Math & Science ⇒ Alignment problem with split

[jjhall]

Hello, I am having a problem getting an equation aligned properly using align and split. The equation doesn't fit on one line, so after much searching I found that using the split environment is the way to go. However, the left hand side of the equation is now lining up between the two lines on the right. Also, the first segment and last segment of the right hand side are not aligned vertically. I am pretty sure I am doing this incorrectly, since I have not used split before.

Code: Select all

\begin{align}
x_{i,j}  & = \begin{split} \left\{\frac{a}{(\Delta\xi)^2}\left(x_{i+1,j}+
  x_{i-1,j}\right) + \frac{c}{(\Delta\eta)^2}
  \left(x_{i,j+1}+x_{i,j-1}\right){} \right.\\ 
  \left. {}- \frac{b}{2\Delta\xi\Delta\eta}\left(x_{i+1,j+1}-x_{i+1,j-1}
  -x_{i-1,j+1}+x_{i-1,j-1}\right) \right\}/2\left[\frac{a}{(\Delta\xi)^2}+
    \frac{c}{(\Delta\eta)^2}\right] \end{split}
\end{align}

FYI, I am not exactly a newbie, but I am also far from being a LaTeX expert.

Thanks,
Jeremiah

[localghost]

You can put the alignment character (&) directly into the split environment to get the split lines aligned to the equal sign.

Code: Select all

\begin{align}
  x_{i,j}=
  \begin{split}
    &\left\{\frac{a}{(\Delta\xi)^2}\left(x_{i+1,j}+x_{i-1,j}\right)+\frac{c}{(\Delta\eta)^2}\left(x_{i,j+1}+x_{i,j-1}\right){} \right.\\
    &\left.-\frac{b}{2\Delta\xi\Delta\eta}\left(x_{i+1,j+1}-x_{i+1,j-1}-x_{i-1,j+1}+x_{i-1,j-1}\right)\right\}/2\left[\frac{a}{(\Delta\xi)^2}+\frac{c}{(\Delta\eta)^2}\right]
  \end{split}
\end{align}

Best regards and welcome to the board
Thorsten¹

[jjhall]

Thanks! That worked. Now how do I get the left hand side to line up with the top row of the right hand side?

Jeremiah

[localghost]

[…] Now how do I get the left hand side to line up with the top row of the right hand side? […]

At first I was a little bit astonished about your answer because it worked as desired when I copied the environment into an existing document. So I built a minimal working example (MWE) and got the same output as you with an alignment of the left side to the middle of the right side. Then I remembered that I have to include amsmath with an option.

Code: Select all

\documentclass[11pt,a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}
\usepackage[margin=2cm]{geometry}
\usepackage[tbtags]{amsmath}

\begin{document}
  \begin{align}
    x_{i,j}=
    \begin{split}
      &\left\{\frac{a}{(\Delta\xi)^2}\left(x_{i+1,j}+x_{i-1,j}\right)+\frac{c}{(\Delta\eta)^2}\left(x_{i,j+1}+x_{i,j-1}\right){} \right.\\
      &\left.-\frac{b}{2\Delta\xi\Delta\eta}\left(x_{i+1,j+1}-x_{i+1,j-1}-x_{i-1,j+1}+x_{i-1,j-1}\right)\right\}/2\left[\frac{a}{(\Delta\xi)^2}+\frac{c}{(\Delta\eta)^2}\right]
    \end{split}
  \end{align}
\end{document}

This makes it look like desired. See the final output in the attachment.

[phi]

The LHS should be inside the split environment:

Code: Select all

  \begin{align}
    \begin{split}
      x_{i,j}=&\left\{\frac{a}{(\Delta\xi)^2}\left(x_{i+1,j}+x_{i-1,j}\right)+\frac{c}{(\Delta\eta)^2}\left(x_{i,j+1}+x_{i,j-1}\right){} \right.\\
      &\left.-\frac{b}{2\Delta\xi\Delta\eta}\left(x_{i+1,j+1}-x_{i+1,j-1}-x_{i-1,j+1}+x_{i-1,j-1}\right)\right\}/2\left[\frac{a}{(\Delta\xi)^2}+\frac{c}{(\Delta\eta)^2}\right]
    \end{split}
  \end{align}

And if you have a single equation with multiple lines, you should use an equation environment rather than an align environment.