\label 和 \ref 在我编译两次后仍然不起作用

Question

这是我的简单文档

\textbf{Theorem 3. }
\it{Let $M_R$ be a $C3$-module. If $M=X\s Y$ and $f:X\to Y$ is a homomorphism with 
$\ker f \ds X$, then $\im f \ds Y$.}\label{theorem3}


An immediate consequence of Theorem \ref{theorem3} is the following. 

我尝试引用该定理，但是当我\ref第一次插入并编译时出现两个问号，而当我第二次编译时两个问号消失并且没有出现数字。

错误在哪里？有没有合适的方法来引用这样的文本？

Answer 1

您可以使用定理的标准界面，而不必为每个定理进行容易出错的手动格式化。

我定义mthm（米年度日核心米) 环境，即一个类似于标准定理的环境（称为mthminner），它设置了一个计数器，我们可以通过重新定义相关表示\themthminner来使用它。可选参数可用于将标题（默认“定理”）设置为“推论”、“引理”或其他。

\documentclass{article}
\usepackage{amsmath}
\usepackage{amsthm}

\newtheorem{mthminner}{\mthminnername}
\newcommand{\mthminnername}{}
\newenvironment{mthm}[2][Theorem]
 {%
  \renewcommand\themthminner{#2}%
  \renewcommand{\mthminnername}{#1}%
  \begin{mthminner}%
 }
 {\end{mthminner}}

\newcommand{\s}{\oplus}
\newcommand{\ds}{\leq^{\oplus}}
\DeclareMathOperator{\im}{im}

\begin{document}

\begin{mthm}{3}\label{theorem3}
Let $M_R$ be a $C3$-module. If $M=X\s Y$ and $f:X\to Y$ is a homomorphism with 
$\ker f \ds X$, then $\im f \ds Y$.
\end{mthm}

An immediate consequence of Theorem \ref{theorem3} is the following corollary.

\begin{mthm}[Corollary]{3.1}\label{corollary3.1}
Corollaries are easy.
\end{mthm}

\end{document}

针对多种语句类型的不同方法。

\documentclass{article}
\usepackage{amsmath}
\usepackage{amsthm}

\newcounter{mthm}
\newtheorem{mthmitalic}[mthm]{\mthmname}
\theoremstyle{definition}
\newtheorem{mthmroman}[mthm]{\mthmname}
\newcommand{\mthmname}{}

\newcommand{\varnewtheorem}[3]{%
  \newenvironment{#1}[1]{%
    \renewcommand{\mthmname}{#2}%
    \renewcommand{\themthm}{##1}%
    \csname mthm#3\endcsname
  }{\csname endmthm#3\endcsname}%
}

\varnewtheorem{theorem}{Theorem}{italic}
\varnewtheorem{corollary}{Corollary}{italic}
\varnewtheorem{definition}{Definition}{roman}
\varnewtheorem{example}{Example}{roman}

\newcommand{\s}{\oplus}
\newcommand{\ds}{\leq^{\oplus}}
\DeclareMathOperator{\im}{im}

\begin{document}

\begin{definition}{A}
This is a definition of $X\ds Y$.
\end{definition}

\begin{theorem}{3}\label{theorem3}
Let $M_R$ be a $C3$-module. If $M=X\s Y$ and $f:X\to Y$ is a homomorphism with 
$\ker f \ds X$, then $\im f \ds Y$.
\end{theorem}

An immediate consequence of Theorem \ref{theorem3} is the following corollary.

\begin{corollary}{3.1}\label{corollary3.1}
Corollaries are easy.
\end{corollary}

And we can even give examples.

\begin{example}{42}
This is an example related to Corollary~\ref{corollary3.1}
\end{example}

\end{document}

Answer 2

\@currentlabel需要更新才能使您的方法发挥作用。以下示例定义了arbitrarytheorem采用单个参数（数字）的环境，并进行了相应更新。这允许您按预期\@currentlabel使用\label- 。\ref

\documentclass{article}

\makeatletter
\newenvironment{arbitrarytheorem}[1]{%
  \par
  \addvspace{.25\baselineskip}% Space above (arbitrary) theorem
  \noindent\textbf{Theorem~#1.}% Theorem heading
  \edef\@currentlabel{#1}% Store current label
  \itshape% Default shape used within theorem
  \ignorespaces
}{%
  \par
  \addvspace{.25\baselineskip}% Space below (arbitrary) theorem
}

\begin{document}

\begin{arbitrarytheorem}{3}
Let $M_R$ be a $C3$-module. If $M = X \setminus Y$ and $f : X \to Y$ is a homomorphism with $\int f \mathrm{d}s X$, then $\int f \mathrm{d}s Y$.
\label{theorem3}
\end{arbitrarytheorem}

An immediate consequence of Theorem~\ref{theorem3} is the following. 

\end{document}