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)