如何在TeX的lem定理环境中将圆括号替换为方括号

下面提供两种可行的解决方法，都能实现你想要的Lem 1 [Yoneda, 1996]格式效果：

方法1：使用amsthm包自定义定理样式 #

amsthm包提供了更灵活的定理格式控制能力，你可以通过自定义样式指定备注的括号类型：

\documentclass{article}
\usepackage{amsthm}

% 定义新定理样式，将备注的圆括号替换为方括号
\newtheoremstyle{mythmstyle}
  {3pt}% 定理内容上方间距
  {3pt}% 定理内容下方间距
  {\itshape}% 定理内容字体样式
  {}% 内容缩进（无）
  {\bfseries}% 定理标题字体样式
  {.}% 标题后的标点
  {.5em}% 标题与内容的间距
  {\thmname{#1}\thmnumber{ #2}\thmnote{ [#3]}}% 标题格式，这里把括号改为[]

% 应用自定义样式创建lem环境
\theoremstyle{mythmstyle}
\newtheorem{lem}{Lem}

\begin{document}

\begin{lem}[Yoneda, 1996]
Let $F$ be a functor from a locally small category $\mathcal{C}$ to
$\mathbf{Set}$. Then for each object $A$ of $\mathcal{C}$, the
natural transformations $\mathrm{Nat}(h_{A},F) = \mathrm{Hom}
(\mathrm{Hom}(A,-),F)$ from $h_{A}$ to $F$ are in one-to-one
correspondence with the elements of $F(A)$. That is,
$\mathrm{Nat}(h_{A},F) \cong F(A)$. Moreover, this isomorphism is
natural in $A$ and $F$ when both sides are regarded as functors from
$\mathcal{C} \times \mathbf{Set}^{\mathcal{C}}$ to
$\mathbf{Set}$.
\end{lem}

\end{document}

方法2：不依赖额外包，重定义\thmnote命令 #

如果你不想引入新包，直接修改LaTeX默认定理环境的\thmnote命令即可：

\documentclass{article}

\newtheorem{lem}{Lem}
% 重定义\thmnote，将默认的圆括号替换为方括号
\renewcommand{\thmnote}[1]{ [#1]}

\begin{document}

\begin{lem}[Yoneda, 1996]
Let $F$ be a functor from a locally small category $\mathcal{C}$ to
$\mathbf{Set}$. Then for each object $A$ of $\mathcal{C}$, the
natural transformations $\mathrm{Nat}(h_{A},F) = \mathrm{Hom}
(\mathrm{Hom}(A,-),F)$ from $h_{A}$ to $F$ are in one-to-one
correspondence with the elements of $F(A)$. That is,
$\mathrm{Nat}(h_{A},F) \cong F(A)$. Moreover, this isomorphism is
natural in $A$ and $F$ when both sides are regarded as functors from
$\mathcal{C} \times \mathbf{Set}^{\mathcal{C}}$ to
$\mathbf{Set}$.
\end{lem}

\end{document}

内容的提问来源于stack exchange，提问作者innokentijglum