LaTex, alignment of alignment characters between alignment blocks

Question

I would like to align two alignment characters between two alignment blocks so that I can have some text in the middle of the output with equations that support horizontal alignment. For example, the following latex excerpt using align

\begin{align*} \frac{\delta \phi}{\delta x_1} = {} &\frac{9}{8}\frac{\delta_1\phi}{\delta_1x_1}-\frac{1}{8}\frac{\delta_3\phi}{\delta_3x_1} \\ & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right] \end{align*} some text in the middle \begin{align*} & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right] \end{align*} 

Ideally, I would like the left side of the equation in the second block to coincide with the equation of the second equation in the first block. I could make a workaround without having the text in the middle, however I would like this functionality.

EDIT

I would like to have a good text. Say three to four lines that line up like regular paragraphs. Adding text to the alignment block is a workaround that I mentioned poorly.

Answer 1

Use \noalign :

 \begin{align*} \frac{\delta \phi}{\delta x_1} = {} &\frac{9}{8}\frac{\delta_1\phi}{\delta_1x_1}-\frac{1}{8}\frac{\delta_3\phi}{\delta_3x_1} \\ & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right]\\ \noalign{\noindent some text in the middle.} & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right] \end{align*} 

Answer 2

There is a very convenient command:

 \begin{align*} \int_0^1 x^2 &= \frac{1}{3} \\ \intertext{I am the intertext. I am typesetted as normal text but dude, the tabbing/alignment is carried over to the next slide. Check it out!} \int_0^1 x^3 &= \frac{1}{4} \end{align*} 

This is described in ams userguide, as far as I remember. But then I discovered it recently.

Answer 3

Try the following:

 \begin{align*} \frac{\delta \phi}{\delta x_1} = {} &\frac{9}{8}\frac{\delta_1\phi}{\delta_1x_1}-\frac{1}{8}\frac{\delta_3\phi}{\delta_3x_1} \\ & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right]\\ & \mbox{some text in the middle} \\ & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right] \end{align*} 

If you have a lot of text, you probably want to use \phantom :

 \begin{align*} \frac{\delta \phi}{\delta x_1} = {} &\frac{9}{8}\frac{\delta_1\phi}{\delta_1x_1}-\frac{1}{8}\frac{\delta_3\phi}{\delta_3x_1} \\ & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right] \end{align*} Some text in the middle, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, \begin{align*} \phantom{\frac{\delta \phi}{\delta x_1} = {}} & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right] \end{align*} 

If you don't want to use \phantom , the only solution I can come up with is the following:

 \usepackage{multirow} .... \begin{document} \[ \begin{array}{rl} \frac{\delta \phi}{\delta x_1} = {} &\frac{9}{8}\frac{\delta_1\phi}{\delta_1x_1}-\frac{1}{8}\frac{\delta_3\phi}{\delta_3x_1} \\ & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right]\\[5mm] \multicolumn{2}{l}{\parbox{\linewidth}{Some text in the middle, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text, more text}}\\[1cm] & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right] \end{array} \] \end{document} 

I don’t think there is a direct way to “bind” the width of one column of one table / eqnarray / align to another.

Answer 4

If you don't need a lot of text, you can use the \text command from the math environment to display the text inside the same align* environment.

 \begin{align*} \frac{\delta \phi}{\delta x_1} = {} &\frac{9}{8}\frac{\delta_1\phi}{\delta_1x_1}-\frac{1}{8}\frac{\delta_3\phi}{\delta_3x_1} \\ & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right]\\ \text{some text in the middle.}\\ & \frac{9}{8}\frac{1}{h_1}\left[\phi(x_1+h_1/2)-\phi(x_i-h_1/2)\right]-\frac{1}{8}\frac{1}{3h_1}\left[\phi(x_i+3h_1/2)-\phi(x_1-3h_1/2)\right] \end{align*}