getting the equation numbering right using align and a long equation that splits

Question

So I've got a system of equations. One equation is very long. I want them all aligned, but I want the long equation to split over two lines.

\begin{align}
   \theta &= \theta(t-1) -p \theta(t-1) +p
             \frac{\hat{\psi}'(\theta(t-1))}{\ave{K}} +p \phi_R(0)\nonumber\\
          &\qquad + (1-p)(1-\theta(t-1))\\
   R   &= R(t-1)+I(t-1) 
\end{align}

As it stands, the equation for \theta spans two lines and is numbered on the second line. I'd like the number to be centered (rather like split appears to do). I haven't found a good way to do this.

Answer 1

split ought to work inside align, but it has some problems; a trick solves the issue:

\documentclass{article}
\usepackage{amsmath,mathtools}

\newcommand{\ave}[1]{\langle #1\rangle}% <--- guess

\begin{document}

\begin{align}
\begin{split}
\mathllap{\theta}
  &= \theta(t-1) -p \theta(t-1) +p
     \frac{\hat{\psi}'(\theta(t-1))}{\ave{K}} +p \phi_R(0)\\
  &\qquad + (1-p)(1-\theta(t-1))\\
\end{split}
\\[1.5ex]
R &= R(t-1)+I(t-1)
\end{align}

\end{document}

Some space is needed to make clearer what the numbers refer to, in my opinion.

Answer 2

The trick is to use the aligned environment, which acts as a single line for the purposes of equation numbering, alignment inside other environments, etc. You'll also want to remove the \nonumber:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{aligned}
   \theta &= \theta(t-1) -p \theta(t-1) +p
             \frac{\hat{\psi}'(\theta(t-1))}{K} +p \phi_R(0)\\
          &\qquad + (1-p)(1-\theta(t-1))\\
   R   &= R(t-1)+I(t-1) 
\end{aligned}
\end{equation}
\end{document}

Answer 3

it's possible to use aligned within align, as follows:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
 \theta & =
\begin{aligned}[t]
   & \! \theta(t-1) -p \theta(t-1) +p
             \frac{\hat{\psi}'(\theta(t-1))}{K} +p \phi_R(0)\\
          &\qquad + (1-p)(1-\theta(t-1))\\
\end{aligned}\\
   R   &= R(t-1)+I(t-1) 
\end{align}
\end{document}

two things to note here:

• to get the top line of the aligned block even with the left side, it's necessary to specify [t].

• for reasons lost in the mists of time, an aligned block starts with \,, and this must be reversed with the negative equivalent, \!. (i didn't bother doing that on the second line, since it would probably not be obvious in the presence of the \qquad.)