Hanging brackets in an align environment

Question

I have two formulas in an align environment, and the second one starts with a bracket (this particular example is simplified):

\begin{align}
& x_\alpha(s) x_\alpha(t) \\
& \left[ x_\alpha(s),  x_\beta(t) \right]
\end{align}

The result is terrible, apparently because of the spacing before the bracket:

Since I can't put an alignment point between the brackets, I tried to put a phantom of a bracket in the beginning of the first line as follows:

\begin{align}
& \phantom{\left[{}\vphantom{x_\beta(t)}\right.}
  x_\alpha(s) x_\alpha(t) \\
& \left[ x_\alpha(s),  x_\beta(t)\right]
\end{align}

The result is better, but still not perfect (my trick did not work as I expected):

So how can I achieve the perfect alignment?

Update. By the "perfect alignment" I mean that the upper x stands straight above the lower one:

Consider totally dropping the left right constructions and use the manual scallers, then you can align inside. — daleif, Aug 01 '15 at 10:41

AboAmmar · Answer 1 · 2015-08-01T11:25:47.113

5

\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}

\begin{align}
& \left. x_\alpha(s) x_\alpha(t)\right . \\
& \left[ x_\alpha(s),  x_\beta(t)\right]
\end{align}

\end{document}

Just putting a virtual bracket (\left.) will do the trick, because \left. and \left[ should have the same spacing.

If you like the following alignment,

then, you can use the following (also pointed out by @daleif):

\begin{align}
& x_\alpha(s) x_\alpha(t) \\
& \bigl[ x_\alpha(s),x_\beta(t)\bigr]
\end{align}

Update

Aligning at the x's as per the OP's request:

\begin{align}
       & x_\alpha(s) x_\alpha(t) \\
\bigl[ & x_\alpha(s),x_\beta(t) \bigr]
\end{align}

Remark

Mathematicians often prefer the outer fences being just as tall as the tallest inner object excluding the exponents. As such, I personally prefer dropping all \lefts or \rights and just putting only the brackets [ (assuming no tall objects exist) like this:

\begin{align}
 & x_\alpha(s) x_\alpha(t) \\
[& x_\alpha(s),x_\beta(t)]
\end{align}

edited Aug 01 '15 at 11:25

answered Aug 01 '15 at 10:26

AboAmmar

46,352
4
58
127

That is not what I meant. I made an edit to the question. – Andrei Smolensky Aug 01 '15 at 10:55
@AndreiSmolensky Please see my update! – AboAmmar Aug 01 '15 at 11:03
The \bigl.\bigr. bit in the first equation and the \bigr. and \bigl. parts in the second aren't needed. – Mico Aug 01 '15 at 11:14
@Mico Right, just forgot them from the previous \left[ and \right]. I edited my answer, thanks. – AboAmmar Aug 01 '15 at 11:17

score 1 · Answer 2 · answered Aug 01 '15 at 11:09

1

One more solution:

\begin{align}
      x_\alpha(s)       & x_\alpha(t) \\
\left[x_\alpha(s)\right.&,\left.x_\beta(t) \right]
\end{align}

answered Aug 01 '15 at 11:09

Zarko

296,517

the "x"s aren't aligned, and worse, the spaces around the comma look wider than they should. – barbara beeton Aug 01 '15 at 13:36
@barbarabeeton, you have right. – Zarko Aug 01 '15 at 15:09

score 1 · Answer 3 · edited Apr 13 '17 at 12:35

1

I have found a way to do this while not throwing away the \left[ command, thank to this answer. It goes like this (perhaps my code is too complicated and can be simplified):

\begin{align}
& \hphantom{ \left[ \vphantom{\dfrac{ABCD}{EFGH}} \right.\kern\nulldelimiterspace }
  x_\alpha(s) x_\alpha(t) \\
& \left[ x_\alpha(s), \dfrac{ABCD}{EFGH}\right]
\end{align}

Although looking promising, the mathopen and mathclose commands suggested by the other answers to the question in the link above do not solve the problem.

edited Apr 13 '17 at 12:35

Community

1

answered Aug 01 '15 at 12:09

Andrei Smolensky

681
4
11

If the other answers "do not solve the problem", then maybe the problem wasn't stated clearly enough? – Mico Aug 01 '15 at 12:21

Hanging brackets in an align environment

3 Answers3