# Horizontal alignment from align with numbering from equation

I would like to make the horizontal alignment of the aligned environment match that of align. I like to group and align a set of equations that belong together, and I like that equation+aligned gives my one vertically centered equation number. However, the horizontal spacing of align is better. In the particular example below, I could of course use two \notag's but what if there were eight lines of equations? Is there a simple way to achieve the best of both worlds?

\documentclass{article}

\usepackage{lmodern}
\usepackage{mathtools}

\begin{document}
\noindent This display has nice alignment:
\begin{align}
E(y) &= \mu_y, & V(y) &= \sigma_y^2,\\
E(x) &=\mu_x,  & C(x) &= \Sigma_x,\\
E(\epsilon) &= 0 \quad\text{and} & V(\epsilon) &= \sigma_\epsilon^2.
\end{align}
This display has a single number:
\begin{equation}
\begin{aligned}
E(y) &= \mu_y, & V(y) &= \sigma_y^2,\\
E(x) &=\mu_x,  & C(x) &= \Sigma_x,\\
E(\epsilon) &= 0 \quad\text{and} & V(\epsilon) &= \sigma_\epsilon^2.
\end{aligned}
\end{equation}
\end{document} • The internal forms of the alignment environments do not have the generous horizontal spacing of the normal forms. Just put a \qquad at the beginning of the second column. – campa May 23 '18 at 7:59
• @campa Actually, \qquad\qquad\qquad\quad is what gets me there in this example. :) – Mankka May 23 '18 at 8:02
• \hspace{7em} is easier. But you got the idea. – campa May 23 '18 at 8:02

Note alignedat gives you full control on the spacing between alignment columns. Here are three possibilities:

\documentclass{article}

\usepackage{lmodern}
\usepackage{mathtools}

\begin{document}

\noindent This display has nice alignment:
\begin{align}
E(y) &= \mu_y, & V(y) &= \sigma_y^2,\\
E(x) &=\mu_x, & C(x) &= \Sigma_x,\\
E(\epsilon) &= 0 \quad\text{and} & V(\epsilon) &= \sigma_\epsilon^2.
\end{align}
These displays have a single number (each):
\begin{equation}
\begin{alignedat}{2}
E(y) &= \mu_y, & V(y) &= \sigma_y^2,\\
E(x) &=\mu_x, & C(x) &= \Sigma_x,\\
E(\epsilon) &= 0 \quad\text{and} &\hspace{8em} V(\epsilon) &= \sigma_\epsilon^2.
\end{alignedat}
\end{equation}
%
\begin{equation}
\begin{aligned}
E(y) &= \mu_y, & V(y) &= \sigma_y^2,\\
E(x) &=\mu_x, & C(x) &= \Sigma_x,\\
E(\epsilon) &= 0 & \makebox[8em]{and\quad} V(\epsilon) &= \sigma_\epsilon^2.
\end{aligned}
\end{equation}
%
\begin{equation}
\begin{alignedat}{3}
E(y) &= \mu_y, & & & V(y) &= \sigma_y^2,\\
E(x) &=\mu_x, & & & C(x) &= \Sigma_x,\\
E(\epsilon) &= 0 & \makebox[9em]{and} & & V(\epsilon) &= \sigma_\epsilon^2.
\end{alignedat}
\end{equation}

\end{document} • This is very similar to what @campa suggested in a comment to the question. If I typeset both align and equation+aligned I can manually tune the hspace to make them match. This is exactly what I don't want to do. – Mankka May 23 '18 at 11:59
• I'm afraid the only choice is between a manual space adjustment and a manual deleting of unwanted equation tags. – Bernard May 23 '18 at 13:33