总是让张量积的环位于 \otimes 之下

Question

我宁愿我的张量积看起来像$A\underset{R}{\otimes}B$但仍然写$A\otimes_R B$。有没有办法通过序言中的一些命令来实现这一点？（如果它只在显示模式下工作就可以了）

Answer 1

也许你喜欢把环放在张量积符号下面，但排版却不这么认为。原因如下：

\documentclass{article}
\usepackage{amsmath}

\newcommand{\tens}[1]{%
  \mathbin{\mathop{\otimes}\limits_{#1}}%
}

\begin{document}

Here is a tensor product $M\tens{R}N$, but note that
it will have a very bad influence on the spacing of
lines.
Here is a tensor product $M\tens{R}N$, but note that
it will have a very bad influence on the spacing of
lines.
Here is a tensor product $M\tens{R}N$, but note that
it will have a very bad influence on the spacing of
lines.
Here is a tensor product $M\tens{R}N$, but note that
it will have a very bad influence on the spacing of
lines.
Here is a tensor product $M\tens{R}N$, but note that
it will have a very bad influence on the spacing of
lines.
Here is a tensor product $M\tens{R}N$, but note that
it will have a very bad influence on the spacing of
lines.

\end{document}

简单的改变也许会让您喜欢它，但我不确定是否会非常喜欢它。

\documentclass{article}
\usepackage{amsmath}

\newcommand{\tens}[1]{%
  \mathbin{\mathop{\otimes}\displaylimits_{#1}}%
}

\begin{document}

Here is a tensor product $M\tens{R}N$, note that
it won't have a very bad influence on the spacing of
lines.
Here is a tensor product $M\tens{R}N$, note that
it won't have a very bad influence on the spacing of
lines.
Here is a tensor product $M\tens{R}N$, note that
it won't have a very bad influence on the spacing of
lines.
Here is a tensor product $M\tens{R}N$, note that
it won't have a very bad influence on the spacing of
lines.
Here is a tensor product $M\tens{R}N$, note that
it won't have a very bad influence on the spacing of
lines.
Here is a tensor product $M\tens{R}N$, note that
it won't have a very bad influence on the spacing of
lines.
\[
M\tens{R}N
\]
Here is a tensor product $M\tens{R}N$, note that
it won't have a very bad influence on the spacing of
lines.

\end{document}

---

您可以使用\tens通常的语法：

\documentclass{article}
\usepackage{xparse}

\NewDocumentCommand{\tens}{t_}
 {%
  \IfBooleanTF{#1}
   {\tensop}
   {\otimes}%
 }
\NewDocumentCommand{\tensop}{m}
 {%
  \mathbin{\mathop{\otimes}\displaylimits_{#1}}%
 }

\begin{document}
In line we have $M\tens N=M\tens_{R}N$, but displayed we have
\[
M\tens N=M\tens_{R}N
\]
\end{document}

更符合 LaTeX3 的实现，使用（不太）实验性的e参数说明符（装饰)：

\documentclass{article}
\usepackage{xparse}

\NewDocumentCommand{\tens}{e{_^}}{%
  \mathbin{\mathop{\otimes}\displaylimits
    \IfValueT{#1}{_{#1}}
    \IfValueT{#2}{^{#2}}
  }%
}

\begin{document}
In line we have $M\tens N=M\tens_{R}N$, but displayed we have
\[
M\tens N=M\tens_{R}N
\]
For a derived functor $M\tens^{L}N=M\tens_{R}^{L}N$ or
\[
M\tens^{L}N=M\tens_{R}^{L}N
\]
\end{document}