# 2 x 2 lines equation on a single line separated by a text in the middle

How to write 2 x 2 rows equation on a single line separated by a text in the middle of the form:

The picture example is the result that I want to reproduce and can't (which I found in a textbook)

An imperfect try to reproduce the output would be:

\begin{flalign*}
a_n &= c_n + c_{-n} && && c_n &= \frac{1}{2} (a_n - ib_n)\\
&& \text{or} && \\
b_n &= i(c_n - c_{-n}) && && c{_n} &= \frac{1}{2} (a_n + ib_n)
\end{flalign*}


with the following rendering:

However:

• the or separating the two sides takes an entire row (instead of sitting on the half row between the 2 row equations)
• the 2 row equation of the right is put way to far on the right
• I added a MWE @CarLaTeX – ecjb Nov 18 '18 at 9:40

As Mico advised in the comments that the spacing is not good, I edited my answer. Thanks Mico!

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$\begin{array}{rcl} a_n & = & c_n+c_{-n}\\[1ex] b_n & = & i(c_n-c_{-n}) \end{array} \qquad \text{or} \qquad \begin{array}{rcl} c_n & = & \frac{1}{2}(a_n-ib_n)\\[1ex] c_{-n}&=& \frac{1}{2}(a_n+ib_n) \end{array}.$
\end{document}


• If there is only one line, what is the purpose of using align here? – Artificial Odorless Armpit Nov 18 '18 at 11:06
• The horizontal spacing around the four = symbols is too loose with this solution. – Mico Nov 18 '18 at 11:51
• @ArtificialStupidity Actually I tried \begin{align*} Hello & \text{or} & Hello \end{align*} at first. But it failed. I changed to this one and forget changing \begin\end{align*} to something like . Thanks for your feedback! – JouleV Nov 18 '18 at 12:53
• @Mico You know, one can always change the spacing between lines by \$1ex], for example. Thanks for your feedback! I will edit my answer now. – JouleV Nov 18 '18 at 12:54 • My earlier comment was meant to address the horizontal spacing around the = symbols. The vertical distance between the rows is a separate matter. – Mico Nov 18 '18 at 13:06 Here's a version of @DüngVü's answer that applies the correct amount of whitespace around the four = symbols. \documentclass{article} \usepackage{array} % for "\newcolumntype" macro \newcolumntype{C}{>{{}}c<{{}}} \begin{document} \[ \setlength\arraycolsep{0pt} \begin{array}{rCl} a_n & = & c_n+c_{-n} \\[1ex] b_n & = & i(c_n-c_{-n}) \end{array} \qquad \mbox{or} \qquad \begin{array}{rCl} c_n & = & \frac{1}{2}(a_n-ib_n) \\[1ex] c_{-n}& = & \frac{1}{2}(a_n+ib_n) \end{array}$
\end{document}

\documentclass[preview,border=12pt,varwidth]{standalone}
\usepackage{amsmath}
\begin{document}
\begin{aligned} a_n &= c_n + c_{-n}\\ b_n &= i(c_n - c_{-n}) \end{aligned} \qquad \text{or} \qquad \begin{aligned} c_n &= \frac{a_n-ib_n}{2}\\ c_{-n} &= \frac{a_n+ib_n}{2} \end{aligned}
Is it what you are looking for?
\end{document}


# My proposal

\documentclass[preview,border=12pt]{standalone}
\usepackage[a6paper,landscape,margin=2cm]{geometry}
\usepackage{mathtools}
\begin{document}
I prefer the following style because it is clearer what we are grouping, how about you?
\left\{ \begin{aligned} a_n &= c_n + c_{-n}\\ b_n &= i(c_n - c_{-n}) \end{aligned}\right. \qquad \text{or} \qquad \left\{ \begin{aligned} c_n &= \tfrac12(a_n-ib_n)\\ c_{-n} &= \tfrac12(a_n+ib_n) \end{aligned}\right.
\end{document}


I wouldn't use flalign* that spreads out the equations too much.

\documentclass{article}
\usepackage{amsmath,mathtools}

\begin{document}

\begin{flalign*}
a_n &= c_n + c_{-n} & c_n &= \frac{1}{2} (a_n - ib_n)\\
\shortintertext{\centering or}
b_n &= i(c_n - c_{-n}) & c{_n} &= \frac{1}{2} (a_n + ib_n)
\end{flalign*}

\begin{alignat*}{2}
a_n &= c_n + c_{-n} &\hspace{5em} c_n &= \frac{1}{2} (a_n - ib_n)\\
\shortintertext{\centering or}
b_n &= i(c_n - c_{-n}) & c{_n} &= \frac{1}{2} (a_n + ib_n)
\end{alignat*}

\end{document}


On the other hand, it is not really clear what “or” refers to; I'd suggest placing “or” at the left margin. Just remove \centering from the second example to get