如何对数学模式方程进行编号？

Question

是否可以对在数学模式下编写的方程式进行编号？我尝试在数学模式下使用“\begin{equation}”，但会出现错误。

\documentclass{article}
\usepackage[french,USenglish]{babel}
\usepackage[mmddyyyy]{datetime}
\usepackage{amsmath,amssymb}
\begin{document}

    I can number the equations using ``begin{equation}''.
    \begin{equation}
        A = \frac{\pi r^2}{2} 
    \end{equation}

    But, how to number the equations in written using math mode?
    \[\mathrm{ \frac {P \models Q\stackrel{Y} {\rightleftharpoons} P \mbox{, } P \lhd \langle X \rangle _Y} {P \models Q \rhd X} } \]

    \[\mathrm{ \frac {P \models Q, P \lhd \langle X \rangle _Y} {Q \rhd X} } \]
    
\end{document}

Answer 1

我认为（但我可能错了）gather环境可能就是您要找的东西。请注意，显示样式的数学在环境内有效gather。

\documentclass{article}
\usepackage[french,USenglish]{babel}
\usepackage[mmddyyyy]{datetime}
\usepackage{amsmath,amssymb}
\begin{document}

I can number a single-line equation using an \texttt{equation} environment.
\begin{equation}
   A = \frac{\pi r^2}{2} 
\end{equation}

I can number multi-line mathmode material with a \texttt{gather} environment.
\begin{gather}
    \mathrm{ \frac {P \models Q\stackrel{Y} \rightleftharpoons P ,\ 
       P \mathrel\lhd \langle X \rangle _Y} {P \models Q 
       \mathrel\rhd X} } \\[1ex] % "1ex" inserts a bit more vertical whitespace
    \mathrm{ \frac {P \models Q,\ 
       P \mathrel\lhd \langle X \rangle _Y} {Q \mathrel\rhd X} } 
\end{gather}
\end{document}