up the vertical spacing. –  barbara beeton Aug 27 '12 at 12:57
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2 Answers 2

up vote 9 down vote accepted

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:

enter image description here

\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.

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It's probably easier to use textcolor with white:

\textcolor{white}{sometext}

You need the color package.

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4  
Unusual approach, however I don't consider it good in this case. There are two reasons: (1) unnecessary usage of colours, (2) the hidden text is still copy-pastable, which might be an issue, especially if it's hidden to achieve correct spacing, because then the text does not belong to the output but still would get copied. –  tohecz Aug 27 '12 at 6:18
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