Inalign you use&=, not&=& like ineqnarray. For the numbering, changealign* toalign and add\nonumber to the end of the first line.

\documentclass[12pt,letterpaper]{article}\usepackage[a4paper,left=20mm,top=25mm,right=20mm,bottom=25mm]{geometry}\usepackage[utf8]{inputenc}\usepackage{amsmath}\begin{document}Asymptotic equation of Motion\begin{align}\mathcal{L}_n (\nabla_{\mu} n_{\nu}) &= n^{\alpha} \nabla_{\alpha} ( \nabla_{\mu} n_{\nu}) + \nabla_{\alpha} n_{\nu} \nabla_{\mu} n^{\alpha} + \nabla_{\mu} n_{\alpha} \nabla_{\nu} n^{\alpha} \nonumber \\ % <----  add \nonumber here&= n^{\alpha} \nabla_{\alpha} ( \nabla_{\mu} n_{\nu}) - n^{\alpha} \nabla_{\mu} ( \nabla_{\alpha} n_{\nu}) + n^{\alpha} \nabla_{\mu} ( \nabla_{\alpha} n_{\nu}) + \nabla_{\alpha} n_{\nu} \nabla_{\mu} n^{\alpha} + \nabla_{\mu} n_{\alpha} \nabla_{\nu} n^{\alpha} \end{align}\end{document}

• The answers intex.stackexchange.com/questions/159723 describe how the columns inalign work.

  Torbjørn T.

  – Torbjørn T.

  2018-07-20 07:06:33 +00:00

  CommentedJul 20, 2018 at 7:06

For a single equation, the choice should besplit insideequation:

\documentclass[12pt]{article}\usepackage[a4paper,left=20mm,top=25mm,right=20mm,bottom=25mm]{geometry}\usepackage[utf8]{inputenc}\usepackage{amsmath}\begin{document}Asymptotic equation of Motion\begin{equation}\begin{split}\mathcal{L}_n (\nabla_{\mu} n_{\nu}) &= n^{\alpha} \nabla_{\alpha} ( \nabla_{\mu} n_{\nu})    + \nabla_{\alpha} n_{\nu} \nabla_{\mu} n^{\alpha}      + \nabla_{\mu} n_{\alpha} \nabla_{\nu} n^{\alpha}\\&= n^{\alpha} \nabla_{\alpha} ( \nabla_{\mu} n_{\nu})    - n^{\alpha} \nabla_{\mu} ( \nabla_{\alpha} n_{\nu})    + n^{\alpha} \nabla_{\mu} ( \nabla_{\alpha} n_{\nu})    + \nabla_{\alpha} n_{\nu} \nabla_{\mu} n^{\alpha}     + \nabla_{\mu} n_{\alpha} \nabla_{\nu} n^{\alpha}\end{split}\end{equation}\end{document}

If you prefer the number to be at the bottom, this should be a global decision, that can be accomplished by callingamsmath with thetbtags option:

\usepackage[tbtags]{amsmath}

With this single change, the output would become

Note thatamsmath alignment environments use&= (not&=&). I removed theletterpaper option, that does nothing and might be a bit unclear.

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