Behaviour of the align environment

Question

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?

Answer 1

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} \\
          & \qquad + \frac{dm}{dt} \vec{v}
\end{align}

\end{document}

Note: It is advisable NOT to use eqnarray.

Answer 2

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.

Answer 3

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