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

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}

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}

[

setlengtharraycolsep{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