Align $:=$ with $=$ in align/split mode

Question

I am writing a sequence of inequalities in math mode, where the first line defines the first object. To be clear, I have something like

\documentclass[12pt]{amsart} 
\usepackage{amsfonts,graphics,amsmath,amsthm}
\usepackage{amsfonts,amscd, amssymb,amsmath,latexsym}
\usepackage{mathtools}

\begin{document}
\begin{equation}
\begin{split}
A &:= B - C \\
&= U + V \\
&\geq Z.
\end{split}
\end{equation}
\end{document}

I would like the = from the := in the first line to be aligned with the = in the second line. If I use the code above, it does not happen. Similarly, if I use \coloneqq in place of :=, the same misplacement happen. On the other hand, if I write :&=, I get the alignment I want; yet, in this way the column gets too far from the equal sign and it renders in an unpleasant way.

How can I get the alignment I want without having : too far from =?

Do you mean that you want to have the = in := aligned with the = below ? — marsupilam, May 30 '17 at 17:43
@Manuel But then you have to add the \phantom to all subsequent lines, don't you ? Maybe it is more simple to have : A:&= B-C and then leave all further lines unchanged ? An even better solution probably involves the mathtools command \coloneqq — marsupilam, May 30 '17 at 17:47
@marsupilam Or define \colonalign and use that every instance you need. Or may be :\joinrel\joinrel&= works if you put it just in the first line? (Can't test right now.) — Manuel, May 30 '17 at 17:53

score 10 · Accepted Answer · answered May 30 '17 at 18:17

\documentclass[12pt]{amsart} 
\usepackage{amsfonts,graphics,amsmath,amsthm}
\usepackage{amsfonts,amscd, amssymb,amsmath,latexsym}
\usepackage{mathtools}

\begin{document}
\begin{equation}
\begin{split}
A :={}& B - C \\
={}& U + V \\
\geq{}& Z.
\end{split}
\end{equation}
\end{document}

Steven B. Segletes · Answer 2 · 2017-05-30T18:50:00.293

With a TABstack, one does not need to add the blank groups that the split version needs, because TABstacks automatically, by default, add a group to the right end of each cell. The blank group can be defaulted to the left of each cell with the declaration \TABbinaryLeft (also \TABunaryRight), or blank groups can be added to both ends of each cell with \TABbinary. The original default condition can be restored with \TABbinaryRight (also \TABunaryLeft).

\documentclass[12pt]{amsart} 
\usepackage{amsfonts,graphics,amsmath,amsthm}
\usepackage{amsfonts,amscd, amssymb,amsmath,latexsym}
\usepackage{mathtools}
\usepackage{tabstackengine}
\TABstackMath
\begin{document}
\begin{equation}
\tabbedCenterstack[r]{
A \coloneqq& B - C \\
=& U + V \\
\geq& Z.
}
\end{equation}
\end{document}

Note that I preferably use \coloneqq instead of :=, since \coloneqq comes ready made with vertical symmetry, whereas := does not.

score 3 · Answer 3 · answered May 30 '17 at 18:40

This produces perfect alignment, and only affects the first line.

as shown by replacing the first \\ by \\[-.9\baselineskip].

However, it might be a bit silly...

The output

with `\\[-.9\baselineskip]`

without

The code

\documentclass[12pt]{standalone} 
\usepackage{mathtools}
\newlength\myUselessLength
\begin{document}
\settowidth{\myUselessLength}{${}={}$}
$
\begin{aligned}
  A\coloneqq{}\hspace{-\myUselessLength} & \phantom{{}={}} B - C \\
                                         & = U + V               \\
                                         & \geq Z.               \\
\end{aligned}
$
\end{document}

If someone knows how not to have to use a useless length for doing such a thing, I'm all ears ! — marsupilam, May 30 '17 at 18:46

score 2 · Answer 4 · answered May 30 '17 at 18:54

Another way, same output.

\documentclass[12pt]{article} 
\usepackage{mathtools}
\begin{document}
\begin{equation}
  \begin{split}
    & \phantom{{}={}} \mathllap{A\coloneqq{}} B - C \\
    & = U + V                                       \\
    & \geq Z.                                       \\
  \end{split}
\end{equation}
\end{document}

Align $:=$ with $=$ in align/split mode

4 Answers4

The output

with \\[-.9\baselineskip]

without

The code

with `\\[-.9\baselineskip]`