# How to get only one vertically centered equation number in align environment with two equations

What I have are two equations with align and I want one single equation number to refer to this system of equations. To illustrate, I have

\begin{align}
dr_t &= \kappa ( \theta - r_t ) dt + \sigma_r r^{\xi} dW_t \\
dA_t &= \mu A_t dt + \sigma_A^{\alpha} dZ_t,
\end{align}


and I want neither

\begin{align}
dr_t &= \kappa ( \theta - r_t ) dt + \sigma_r r^{\xi} dW_t \nonumber \\
dA_t &= \mu A_t dt + \sigma_A^{\alpha} dZ_t,
\end{align}


nor

\begin{align}
dr_t &= \kappa ( \theta - r_t ) dt + \sigma_r r^{\xi} dW_t \\
dA_t &= \mu A_t dt + \sigma_A^{\alpha} dZ_t, \nonumber
\end{align}


but having a single number in the middle. I know this can be achieved by nesting an array inside of an equation but then some symbols (sums, etc.) look differently and I thought there had to be a better way of doing this.

-

Use aligned instead.

\begin{aligned} dr_t &= \kappa ( \theta - r_t ) dt + \sigma_r r^{\xi} dW_t \\ dA_t &= \mu A_t dt + \sigma_A^{\alpha} dZ_t, \end{aligned}


-
@Paul: That seems wrong. Hmm. – TH. Mar 14 '11 at 15:11
Oh, by the way, I know this is OT but could you tell me how to get the equations ''compiled'' like you did? thank you – Paul Koer Mar 14 '11 at 15:13
@Paul you should now have enough reputation to upvote TH's answer. – Seamus Mar 14 '11 at 15:14
@Martin: I think it was rejected because most of the TeX code here is designed to display as TeX, not as graphics. – TH. Mar 14 '11 at 17:34
also with the complicated LaTeX constructions sometimes shown here, it may be difficualt to secure a LaTeX to image renderer. Depending on the system it might e.g. be easy to fill a disk/partition or make LaTeX run in an infinite loop. Thus I thinkthe current situation is the best compromise. – daleif Mar 22 '11 at 23:05

Here's a slightly better way to do this (IMO):

Using the split environment within an align will produce one equation number, vertically centered within the split (so long as there's not a page break involved, as discussed here).

Note that you can have individual labels for each equation by adding \label{} to the end of each line before the line break \\

For example:

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}

\begin{align}
E_1&=A+B \label{eq:1}\\
\begin{split}
E_2&=(C-D)E_1 \label{eq:2}\\
\end{split}\\
E_3 &=(\pi\cdot \chi)-(R\cdot E_1)-(RY\delta\cdot E_2) \label{eq:3}
\end{align}

\end{document}


which gives:

-

This is really a comment on TH.'s accepted answer (which should stay that way). Using just one aligned doesn't work if you have multiple columns: they are separated by only a small space, since aligned "shrinks to fit". The workaround is to use one aligned per column, and wrap the whole thing in a larger align, like so:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
\begin{aligned}
x &= y \\       f(x) &= f(y)
\end{aligned}
&&
\begin{aligned}
a &= b \\       g(a) &= g(b)
\end{aligned}
\end{align}
\end{document}


This should produce two pairs of equations, each aligned at the equals sign, and with one equation number centered between the lines.

-