Multiple alignments

Question

If I have a string of equations whose equal signs are aligned and one of the equations is too long and needs to span multiple lines, how do I create a second alignment. For instance, in the following example

\begin{align*}  
  f(x)&=eqn1\\  
      &=eqn2_part1+eqn2_part2\\   
      &           +eqn2_part3\\  
      &=eqn3\\   
      &=eqn4\\  
\end{align*}

How do I align the equals signs and the plus signs.

Answer 1

Using boxes via \phantom is also an option, if alignment characters like && get in the way for long equations:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align*}  
  f(x)&=\text{eqn}_1\\  
      &=\text{eqn}_2\text{-part}_1+\text{eqn}_2\text{-part}_2\\   
      &\phantom{{}=\text{eqn}_2\text{-part}_1}{}+\text{eqn}_2\text{-part}_3\\  
      &=\text{eqn}_3\\   
      &=\text{eqn}_4\\  
\end{align*}
\end{document}

A correction for the spacing around binary operators ({}+) and relations ({}=) is required though. See Herbert's mathmode document for more on AMS environments and alignment.

Answer 2

You could use alignat which is also from the amsmath package.

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{alignat*}{2}
  f(x)&=eqn1                    &&   \\  
      &=eqn2_part1              && +eqn2_part2 \\   
      &                         && +eqn2_part3\\  
      &=eqn3                    &&   \\   
      &=eqn4                    &&   \\  
\end{alignat*}

\end{document}

You'll notice that the alignat environment takes an argument- which is (quoting from the documentation)

the number of “equation columns”: count the maximum number of &s in any row, add 1 and divide by 2.

Answer 3

Please try breqn package. Then use \begin{dmath}...\end{dmath} environment. For example

\documentclass[12pt]{article} %
\usepackage{breqn}
\begin{document}
\begin{dmath}
        g\left[Pr(unrest_{im}=1)\right]=\alpha_{0i}
            +\alpha_{1}.\left|price\_shock_{im}\right|
            +\beta.{year}+\gamma.region.\left|price\_shock_{im}\right|
            +\delta.{region}+\epsilon_{im}
\end{dmath}
\end{document}