[Tex/LaTex] How to align ‘implies that’ symbols neatly in equation array

align

The following is an equation array I have:

\begin{eqnarray}
\Rightarrow\eta_t &=& \eta_x(\omega \eta-(\varphi_x)_s) + (\varphi_y)_s\notag\\
\Rightarrow\eta_t\,(\varphi_y)_s &=& \eta_x(\omega \eta-(\varphi_x)_s)(\varphi_y)_s +      {(\varphi_y)_s}^2\notag\\
&=&\omega\, \eta\,(\varphi_y)_s\, \eta_x-(\varphi_x)_s\, \eta_x(\varphi_y)_s +     {(\varphi_y)_s}^2.
\end{eqnarray}

All I want to do is move the first 'implies that' symbol so that it is aligned with the second one and to not change anything else. I have tried several things but the end result is always that the equations are stretched out and I want to avoid that. (I know use of eqnarray is not recommended but I've done too much to go back and change everything!!)

Best Answer

Note that you only need two alignment points. (Also, Avoid eqnarray!.)

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{alignat}{2}
  &\Rightarrow\quad
  &\eta_{t}
  &= \eta_{x}(\omega\eta - (\varphi_{x})_{s}) + (\varphi_{y})_{s}\notag\\ 
  &\Rightarrow
  &\eta_t(\varphi_{y})_{s}
  &= \eta_{x}(\omega\eta - (\varphi_{x})_{s})(\varphi_{y})_{s} + {(\varphi_{y})_{s}}^{2}\notag\\  
  &&&= \omega\eta(\varphi_{y})_{s}\eta_{x} - (\varphi_{x})_{s}\eta_{x}(\varphi_{y})_{s} + {(\varphi_{y})_{s}}^{2}. 
\end{alignat}

\end{document}

output

Update

I think you have one arrow too many. Either way, here are one version with a single horizontal arrow and another version with a single vertical arrow:

\documentclass{article}

\usepackage{mathtools}

\begin{document}

\begin{alignat}{2}
  &&\eta_{t}
  &= \eta_{x}(\omega\eta - (\varphi_{x})_{s}) + (\varphi_{y})_{s}\notag\\
  &\Rightarrow\quad
  &\eta_t(\varphi_{y})_{s}
  &= \eta_{x}(\omega\eta - (\varphi_{x})_{s})(\varphi_{y})_{s} + {(\varphi_{y})_{s}}^{2}\notag\\  
  &&&= \omega\eta(\varphi_{y})_{s}\eta_{x} - (\varphi_{x})_{s}\eta_{x}(\varphi_{y})_{s} + {(\varphi_{y})_{s}}^{2}. 
\end{alignat}
or
\begin{alignat}{2}
  &&\eta_{t}
  &= \eta_{x}(\omega\eta - (\varphi_{x})_{s}) + (\varphi_{y})_{s}\notag\\
  \ArrowBetweenLines[\Downarrow]
  &&\eta_t(\varphi_{y})_{s}
  &= \eta_{x}(\omega\eta - (\varphi_{x})_{s})(\varphi_{y})_{s} + {(\varphi_{y})_{s}}^{2}\notag\\
  &&&= \omega\eta(\varphi_{y})_{s}\eta_{x} - (\varphi_{x})_{s}\eta_{x}(\varphi_{y})_{s} + {(\varphi_{y})_{s}}^{2}.
\end{alignat}

\end{document}

output2

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