# Behaviour of the align environment

I'm writing a manuscript with long equations. In order to break the equations I prefer the align environment, provided by the amsmath package, over the eqnarray one.

In the below example I show the kind of align I need. The eqnarray gives the correct behaviour, will the align one drops a space after the & symbol.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{eqnarray}
\vec{F} &=& m\vec{a}\\
&& + \frac{dm}{dt} \vec{v}
\end{eqnarray}

\begin{align}
\vec{F} =& m\vec{a}\\
& + \frac{dm}{dt} \vec{v}
\end{align}

\end{document}


Question

Is it possible to replicate the first example using align?

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please note -- the answer, to put = *after the &, is stated clearly in the documentation: texdoc amsmath, near the top of p.3. (but, of course, nobody ever reads documentation ...) – barbara beeton Aug 20 '12 at 18:23
Why eqnarray shouldn't be used: Avoid eqnarray (PDF). – Torbjørn T. Aug 20 '12 at 18:47
@Barbara: has it ever been explained why one need the {} pair then one use ={}& ? – daleif Aug 20 '12 at 20:22
@barbarabeeton I've read it, however the = after the & gives a result I'm not looking for. – Dox Aug 20 '12 at 20:35
@TorbjørnT. Wonderful link, Thx. – Dox Aug 20 '12 at 20:36

okay, this "answer" stretches the bounds of what is proper here, but continued recommendations to place the & in an amsmath alignment after the sign of relation are simply wrong.

the proper placement of the & is before the sign of relation. this is basic to tex itself.

as an exercise, here is the code for the first multi-line display on p.192 of the texbook as (plain) \eqalign with &= and then again with =& as the only differences in the two examples.

\eqalign{% (x+y)(x-y) &= x^2 -xy +yx -y^2\cr &= x^2 -y^2\cr (x+y)^2 &= x^2 +2xy +y^2\cr }
%
now, with ampersands and equal signs interchanged:
%
\eqalign{% (x+y)(x-y) =& x^2 -xy +yx -y^2\cr =& x^2 -y^2\cr (x+y)^2 =& x^2 +2xy +y^2\cr }

\bye


here is the output:

the spacing is governed by the values in the spacing table on p.170. the "null" condition represented by the = sign followed by & isn't in that table. however, the result is clear. amsmath simply followed this model, and depends on the default outcome.

-

In order for align to generate a similar result to eqnarray as in the example,

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{eqnarray}
\vec{F} &=& m\vec{a}\\
&& + \frac{dm}{dt} \vec{v}
\end{eqnarray}

\end{document}


a pair of curly brackets should be used between the = sign and the &, as below,

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
\vec{F} ={}& m\vec{a}\\
& + \frac{dm}{dt} \vec{v}
\end{align}

\end{document}


Acknowledgements

I'd like to thank to daleif and TorbjørnT for pointing out the answer, and for recommending the manuscript by Lars Madsen,

Avoid eqnarray!

The Practex Journal, 2006, No. 4

-

You should put = after &. I have added \phantom{=:} to push the+to right. (IMO it would be nice if we put this+ at the end of first line.)

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{eqnarray}
\vec{F} &=& m\vec{a}\\
&& + \frac{dm}{dt} \vec{v}
\end{eqnarray}

\begin{align}
\vec{F} &= m\vec{a} \\
& \phantom{=:} + \frac{dm}{dt} \vec{v}
\end{align}

\end{document}


Update As per the suggestion of @egreg (who is a mathematician hence I don't say no to what he says :) ) instead of \phantom{=:} one can use \qquad to push things to right.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{eqnarray}
\vec{F} &=& m\vec{a}\\
&& + \frac{dm}{dt} \vec{v}
\end{eqnarray}

\begin{align}
\vec{F} &= m\vec{a} \\

I'd use \qquad instead of \phantom – egreg Aug 20 '12 at 17:05
I usually use = {} & and just &+ instead. You might also find the \MoveEqLeft macro from mathtools useful. Instead of starting a line with &... and indenting the other lines, starting a line with \MoveEqLeft ...` move this line backwards (2ems). This makes it easy to to the equation setup recommended in Mathematics into Type (or see the drawings in my (danish) LaTeX book, chapter 4) – daleif Aug 20 '12 at 18:27