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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}

1 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}

enter image description here 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}

enter image description here

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.

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4 Answers 4

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227

Use aligned instead.

\documentclass{article} \usepackage{amsmath} \begin{document} \begin{equation} \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} \end{equation} \end{document}

enter image description here

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  • @TH. I have a question: Is it possible to place text between these two equations? Commented Sep 11, 2014 at 8:34
  • @TH. @Paul Koer You need \\ after dW_t, otherwise both equations will be concatenated in the same line Commented Mar 17, 2017 at 22:15
  • 1
    Environments equation and align serve similar kinds of purposes. Whereas the purpose of split or alignedis quite distinct: i.e. they typically fit inside equation, align*, etc. Commented Jul 12, 2017 at 5:14
69

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}\\ &\quad +[(1-R)+R(1-Y)\\ &\quad +\pi(1-\delta)]E_2\\ &\quad +F\cdot E_3 \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:

enter image description here

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43

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.

enter image description here

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  • It would seem better to wrap the whole thing in a larger equation, and separated the aligned by using a \quad or two. Commented Nov 13, 2019 at 18:33
  • I disagree; explicit positioning is exactly what we use aligned to avoid. If you want greater control (and I don't think this is visually critical enough to warrant designing your document code around this goal) you can play with the column spacing. Commented Nov 15, 2019 at 18:30
  • One problem is that when the equations on different columns have different heights, the horizontal alignment gets screwed. Commented Feb 11, 2020 at 14:09
10

Using 

\begin{equation} \begin{split} 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{split} \end{equation}

will produce one equation number that is vertically centered on the group of equations.

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