align environment according to *first* character, rather than last

Question

\begin{align*}
    abcd &= abcd &(abcdefghijk)\\
         &= ab   &(ab)
\end{align*}

After the first ampersand the equal signs are aligned. However, after the second ampersand, the closing parentheses ")" are aligned. I'd like the opening parenthesis "(" to be aligned instead. Is there an optional argument like that for align? Am I using the wrong environment? Should I define a new one instead?

Incidentally, how can I avoid having that much white space in between these two?

Answer 1

1. align:

The align environment provides pairs or rl aligned equations. So, in normal usage the first & make the content on the left, be right aligned and the content on the right be left aligned. So, you need to insert an additional & to make the second left aligned:

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2. alignat:

If you want to control the spacing between them, I would recommend you use the alignat environment. Similar, to align, it also provides pairs of rl aligned equations, but unlike align the alignat does not insert additional horizontal spacing in between:

In the above, I inserted a \quad space, without which it looks like:

References:

• An excellent reference for math mode is Herbert Voss' comprehensive review of mathematics in (La)TeX.

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Code:

\documentclass{article}
\usepackage{amsmath}


\begin{document}
\begin{align*}
    abcd &= abcd &&(abcdefghijk)\\
         &= ab   &&(ab)
\end{align*}

\begin{alignat*}{2}
    abcd &= abcd &\quad &(abcdefghijk)\\
         &= ab   &\quad &(ab)
\end{alignat*}

\begin{alignat*}{2}
    abcd &= abcd &&(abcdefghijk)\\
         &= ab   &&(ab)
\end{alignat*}
\end{document}