## LaTeX forum ⇒ Math & Science ⇒ multiple alignment points and \split environment

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Drugbird
Posts: 2
Joined: Tue Aug 31, 2010 3:13 pm

### multiple alignment points and \split environment

Hi there,

I'd like to display a two line equation aligned in more than one place across two lines, but only supply it with one number.

I.e. in an equation like:
a=b=c
ab=bc=c^2

I'd like the = signs to be aligned vertically, and I want 1 equation number to the right halfway between the two lines.

I know alignment can be done with \alignat, and the numbering can be done with \split, but I can't seem to get both things at once.

MWE:
\documentclass[10pt,a4paper]{article}\usepackage{amsmath}\begin{document}\section{Test}\begin{alignat}{3}a&={}&b&={}&c\\ab&={}&bc&={}&c^2\end{alignat} \begin{alignat}{3}a&=b&=c\\ab&=bc&=c^2\end{alignat} \begin{equation}\begin{split}a&=b=c\\ab&=bc=c^2\end{split}\end{equation} \end{document}

Also, alignat seems to require an extra {}& per equal sign that I don't quite understand (top equation in the MWE, since the one below has strange spacing). Could someone explain this to me?

Furthermore, the split environment will produce error messages if I put more & symbols on one line. For example, inserting this in the MWE will produce error messages:
\begin{equation}\begin{split}a&=b&=c\\ab&=bc&=c^2\end{split}\end{equation}

Could someone help me align and number things the way I like? Thanks in advance.

(Some more details I'm not quite sure are useful: I use WinEdt 6.0 with MikTex 2.8 and PDFTexify)
Last edited by Drugbird on Thu Sep 02, 2010 1:00 pm, edited 1 time in total.

localghost
Site Moderator
Posts: 9206
Joined: Fri Feb 02, 2007 12:06 pm
Location: Braunschweig, Germany
It has to look a bit different. The code below shows only the relevant part.
\begin{equation}   \begin{alignedat}{2}     a  &= b  & &= c\\     ab &= bc & &= c^2   \end{alignedat}\end{equation}

See the amsmath manual for details. More stuff about math typesetting can be found in the »Math mode« document.

Best regards and welcome to the board
Thorsten
LaTeX Community Moderator

¹ System: openSUSE 42.2 (Linux 4.4.52), TeX Live 2016 (vanilla), TeXworks 0.6.1

Drugbird
Posts: 2
Joined: Tue Aug 31, 2010 3:13 pm
Thanks a lot, localghost. This is exactly what I was looking for.

I'm still not quite sure though why you need three & symbols per line. My reasoning is that there are only two equal signs that need to be aligned, so you'd also need two & symbols. I tried searching the amsmath documentation, but was unable to find it... Any help?

localghost
Site Moderator
Posts: 9206
Joined: Fri Feb 02, 2007 12:06 pm
Location: Braunschweig, Germany
Drugbird wrote:[…] I'm still not quite sure though why you need three & symbols per line. My reasoning is that there are only two equal signs that need to be aligned, so you'd also need two & symbols. I tried searching the amsmath documentation, but was unable to find it. […]

Consider the alignat environment as an array with special alignment of the columns.
\begin{array}{@{}r@{\,=\,}lr@{\,=\,}l@{}}   a  & b  & c \\   aa & bb & cc\end{array}

If you declare an alignat environment with n as its mandatory argument for the number of equal signs to be aligned, you will need n+1 alignment characters (ampersands) to get the right alignment. Take a look at Sections 3.6 and 3.7 of the amsmath manual. You can find similar descriptions in the »Math mode« document.
LaTeX Community Moderator

¹ System: openSUSE 42.2 (Linux 4.4.52), TeX Live 2016 (vanilla), TeXworks 0.6.1