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I am using aligned environment to write a multiline equation which works out fine if the equation is not too long.

However, this equation is just a bit wider so I want the tag to be still at the middle line (Line 2 in the MWE) which has space. I know the solution can be to squeeze the space between symbols or break the equation even more. But I am curious about how to force the tag position to the center?

Please consider the following MWE:

\documentclass[twocolumn,letterpaper]{article}
\usepackage{amssymb,amsmath,amsfonts,amsthm}
\usepackage{bm}

\begin{document}
  The following equation is a it wider so the tag is squeezed below,
  but there is space on the second line.
  How can I force that tag to that place?
  \begin{equation}
    \begin{aligned}
      \mathrm{vec}(\mathbf{H}{t,k})=&\ (\mathbf{V}_k^*\otimes\mathbf{V}_M),\mathrm{vec}(\bm{\Gamma}{t,k}\bm{\Omega}t\bm{\Sigma}{t,k}) \
      =&\ (\mathbf{V}k^*\otimes\mathbf{V}_M)(\bm{\Sigma}{t,k}^T\otimes\bm{\Gamma}{t,k}),\mathrm{vec}(\bm{\Omega}_t) \
      =&\ (\mathbf{V}_k^*\otimes\mathbf{V}_M)(\bm{\Sigma}{t,k}^T\otimes\bm{\Gamma}_{t,k})(\mathbf{V}_N^T\odot\mathbf{V}_N)\bm{\Psi}^*
    \end{aligned}
  \end{equation}
\end{document}

which produces this:

result

Teddy van Jerry
  • 2,110
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    use align not equation and use notag on the first and last lines? – David Carlisle Feb 14 '22 at 15:05
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    why the unusual =&\ markup, which is forcing non standard space after the = ? The usual markup would be &= – David Carlisle Feb 14 '22 at 15:06
  • @DavidCarlisle Thank you for the workaround and it looks nice! So lucky my equation has odd numbers of lines. Well, using =&\ makes the space looks normal (without which the space looks awkward.) but using &= indeed is a better choice! – Teddy van Jerry Feb 14 '22 at 15:15
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    see https://tex.stackexchange.com/questions/633570/width-of-a-control-space-in-relation-to-a-quad-thin-medium-and-thick-spaces/633572?noredirect=1#comment1580442_633572 \ is quite a bit more than \; (which is the default space around =) – David Carlisle Feb 14 '22 at 15:37

1 Answers1

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A possibility with the \mathrlap command from mathtools and the fleqn environment from nccmath ro ensure the equation starts at the left margin. Note that you don't have to load amsmath in this case, as mathtools does it for you.

\documentclass[twocolumn,letterpaper]{article}
\usepackage{amssymb,mathtools,amsthm}
\usepackage{nccmath}
\usepackage{bm}

\begin{document}


The following equation is a it wider so the tag is squeezed below,
  but there is space on the second line.
  How can I force that tag to that place?
    \begin{fleqn}
    \begin{equation}
    \begin{aligned}
      \mathrm{vec}(\mathbf{H}{t,k}) &=\ (\mathbf{V}_k^*\otimes\mathbf{V}_M),\mathrm{vec}(\bm{\Gamma}{t,k}\bm{\Omega}t\bm{\Sigma}{t,k}) \
      &=\ (\mathbf{V}k^*\otimes\mathbf{V}_M)(\bm{\Sigma}{t,k}^T\otimes\bm{\Gamma}{t,k}),\mathrm{vec}(\bm{\Omega}_t) \
      &=\ (\mathbf{V}_k^*\otimes\mathbf{V}_M)(\bm{\Sigma}{t,k}^T\otimes\bm{\Gamma}_{t,k})(\mathbf{V}_N^T\odot \mathrlap{\mathbf{V}_N)\bm{\Psi}^*}
    \end{aligned}
    \end{equation}
    \end{fleqn}


\end{document}

enter image description here

Bernard
  • 271,350