# combine several align-environments

I want to combine several align-environments but reset the alignment at some point.

Esp., I want to combine these:

\begin{align*}
& \lim(\emptyset) = \ext(\emptyset), \lim(\Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^*) = \ext(\Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^*) \text{(see above)}
\end{align*}
\begin{align*}
& \lim( \neg ( \Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^* ) ) & = & \{ \alpha \in \Sigma^\omega | \exists^\omega n : \alpha [0,n] \notin \Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^* \} \\
& & = & \{ \alpha \in \Sigma^\omega | \forall n: \alpha [0,n] \notin \Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^* \} \\
& & = & \neg \ext( \Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^* )
\end{align*}


But the should not share a common alignment. (Edit: Actually, they should share the same outermost left alignment but nothing more.)

I want to combine them because it otherwise adds an empty line/space between them but I don't want that. Or is there any other way I can avoid that?

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I don't quite understand what you're trying to achieve. Do you really want the first alignment in a single line, or should it be two lines? (It's 2 separate equations, after all!) –  Hendrik Vogt Mar 16 '11 at 14:13
@Hendrik: Well, I could also make it two lines but that is not related to my question here. I want that the lines of the second align start right after. But it adds an empty line between the two align environments. –  Albert Mar 16 '11 at 14:35

You can use the aligned environment inside align*. Note that I added a \  before \lim and before \text so that you get a space there. (This gives an overfull \hbox, but that's another thing.)

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator\ext{ext}
\begin{document}
\begin{align*}
& \lim(\emptyset) = \ext(\emptyset), \
\lim(\Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^*)
= \ext(\Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^*)\ \text{(see above)}, \\
& \begin{aligned}
\lim( \neg ( \Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^* ) )
&= \{ \alpha \in \Sigma^\omega | \exists^\omega n : \alpha [0,n] \notin \Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^* \} \\
&= \{ \alpha \in \Sigma^\omega | \forall n: \alpha [0,n] \notin \Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^* \} \\
&= \neg \ext( \Sigma^* a_1 \Sigma^* a_2 \dotsb a_n \Sigma^* )
\end{aligned}
\end{align*}
\end{document}

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Now it aligns all of them together at the =. But I don't want that. I want the alignment to behave just as if it were two separates environments. –  Albert Mar 16 '11 at 14:45
@Albert: So what do you mean when you say "they should share the same outermost left alignment"? –  Hendrik Vogt Mar 16 '11 at 14:47
@Hendrik: Every line (except those which begin with =) should start at the same x-offset. –  Albert Mar 16 '11 at 14:49
@Albert: Better now? –  Hendrik Vogt Mar 16 '11 at 14:56
@Hendrik: Ah thanks, that is perfect. That answers also the other question. It seems that aligned is actually what I want there instead of align because aligned doesn't seem to center it. –  Albert Mar 16 '11 at 15:06