[Tex/LaTex] How to get only one vertically centered equation number in align environment with two equations

Question

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.

Answer 1

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}