通过 `align` 进行水平对齐，通过 `equation` 进行编号

Question

我想让aligned环境的水平对齐方式与 相匹配align。我喜欢将一组属于同一类的方程式分组并对齐，并且我喜欢equation+aligned给出一个垂直居中的方程式编号。但是， 的水平间距align更好。在下面的特定示例中，我当然可以使用两个 ，\notag但如果有八行方程式怎么办？有没有一种简单的方法可以实现两全其美？

\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}

Answer 1

注意，alignedat您可以完全控制对齐列之间的间距。以下是三种可能性：

\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}