将方程式包裹在列表周围

Question

是否可以将单个居中方程 # 应用于列表？我在第一个列表中使用对齐包裹方程，但 1) 它需要相当多的手动间距才能使其看起来正确，并且 2) 长项目行为不当。项目 (iii) 被注释掉，因为它破坏了格式，我似乎无法控制它。第二个列表中的枚举格式完美，但缺少方程编号。我很感激任何建议。

\documentclass[12pt, leqno]{book}
\usepackage{mathtools,amsthm} 
\usepackage[shortlabels]{enumitem}
\newcommand{\blank}[1]{\hspace*{#1}}  
\newcommand\abs[1]{\lvert#1\rvert}
\newcommand\dvd[2]{#1\!\mid\!#2}

\begin{document}
\noindent Using the equation env, this list is numbered and does what I want, except I'm struggling with
how to treat long lines (as in item (iii) which is commented out).
\begin{equation}
   \begin{aligned}
      &\text{(i) }   \enspace \text{If $a>b$ then $a+c > b+c$ for every $c \in D$.} \blank{15mm}  \\
      &\text{(ii) }  \>       \text{If $a>b$ and $c>0$, then $ac>bc$.}  \\ 
 %      &\text{(iii) }          \text{If If $a \neq 0$ and $\dvd{d}{a}$, then $\abs{d} \leq \abs{a}$. Moreover, if $a \neq 0$, and $d \neq \pm a$, then $\abs{d} < \abs{a}$. } \\
      &\text{(iv) }           \text{If $a>b$ and $b>c$, then $a>c$.}    \\ 
      &\text{(v) }   \>       \text{If $a \neq 0$, then $a^2 > 0$.}     \\ 
   \end{aligned}
\end{equation}

\noindent Using the enumerate env handles the items perfectly with the desired formatting. I just
don't get a centered equation number.

\begin{enumerate}[labelindent=\parindent, leftmargin=*, label=(\roman*),widest=viii, align=left, noitemsep]
   \item If $a>b$ then $a+c > b+c$ for every $c \in D$.
   \item If $a>b$ and $c>0$, then $ac>bc$.
   \item If If $a \neq 0$ and $\dvd{d}{a}$, then $\abs{d} \leq \abs{a}$. Moreover, if $a \neq 0$, and $d \neq \pm a$, then $\abs{d} < \abs{a}$.
   \item If $a>b$ and $b>c$, then $a>c$.
   \item If $a \neq 0$, then $a^2 > 0$.
\end{enumerate}
\end{document}

Answer 1

只需将枚举环境放在 中\parbox。顺便说一句，我认为这里的项目编号右对齐时看起来更好。我还借此机会 \abs用 `mathtools 重新定义（因此它可以具有可变大小）：

\documentclass[12pt, leqno]{book}
\usepackage{mathtools,amsthm}
\usepackage[showframe]{geometry}
\usepackage[shortlabels]{enumitem}
\newcommand{\blank}[1]{\hspace*{#1}}
\DeclarePairedDelimiter\abs\lvert\rvert
%\newcommand\abs[1]{\lvert#1\rvert}
\newcommand\dvd[2]{#1\!\mid\!#2}

\begin{document}

\noindent Using the enumerate env handles the items perfectly with the desired formatting. I just get a centred equation number.
\begin{equation}
  \parbox{0.9\linewidth}{\begin{enumerate}[labelindent=\parindent, leftmargin=*, label=(\roman*),widest=viii, labelsep=0.8em, noitemsep]
    \item If $a>b$ then $a+c > b+c$ for every $c \in D$.
    \item If $a>b$ and $c>0$, then $ac>bc$.
    \item If If $a \neq 0$ and $\dvd{d}{a}$, then $\abs{d} \leq \abs{a}$. Moreover, if $a \neq 0$, and $d \neq \pm a$, then $\abs{d} < \abs{a}$.
    \item If $a>b$ and $b>c$, then $a>c$.
    \item If $a \neq 0$, then $a^2 > 0$.
    \end{enumerate}}
\end{equation}

\end{document}