# How can I make a single-aligned row under two double-aligned rows in an equation?

I have an equation that I would like to align like this (the x is actually \times):

A = BBBBB CCCCCCC
x DDDDD
= EEEEEEE FFFFF


I have tried the align environment, align with nested aligned, and alignat with two columns.

The best I can do with plain align is:

A =&~ widersum &BBBBB &CCCCCCC
&           &      &x DDDDD
=&~ sum      &BBBBB &FFFFF


Which in my current case is fine, because the second sum is only a fraction less wide. But is there any decent way to ignore a second alignment & in one row?

PS: I seem to need extra spaces ~ when I place the alignment after an equals sign. How come?

EDIT: a realistic example

f_ &= \sum_{\mu, \lambda=-\infty}^\infty &blabla bla bla \notag \\
&&\times blabla \notag \\
&= \sum_{\lambda=-\infty}^\infty blabla bla blabla


is what I'm after

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ad "~ at equals sign": There are two options: A &= B or A = {}& B, choose whichever you prefer. But ~ will create certaily a too long space. –  tohecz Feb 13 at 15:42

You could use alignat instead of align:

Additionally you could use \rlap from the mathtools package to not have the last line effect the alignment of the previous lines:

## Notes:

• I inserted an additional {} to ensure that the \times is treated as a binary operator.

## Code: alignat

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{alignat}{3}
f &= \sum_{\mu, \lambda=-\infty}^\infty   & blabla bla bla  \notag \\
&                                       & {}\times blabla \notag \\
&= \sum_{\lambda=-\infty}^\infty blabla bla blabla
\end{alignat}
\end{document}


## Code: alignat and rlap:

\documentclass{article}
\usepackage{math tools} % Already includes amsmath

\begin{document}
\begin{alignat}{3}
f &= \sum_{\mu, \lambda=-\infty}^\infty   & blabla bla bla  \notag \\
&                                       & {}\times blabla \notag \\
&= \rlap{$\displaystyle\sum_{\lambda=-\infty}^\infty blabla bla blabla$}
\end{alignat}
\end{document}

-

One can do that with alignat:

        \documentclass[a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}

\pagestyle{empty}

\begin{document}

\begin{subequations}
\begin{alignat}{2}
f_0 & = & \sum_{\mu, \lambda=-\infty}^\infty blabla bla bla & {}\\
& & \times blabla & \notag\\
& = & \sum_{\lambda=-\infty}^\infty blabla bla blabla & {}
\end{alignat}
\end{subequations}

\end{document}


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You could use an array environment inside an align or align* environment. The first row of the array material should be left-justified, and the second row should be right-justified; the {} and @{} terms in the code below are there to help fine-tune the alignment. A (probably advantageous) side-effect of this approach is that the middle line -- i.e., the second line of the array -- will be a bit closer to the first line than it is to the final line.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
A &= \begin{array}[t]{@{}l}
BBBBB\ CCCCCCC\\
\multicolumn{1}{r}{{}\times DDDDD}
\end{array}\\
&= EEEEEEE\ FFFFF
\end{align*}
\end{document}

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