Centering the align environment on the middle column

I have a set of aligned equations that I would like to provide an \implies for down the left hand side. I used this originally:

\begin{align}
(x-1)^2 &= -1    \\
x-1 &= \pm i \\
x &= 1 \pm i
\end{align}


which results in a centred set of equations. Then added the \implies in a left column:

\begin{align}
&          & (x-1)^2 &= -1    \\
& \implies &     x-1 &= \pm i \\
& \implies &       x &= 1 \pm i
\end{align}


However now the entire environment is shifted to the right. Is there a way to keep the original alignment?

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migrated from stackoverflow.comAug 9 '11 at 22:20

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If you want exactly the same alignment as before, but this time with \implies, this does the trick:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
(x-1)^2 &= -1     \\
x-1 &= \pm i  \\
x &= 1 \pm i
\end{align}

\begin{alignat}{3}
&& (x-1)^2 &= -1      && \phantom{\implies} \\
\implies &&     x-1 &= \pm i   && \\
\implies &&       x &= 1 \pm i &&
\end{alignat}
\end{document}


The idea here is to duplicate the width of \implies on the left with a \phantom{\implies} on the right, thereby evenly spacing the equation/environment again.

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Removing the first alignment character and adding a few \phantoms should do the trick. Alternatively you can use alignat:

\begin{document}
\begin{align}
(x-1)^2 &= -1    \\
x-1 &= \pm i \\
x &= 1 \pm i
\end{align}

\begin{align}
\phantom{\implies}    (x-1)^2 &= -1    \\
\implies\phantom{()^2}    x-1 &= \pm i \\
\implies\phantom{({}-1)^2}  x &= 1 \pm i
\end{align}

\begin{alignat}{3}
&& (x-1)^2 &= -1    \\
\implies &&     x-1 &= \pm i \\
\implies &&       x &= 1 \pm i
\end{alignat}
\end{document}

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