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

It's probably easier to use textcolor with white:

\textcolor{white}{sometext}

You need the color package.