[Tex/LaTex] Combine split and align environment

Question

The split environment allows you to give one number to two equations, as in

\begin{equation}
\begin{split}
a&=1+2+3+4\\ 
b&=5+6+7+8
\end{split}
\end{equation}

which gives

Note that the equation numbre is nicely centered in the middle of the two equations.

The align environment allows you to put multiple equations ("tabs") in the same line, as in

\begin{align}
 a&=1+2+3 & b&=4+5+6\\
 c&=2+4+6 & d&=3+5+7
\end{align}

which gives

Is there a way to use the multi-tab functionality of align together with a single equation number like in split?

Answer 1

This is the aligned environment:

\begin{equation}
\begin{aligned}
 a&=1+2+3 & b&=4+5+6\\
 c&=2+4+6 & d&=3+5+7
\end{aligned}
\end{equation}

If you want to have control on the gap between groups of alignments, you can use the alignedat{n} environment, where n is the number of groups, requiring 2n-1 ampersands.

\begin{equation}
\begin{alignedat}{2}
 a&=1+2+3 &\hspace{1in}b&=4+5+6\\
 c&=2+4+6 & d&=3+5+7
\end{alignedat}
\end{equation}

As pointed by egreg, the tbtags option doesn't work in that case, so that one has either to use the [t] or [b] option of the aligned/alignedat environments, or to use an align/aligned environment and the \notag command at the relevant places:

\begin{equation}
\begin{alignedat}[b]{2}
 a&=1+2+3 &\hspace{1in} b&=4+5+6\\
 c&=2+4+6 & d&=3+5+7
\end{alignedat}
\end{equation}

\begin{alignat}{2}
 a&=1+2+3 &\hspace{1in} b&=4+5+6\notag\\
 c&=2+4+6 & d&=3+5+7
\end{alignat}