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Is it possible to type something such as "and" in between these two aligned equations where the arrow mentions?. If so, can anyone kindly tell me how to achieve this?

Here is the screenshot

enter image description here

and here is my code: 

\begin{align*}
\bigl(Sf\bigr)\bigl(v_{1},\ldots,v_{k}\bigr) &:= \sum_{\sigma\in S_{k}}
f\bigl(v_{\sigma(1)},\ldots,v_{\sigma(k)} \bigr), \\ 
\bigl(Af \bigr) \bigl(v_{1},\ldots,v_{k}\bigr) 
  &:=\sum_{\sigma\in S_{k}} \bigl( \text{sgn\,}\sigma\bigr) 
f\bigl( v_{\sigma(1)}, \ldots,v_{\sigma(k)} \bigr).
\end{align*}
Hussein Eid
  • 633

2 Answers2

1

The mathtools package has \shortintertext for this purpose:

\documentclass{article}
\usepackage{mathtools}
\usepackage{lipsum}

\begin{document}

\lipsum[1]
\begin{align*}
\bigl(Sf\bigr)\bigl(v_{1},\ldots,v_{k}\bigr) &:= \sum_{\sigma\in S_{k}}
f\bigl(v_{\sigma(1)},\ldots,v_{\sigma(k)} \bigr), \\
\shortintertext{and}
\bigl(Af \bigr) &:=\sum_{\sigma\in S_{k}} \bigl( \text{sgn\,}\sigma\bigr) 
f\bigl( v_{\sigma(1)}, \ldots,v_{\sigma(k)} \bigr).
\end{align*}

\end{document}

enter image description here

siracusa
  • 13,411
  • This is good but the distance between "and" and the two aligned equation is a bit longer. Is it possible to make it nearer ? – Hussein Eid Dec 28 '16 at 05:50
  • What do you mean, longer than what? – siracusa Dec 28 '16 at 05:51
  • Longer than the expected. I want the word "and" a bit nearer to the two aligned equations. Is it clear ? – Hussein Eid Dec 28 '16 at 05:53
  • 1
    You could use something like \shortintertext{\hspace*{1cm}and} to bring it nearer to the center, but I'd vote against this because it will look strange if you have surrounding text as in the example – siracusa Dec 28 '16 at 05:56
1

Another solution: the \ArrowBetweenLines command, from mathtools, which accepts an optional argument. It has to be used within the alignedat environment. Note I replaced := (where the colon is not centred on the math axis) with coloneqq.

\documentclass{article}
\usepackage{mathtools}

\begin{document}

Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text.

\documentclass{article}
\usepackage{mathtools}

\begin{document}

Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text.

\begin{alignat*}{2}
    & & \bigl(Sf\bigr)\bigl(v_{1},\ldots,v_{k}\bigr) & \coloneqq \sum_{\sigma \in S_{k}}
  f\bigl(v_{\sigma (1)},\ldots,v_{\sigma (k)} \bigr), \\
  \ArrowBetweenLines[\text{and }]
    & & \bigl(Af \bigr) & \coloneqq \sum_{\sigma \in S_{k}} \bigl( \text{sgn\,}\sigma \bigr)
  f\bigl( v_{\sigma (1)}, \ldots,v_{\sigma (k)} \bigr).
\end{alignat*}

\end{document}

enter image description here

Bernard
  • 271,350